Tag Archives: heat loss

  • The worst building you can build by law

    "We live in an era when our homes have the potential to be energy efficient, comfortable and affordable places to live, despite ever increasing fuel prices. In the past it could be argued that we didn't know how to achieve this but today we have no excuses. Yet, on the whole, we continue to build new houses to a pathetic standard, and our refurbishments commonly concentrate on kitchens and bathrooms, rather than investing to provide warm and pleasant places to live without ruinously high fuel bills." this is an extract from the blog of Ben Adam-Smith a campaigning documentary film maker and creator of a film about poor quality building called 'The Future of Housing ' there is a short  clip below to help illustrate how poor quality construction can directly affect us all.

    There is mounting evidence to suggest that buildings that are being designed to achieve thermal performance standards, including the Building Regulations, are in some cases consuming in excess of 70-100% more energy than the predicted values.[1]

    Plus, some would say more worryingly, that the building Regulations do not set an adequate base level of performance and facilitate the delivery of many buildings of poor quality into the UK housing market.

    But….don’t blame the Building Regulations they are not designed to deliver quality they are a set of minimum standards, all too often they are described, used and promoted as a benchmark; something to aim for:  we must remove the presumption prevalent in the UK that they provide some form of performance guarantee……….they don’t.

    The Building Regulations contain 14 individual sections that, in their own words, ‘contain the rules for building work in new and altered buildings to make them safe and accessible and limit waste and environmental damage’[2] nothing there about quality or performance then.

    airtight-house-diagramWithin the Building regulations Part L is the section on ‘Conservation of Fuel and Power’ which relates to the thermal efficiency / performance of buildings. This is important because it covers the buildings potential (target) level of efficiency and therefore comfort, it also has implications for running costs and is linked to (largely erstwhile) carbon reduction targets.

    In particular section 43 of Part L deals specifically with a buildings air permeability, or airtightness, recognising this as a major factor in a buildings energy performance Interestingly the word “airtightness” wasn’t even a word used in connection with domestic buildings, until it was introduced and formalised (through building regulations in the early 2000’s) and has now become a key part of Part L, section 43 goes on to describe how this the means by which a buildings efficiency can be measured. Compliance with Part L is mandatory throughout the UK[3] it applies to new buildings and certain types of work in existing buildings, and is there to enforce minimum standards of energy efficiency.

    air-test-graphicAirtightness is measured as m3/ (h.m2)@50Pa given as the flow of air (m3/hour) in or out of the building, per square metre of the building internal envelope at a reference pressure of 50 Pascal’s between the inside and outside of the building.  Current Building Regulations require 10 m3/ (h.m2)@50Pa, for a new build property, that’s 10 cubic metres of exchanged air per hour at 50 Pascal’s. This is a low standard by anyone’s reckoning, examples of just how bad this is in reality are easily found. A new build terraced house with a tested result of 9.1 m3/ (h.m2)@50Pa passes Building Regulations but the tester pointed out (see clip below) that with external wind at an average of 20mph it would take just 6 and a half minutes to exchange all the heated air from within the building resulting in an expected increase in annual heating bills of around 50%.[4]

    The short video below is an extract from The Future of Housing which clearly illustrates the point made above, its well worth a watch, most people who watch this have exactly the same astonished and angry reaction as the owner of the house.

    future-of-housing-clip

    So how do you reduce wastage and increase the energy efficiency of a building, in simple terms insulate well and prevent leakage..... and that sits right at the heart of what needs doing.

    In order to arrive at some basic elements that would apply to most building situations we asked the technical team at Ecological Building Systems if they could come up with a simple list of the “workhorses” of airtightness? Could they find six to eight products that would cover most eventualities? The answer came back as a resounding yes.

    First two very pared down observations about airtightness.

    • Airtightness is effected on the inside of a building, on the warm or internal side of the insulation; the function is to prevent leakage. Variously called vapour check, vapour barrier, vapour control, or airtight membrane
    • Wind and weather proofing is effected on the outside of a building the function is to prevent adverse weather penetrating the building fabric and reducing the insulation's capacity to perform and prevent deterioration in the building fabric. Variously called vapour control, breather membrane, vapour open membrane, vapour permeable membrane, vapour open underlay and sometimes additionally described as being diffusion open.

    air-movment-and-insulationTogether this ‘wrap’ for the insulated layer allows moisture control geared to our climate with protection from the elements on the outside and leakage prevention on the inside. The airtightness won’t increase the U value of the insulation but it does ensure that the insulation functions to its optimum performance and more likely to achieve designed U Value. It cannot be over emphasised that airtightness and vapour control go hand in hand they work together to solve different problems.

    Airtightness means designing and installing a continuous seal around the internal fabric of the external envelope to eliminate unwanted draughts. Once the airtightness layer is in place and sealed with flexible and durable tapes, seals and glues, it ensures that the insulation functions to its optimum performance, saving energy and drastically reducing carbon emissions for the lifetime of the building. The airtight layer also ensures that interstitial condensation risk is minimised, ensuring no structural damage from moisture, mould, rot and damp.

    Here is the workhorse list

    1. Internal airtightness membrane Intello Plus
    2. External roofing membrane Solitex Plus
    3. External wall membrane for use with timber frame Solitex Fronta
    4. Universal jointing tape Tescon Vana
    5. Sealing tape for windows Tescon Profil
    6. Sealing tape for masonry and integrating into plaster Contega Solido

    Six products that cover pretty much all the basic requirements, there are various accessories such as grommets, stoppers and glue that will be required but the bulk of the work is done with these six products.

    How to use these products is neatly described in our “Making Airtightness Simple” [sic] guide available to download here

    If you start with the basic principles and keep the products to a proven few then you will be less susceptible to industry ‘noise’ creating confusion or quandary over what product to choose. It would be disingenuous to say the there won’t be times when technical advice is be needed and that is easily available through our technical support team. What it does mean is that for those manufacturers who do it well and make it look simple, take them on trust you are benefiting from years of robust and thorough testing and R&D arriving at a proven and purpose specific product. Their expertise should be your comfort.

    Our thanks to Ben Adam-Smith at Regen Media for his quotes and permission to use a clip from the programme see the whole film by clicking here

    [1] Lessons from Stamford Brook, Understanding the Gap between Designed and Real Performance, Evaluating The Impact Of An Enhanced Energy Performance Standard On Load-Bearing Masonry Domestic Construction, Partners in Innovation Project: CI 39/3/663, Report Number 8 – Final Report, Leeds Metropolitan University

    [2] DCLG Policy paper 2010 to 2015 government policy: building regulation

    [3] Check local variations for Scotland, Wales & Northern Ireland

    [4] The Future of Housing Paul Jennings 2016

  • A brief history of Building Regulation U-Values – with examples

    In this post, we look at how U-Values within Building Regulations have changed over that last 50 years and how that has affected the thermal efficiency of a new build.

    The theme of Building Regulations and U-Values is something we have commented on for many years, the Regulations, especially Part L1A is designed to set minimum standards for the conservation of fuel and power, but they are just that. For those who are looking to build energy efficient homes with good indoor air quality, the Regulations are simply part of the process as the design of such buildings will necessarily far exceed these minimum standards. We have looked at this subject many times in our blog, one of the most popular was an article that described ‘The worst building you can build by law’ subtitled Building Regulations are no guarantee of quality. However it is a fact that the Building Regulations do influence behaviour across the sector and can act as a lowest legal common denominator for any building, exceeding them for many developers has to be a considered choice, one that carries the certainty of return on investment so many still default to ‘Regs compliant’ only building. For this reason, we thought it interesting to look at how the regulations have changed over the past 50 years and how this has affected the baseline performance of our housing stock.

    The theme for this article came from a piece originally written by Jon Davies and published on www.great-home.co.uk  in this post we look at his observations and examples with comment and opinion by Paul Kalbskopf MRICS Senior Building Control Surveyor Wiltshire County Council and ATTMA Level 2 Airtightness Tester Paul Jennings of Aldas.

    In his original article Jon Davies points out that the age of your house is a fairly reliable indicator of its likely thermal efficiency especially if no upgrades have been made to the fabric of the building – although many houses have been extended or otherwise significantly modified over the years. Over the last 50 years, the level of insulation required in Building Regulations has changed drastically to reflect both the need to reduce heating bills and increasing demand for comfort from homeowners. Below we consider how insulation levels have changed and what difference it has made to energy use.

     Determining Factors on Space Heating Demand
    When heating any space there are two critical factors that help determine how efficiently it can be done.  The two factors also interact, typically in a detrimental way, and this has been a major cause of the “performance gap”, about which so much has been written in recent years.

    The first factor is the thermal efficiency of the structure, and within Building Regulations this is largely defined by U-Values for which there are target ‘pass’ values for the key elements: walls, floors, roofs & windows and doors. U-values are arrived at by calculating the combined individual lambda values of all the section components to arrive at an overall number, the number given in the Regulations will be the minimum required.  It is generally accepted that the lower the U-value of an element of a building's fabric, the more slowly heat can pass through it, and so the better it performs as an insulator. The u-value is measured in W/m²K (Watts per square metre per Kelvin) the figure tells you how much energy is lost for every 1°C difference between the two sides of the material. Very broadly, the better (i.e. lower) the U-value of a buildings’ fabric, the less energy is required to maintain comfortable conditions inside the building. If you have a material with a poor u-value then you can generally improve it with insulation - broadly speaking the more insulation you put in the lower the u-value although it’s important to note that thermal bridging can ruin the performance of any installed insulation, make sure these are checked.

    The second factor is air leakage, whether uncontrolled draughts or ventilation heat losses. Heating an insulated space may keep it warm but (and possibly more importantly) if the building leaks then the heat will be lost and the efficiency plummets. Too much air leakage and we lose our expensively heated air much faster than we need to, resulting in bigger heating bills and colder rooms. The circulation of fresh air to maintain good indoor air quality is essential,  so the elimination of leaks coupled with controlled ventilation is one factor in determining an associated requirement within Building Regulations for a certain number of air changes per hour as houses get better insulated to reduce heat loss through the fabric then the heat lost through air leakage starts to become more significant.

    Changes in u-values over the years

    The table below shows the u-values required by Building Regulations for each building component in each decade. Building Regulations actually change more frequently than that (about every 5 years or so and each part of the regulations may be updated at a different time) but it gives a good guide to what has happened over the last 50 years. Highlighted cells indicate the first time the U-Value requirement for a component was strengthened.[1]

    Building Regulations U-Value minimum standards 1970 – present*


    *
    New Build Regulations Part L1A https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/540326/BR_PDF_AD__L1A__2013_with_2016_amendments.pd 

    See also Appendix 1. Below on ‘Limiting fabric parameters’

    Typical 1970s built terraced houses, despite the small size of the house they have front and back gardens with ample parking bays nearby. many of the houses on this estate also have separate garages.

    The oil crisis of the 1970s forced the government to think seriously for the first time about reducing energy usage through regulation. The crisis drove changes in the 1976 Building Regulations which set minimum insulation levels for the first time. Before 1976 the standard cavity wall had not changed much since the end of the 19th Century, although solid, thermally less efficient, walls were still being built into the 1930s. Walls and ceilings were the first target areas with floors and windows following on later. Until 1994 you could still put a single glazed window in a house (u-value 4.8). When double glazing became a requirement the standard was set at 3.1 (what the double glazing industry could achieve at the time). In 2002 stricter regulations were introduced for windows for both new houses and also for replacement windows in existing houses.[2]

     In his article ‘Building Regulations and U-values: How have they changed?’ Jon Davies gives a good example of the impact of the changes in u-values over the years, he provides the following example for a house built in each decade. He uses, in this example, a simplified 3-bedroomed two storey house, a rectangular box 10 metres long x 5 metres wide x 5 metres high (the pitched roof is not counted as part of the house as the floor of the loft is insulated).

    Jon’s example works through the consequences and comparative outcomes for each decade when building to prevailing Building Regulations, he also takes into account air leakage as described in the Regulations as this has an effect on the overall performance and subsequent heating cost. Needless to say, this is an ‘on paper’ exercise and as both he and Paul Kalbskopf points out later ‘as built’ will generally provide poorer results.

    Jon assumes that the occupants try to keep all the rooms at 20°C for 15 hours per day whilst the outside temperature is at zero (0°C); that the floor is a suspended timber floor with clay underneath. The house is constructed with a cavity wall construction insulated to the standard prevailing at the time of build; he also assumes that air leakage is equivalent to an optimistic 1.5 air changes per hour (using this figure to compare ventilation heat loss with fabric heat loss).

    Using the dimensions above gives the following for the simplified 3-bedroomed house:

    • Ground Floor Area: 50m²
    • Upstairs Ceiling Area:50m²
    • External wall area: 125m² (excluding windows/doors)
    • Windows/doors area: 25m²
    • House volume: 250m³

    This simplified house is not that different to a house built today; today’s average house has shrunk a little over the years in length to be 8.6m with a 5.2m width.

    So what is the impact of improving u-values on the heat loss from this house? 

    Building Fabric Heat Loss by Decade Built To Building Regulations

    Jon Davies makes the calculations in the table below in order to show the comparative difference that each change in building regulation has made. The calculations he uses are necessarily simplified based on pure fabric heat loss with no additional factors included.

    He assumes the gas cost at 4p per kWh which means the occupants of the simplified 3-bedroomed house built to today’s Building Regulations would be spending around 16% of what someone living in an unmodified 1970s built house would spend to mitigate fabric heat loss when the outside temperature was a constant 0°C.

    A small terrace of 3 houses part of a 1980s development note how the front garden is now a parking space.

    Jon's figure ignores the cost of heat loss due to air leakage which is worth briefly mentioning. The ventilation heat loss (air leakage) of a 1970’s house would be about 2,475 Watts when the temperature outside was 0°C. That would add about 37kWh and £1.49 to the day’s demand, taking the total heating cost from £5.46 to £6.95.

    Moreover, the air leakage and insulation choices interact to give rise to a significant part of the performance gap mentioned earlier.  Put simply, most of our bulk insulants (mineral fibre, glass fibre and more recently recycled plastic bottles or cellulose fibre) rely upon trapped air for the majority of the insulation they provide. If air is whistling through the insulation in a leaky house, most of the heat we expect to be retained by the insulation is actually being blown away!  Especially in a loft with rolled out batts of fibrous insulation and warm air rising through it, the insulation might only be 1/3rd as effective as theoretically calculated.

    A 2016-built house is expected to have less air leakage but it will not be dramatically better.  A requirement to restrict air leakage was only introduced in the Building Regulations issued in 2002 (see updated 2016 edition, Approved Document L1A: Conservation of Fuel and Power in New Dwellings) - and even now Building Regulations airtightness requirements are sadly weak. Airtightness is checked by an air leakage test when portable fan equipment is used to apply a pressure difference of at least 50Pa relative to atmospheric pressure and the amount of air required to maintain this pressure is measured. But sample testing is still the norm when large developments are being tested, and the whole industry is well aware that those houses chosen for testing tend to get finished to a significantly better standard – draw your own conclusions!

    For this example calculation, the Building Regulations airtightness target is 5m3/(h.m2) and the maximum allowable is 10m3/(h.m2) when tested at 50 pascals, roughly equal to the pressure of an external wind of 20mph.  If you compare this with a Passivhouse air leakage is an order of magnitude lower at less than 0.6 ac/hr, which equates to approximately 0.5 m3/(m2.h) this being one-tenth of the standard target (or 10 times better than it) and one-twentieth of the maximum allowable (or 20 times better than it).

    In a 2016 built house, it is likely that air leakage is a bigger cost than heat loss through the fabric. This is the big opportunity for future improvements in energy efficiency in Building Regulations. However, once you go much below 3.5m³/ (hr per m²) then other features should be added to the house design such as Mechanical Ventilation and Heat Recovery Systems (MVHR systems).

    Jon continues to provide a little more detail about how the heat loss figures are calculated, below are his fabric heat loss workings for the 1970’s house.

    U-value and Fabric Heat Loss for a 1970s Built House

    This tells us that ignoring air leakage, on a cold day 9,100 Watts or 9.1kW will be required to maintain a 20°C temperature. For a 15 hour period, this figure is multiplied by 15 to give 136,500 Watt-hours which is best expressed as 136.5kWh or kilowatt hours. Paying 4p per kWh for gas equates to about £5.46 uplift on the gas bill per day.

    A modern mixed development from 2016. Note the small strip as a front garden compared to previous decades plus the houses have parking for one car or offroad parking.

    Jon calculates the heat loss because of air leakage (ventilation heat loss) by multiplying the volume of the house (250 m³) by the air changes per hour (1.5) by the temperature difference (20°C) by 0.33 (energy required to heat 1m³ of air). This gives 2,475 Watts of heat loss that converts to an extra 37.12kWh of energy and £1.49 per day. Air leakage probably reduced as construction standards have generally improved between 1970 and 2016 but it is hard to quantify without air leakage testing on individual houses

    Paul Kalbskopf adds that Jon Davies’ excellent example has set out the theoretical requirements and costings for work carried out perfectly (as Jon notes).  As we are all too well aware, the practical outcomes in this fallen world are very different.  Practically all our buildings are prototypes, and, what is erected even in the controlled conditions of a test centre is rarely reproduced in the mucky reality of building sites affected by the daily changes of the UK climate.

    This has been recently confirmed by work carried out by Colin King and his colleagues from the BRE (Building Research Establishment) who, by performing in-situ U-value tests on a range of buildings of all ages across the country, have revealed some startling results[i].  (The pre-1900’s solid walled homes are performing twice as efficiently as we had assumed and the more recent, so-called low U-value elemental homes being compromised by poor construction standards; the 1940’s/50’s cavity-walled homes being worst of all!) Moreover, re-evaluation of condensation risk analysis methods, thermal storage capacity (and hence thermal inertia), microclimatology and air handling/air movement, should be causing us to consider buildings in a different light.

    While a combination of materials in any given thermal element will result in virtually the same U-value irrespective of the order of the layers in the ‘sandwich’, it will have a dramatic effect on thermal capacity, interstitial condensation risk and ultimately, comfort and health factors.

    The requirement for higher air-tightness requirements is a double-edged sword: whilst it reduces the energy loss due to uncontrolled ex- and infiltration, without addressing air quality, an increase of house dust mites, condensation, mould growth and ultimately rot can increase.  While there are four different systems that may be used to comply with the performance specification requirements, the blinkered view of most designers and builders is to stay secure and comfortable in the old ways of doing things.  Changing the way we do anything in our industry is akin to changing the direction of a fully laden supertanker!

    Therefore, the suggestion in Building Regulations Part F section 5 of employing something other than locally operated isolated extract fans is just that – a suggestion.  SAP calculations do not often allow for an air leakage rate of <3m3/(h.m2) at 50Pa, as most assessors will try to achieve design compliance at levels above that due to construction vagaries and quality.  However, on pre-completion testing, if a figure of <3m3/(h.m2) is achieved and an MVHR (mechanical ventilation with heat recovery) system has not been installed, there is huge potential for problems as outlined above, as a retro installation will be inordinately expensive and disruptive.

    Passive stack ventilation (PSV) only works if there is constant air input at a low level.  Any visible openings are often closed as the occupier views the incoming air as a cold nuisance.

    Mechanical extract ventilation (MEV) systems without heat recovery would appear to be literally missing the point if the priority is to maximise thermal efficiency, however, the fundamental reason to ventilate a building is moisture management (including odours) a critical factor in accelerated building decay.

    The other major factor is that the standards set out in the Approved Documents are only minimum standards.  Sadly most people – developers, builders, homeowners, see this as a maximum to be built up to, rather than a minimum to be built from. This attitude has sadly been disseminated throughout the industry by virtue of the major house builders who, in the name of maximising profit, build down to the minimum provisions required by Building Regulations, at minimal quality standards.  Paul Kalbskopf comments that in his 40 years in the industry, he has rarely seen a multi-house developer do anything more than the (Criminal) law requires.  The attitude is one of immediate expediency for short-term gain (profit for the shareholders) and for the foreseeable future, this looks set to remain unchanged

    FOOTNOTE

    Here's another statistic that affects our comfort and our pockets, we are building smaller and smaller houses, smaller houses you would think will require less heat (not necessarily the case as we have set out above) but the size of the building and plot does have a direct bearing on occupant comfort and well being, according to research published by www.onthemarket.com new build homes today are often 20% smaller than homes built in the 1970s. Forty years ago there was room for a garage and two cars on the drive of most semis commonly built 12 to the acre. Today, buyers are lucky to get one parking space outside a terrace, built up to 24 to the acre.

     

    Appendix 1.

    Paul Kalbskopf notes, within the current Building Regulations[ii] there is now a requirement to insulate/ seal party walls to achieve U-values as set out in Table 3 of L1A, which vary from 0.0 to 0.5, depending on whether solid, filled or unfilled or sealed or unsealed. See Building Regulations Part L1A Online Version pages 14/15

    Introduced in L1a 2014 is a requirement for swimming pool basins, if one is so fortunate, max. U-value 0.25.

    Paragraph 2.33 alludes to Table 2, which gives the ‘Limiting fabric parameters’.  The vital sentence is the third: The shame is that the word ‘likely’ is used, rather than ‘should’ or ‘must’.

    Table 3 does give values to which we try to achieve, but, given we are dealing in area-weighted averages, there will still be scope for variations to be at the (poorest performing) limit.

    Windows are a case in point where, for example, the insertion of an unsealed trickle vent into a hollow profile frame will make a mockery of the profile U-value and hence the ‘whole window U-value’ as given in Table 4.

    The original article can be found at http://great-home.co.uk/building-regulations-u-values-how-have-they-changed/ January 19, 2016 Jon Davies

    [1] http://great-home.co.uk/building-regulations-u-values-how-have-they-changed/ January 19, 2016 Jon Davies

    [2] http://great-home.co.uk/building-regulations-u-values-how-have-they-changed/ January 19, 2016 Jon Davies

    [i] https://www.bre.co.uk/filelibrary/pdf/projects/swi/UnintendedConsequencesRoutemap_v4.0_160316_final.pdf

    [ii] Building Regulations 2013 Edition Incorporating 2016 Amendments for Use in England.  Approved Document Building Regulations Part L1A the Conservation of Fuel and Power. Online Version.

    Further reading.

    LABC Warranty Survey 2018 ‘What is the average house size in the UK?’

    https://www.labc.co.uk/news/what-average-house-size-uk?language_content_entity=en

     

     

     

     

     

     

     

     

     

     

     

    Facts

     

    New build homes today are often 20% smaller than homes built in the 1970s. Forty years ago there was room for a garage and two cars on the drive of most semis, commonly built 12 to the acre. Today, buyers are lucky to get one parking space outside a terrace, built up to 24 to the acre.

    Source www.onthemarket.com

     

    Interesting article

     

    LABC Warranty Survey 2018 ‘What is the average house size in the UK?’

    https://www.labc.co.uk/news/what-average-house-size-uk?language_content_entity=en

     

     

  • Designing and building with natural insulation materials

    Why what we build from matters

    The best buildings are life-enhancing and support our physical and mental health. Great design and healthy products enable the delivery of a healthy internal environment - meaning good indoor air quality, natural lighting as well as excellent thermal and acoustic comfort. In order to do this, we have to ensure we practise good decision making which requires an informed and holistic approach. Products that are low embodied carbon, natural, non-toxic, and healthy such as natural insulation have an important part in delivering better buildings.

    But it’s not only what a building is made of that contributes to a healthier living environment ventilation also plays a significant role. Where non-sorptive materials (i.e. in this instance ones that cannot absorb and release water as a vapour or liquid as opposed to sorptive ones that can) are used such as Polyisocyanurate (PIR) insulation moisture needs to be ‘managed out’ of a building to prevent poor air quality or potential condensation within the structure, non-sorptive materials are highly prevalent in modern construction methods meaning that the ‘fabric’ itself cannot help buffer and moderate the internal environment and ventilation is the only strategy to remove water vapour or pollutants. However, emerging evidence suggests that relying on ventilation strategies alone to provide healthy air inside low energy buildings is, in many cases, presenting significant risks to the health of occupants as well as the health of the building fabric.[i]

    In order to build better, healthier more efficient buildings and taking a holistic approach the inevitable conclusion is that alternative strategies and materials should be seriously considered in order to achieve these elevated levels of performance. This presents a real opportunity to leverage design and natural building materials to deliver better standards. As insulation by volume is a significant part of any build cost, plus it has a direct correlation to building performance and occupant health, this is where the focus of building designers, architects, developers and owners is moving.

    The 'Protexion Campaign' to promote natural insulation materials.

    To address these issues and help promote the already growing market for natural insulation materials in the summer of 2018 Ecomerchant and Steico UK joined forces to launch a campaign to champion the benefits of using natural insulation products.  The same principles that sit behind the promotion of natural insulation products were echoed by the Alliance for Sustainable Building Products and the Natural Fibre Insulation Group, the members of which, originally proposed that more work needed to be done to highlight the considerable benefits of natural insulation to a market that has largely ignored them in favour of cheap synthetic materials. Despite the clearly defined, tested and verified performance natural insulation worldwide it has not been taken up in the UK as much as in other countries. In the UK the default insulation materials are still mineral (glass) wool and foil backed Polyisocyanurate (PIR) however previous cost savings afforded by synthetic insulation have largely been lost and the price differential assumed before in favour of synthetic insulation has narrowed to the extent that natural insulation options can now be less expensive than synthetic ones plus the increase in timber frame and the desire for better airtightness is driving constructors towards natural solutions. Year on year sales in natural insulations have seen double-digit growth and a widening of the customer base to include modular construction, custom and self-build and social housing.

    To help inform potential users of natural insulation materials the Protexion campaign developed a dedicated website www.ecomerchant.co.uk/protexion  where you will find the wheel (illustrated below) the wheel has dynamic segments (links) e.g. health, fire and acoustic which click through to more information on each subject, you can also download wood fibre insulation certifications and find toxicology reports and environmental product declarations, this is the type of clear unambiguous information that allows us to make informed and better design choices.

    The Protexion wheel, each segment links to the relevant role with supporting information, the wheel also links to accreditations, EPD's and toxicology reports. Click the image above to link to the Protexion site.

    The appeal of natural insulation materials

    How we select insulation needs to be about having a real choice and for specifiers to be equipped with the right knowledge to compare materials on a like-for-like basis plus different parts of the building will require different performance criteria no one insulation type will be the best for all applications.

    To design a well-insulated building, you need to make informed decisions throughout all phases of a construction project to ensure your building performs as you envisage as mentioned above.

    However, selecting the right insulation is about more than just reaching building regulation compliance or ‘keeping in the heat’. It’s about ensuring a building protects its occupants’ entire well-being and comfort, the following list covers most of the core benefits and features of natural insulation and highlights the role they can play in delivering better, healthier and low impact buildings.

    How well does insulation keep the heat out?

    Summer overheating

    High internal temperatures can cause respiratory or cardiovascular problems. Work by CIBSE and Arup suggests that most people begin to feel ‘warm’ at 25°C and ‘hot’ at 28°C. Their report also defines 35°C as the internal temperature above which there is a significant danger of heat stress. For vulnerable occupant groups, the impact of overheating can take effect much sooner with potentially much poorer outcomes.

    Low fabric thermal mass leaves buildings more vulnerable to uncomfortably high, and in some instances, dangerously high internal temperatures in summer. This problem of summer overheating has been identified, by the NHBC and others, as a particular problem in buildings vulnerable to excessive heat gain with inadequate ventilation.

    In the UK, thermal insulation to protect from the cold is essential, particularly given ever-increasing energy costs. However, as demand for the usable square footage of buildings increases, basement and loft conversions are the routes many now take. However, these parts of a home or office, are the spaces most prone to extremes in temperature. They, therefore, need more thought – i.e. how do you keep a space warm in winter but, for a loft, how to keep it cool come summer.

    Compared with synthetic insulation materials, wood fibre insulation has a much higher density. This higher density means that natural insulation makes for a better heat buffer as the high midday temperature will only reach the internal side and be lost at night when the temperature is already cooler outside.

    High internal summer temperatures are caused by heat from appliances and occupants, solar gain through windows and external heat penetrating through the fabric. It is the latter issue of penetrating heat where the thermal mass of natural insulation systems can delay the arrival of this heat energy so that it is emitted internally in the relative cool of the night. Perhaps good design with natural systems can hit a ‘Goldilocks zone’ of just the right levels of thermal mass and thermal conductivity.

    Thermal comfort

    Maintaining internal temperature around a comfortable mean is at the root of good fabric first low energy design. In lightweight constructions, some degree of thermal mass provided by the fabric helps to smooth out the internal temperature fluctuations which may be caused by heating systems or the opening and closing of windows and doors, for example. Natural insulation and systems tend to have high thermal mass relative to other types of insulation. This is due to the inherent physical properties of the cellulose or protein-based fibres and significantly enhanced by the presence of chemically bound water contained in these fibres. Water has a very high heat capacity which is twice that of concrete so its presence in natural fibres adds to the ability of the insulation to absorb heat energy.

    How a building’s lack of breathability is hurting our health

    A breathable structure is one that allows the passage of moisture.

    Those of us committed to the development of natural insulation products and systems view fabric breathability, or more accurately, the dry transport of moisture, as an important component in overall fabric performance. The ability of natural and hygroscopic materials to absorb and release water whilst remaining dry reduces the risk of interstitial condensation and ultimate fabric failure.

    Natural fibres constantly adjust humidity levels away from extremes of damp and dryness helping maintain air moisture at comfortable levels, reducing the risk of both surface condensation and the negative health risks from moulds, mites and viruses. Of course, fabric breathability is not an alternative to a good ventilation strategy but should be considered as part of a robust and healthy building strategy.

    In a report titled ‘Health and Moisture in Buildings,’ the authors conclude that ‘these risks [moisture in buildings] combine with the other more clearly defined risks to the durability and value of the building fabric. It is relatively easy to see and to cost the damage done to buildings where moisture imbalance occurs. It is estimated that perhaps 70 to 80% of all building damage is due to excessive or trapped moisture’  With such a large percentage of all building construction problems associated with water in some way, breathability is an essential component in preventing the accumulation of harmful water within the building’s fabric.  This is fundamental in reducing health risks from mould and mites that those suffering from respiratory illnesses such as asthma and chronic obstructive pulmonary disease (COPD) are particularly susceptible to.

    For effective breathability, there are four essential components that need to be considered:

    • a moisture pathway
    • a driving force
    • a sorptive fabric
    • vapour control.

    Natural fibre insulation is most effective as it suppresses potentially harmful water by binding and releasing moisture which helps regulate humidity levels as the moisture moves.

    Easy-to-fit insulation

    A well-designed building takes into consideration how a material performs throughout the building’s entire life cycle. This includes ease of installation. Steico’s wood fibre insulation is simple and easy to fit (either packed or friction-fitted), eliminating installer error, keeping construction programmes tight and costs low.

    How insulation is fitted into or onto the building also has an impact on performance, poorly fitted insulation will allow the passage of air through the structure which can quickly strip out the heat from a building. Tests by Paul Jennings from Aldas featured in the documentary The Future of Housing demonstrated that a building with an air change of 9 m3/hour/m2 @ 50 pascals  (Building Regs stipulates 10) when subject to a modest 20 miles/hr wind will take just 7 minutes to remove the heat from the building, what this shows is that regulatory compliance is not a good indicator of building efficiency, a guarantee of lower bills or occupant comfort.  Minimising air movement through insulation is helped if insulation is designed to help restrict airflow, features such as tongue and groove profiles and dense fibre friction fit batts help to eliminate and reduce air pathways through the building.

    Indoor air and occupant health

    Creating and maintaining a healthy and comfortable indoor environment is a complex and difficult challenge. Temperature, humidity and carbon dioxide (CO2) must be maintained at safe and comfortable levels. Moreover, the introduction of pollutants such as particulates and volatile organic compounds (VOCs) greatly influences indoor air quality. A robust ventilation strategy is clearly critical to CO2 levels, but the building fabric can play an important role in helping to manage temperature, humidity and pollution levels. Sheep’s wool insulation, in particular, can mitigate and absorb harmful indoor emissions including formaldehyde; the high levels of Keratin based in sheep’s wool are known to react and eliminate formaldehyde test results[ii] showed that Thermafleece sheep’s wool insulation absorbed 90mg formaldehyde per 1kg of insulation.

    Internally generated air pollution

    Finally, there is a very real and growing problem of indoor air pollution. The problem of poor external air is now well documented with a recent report from the Royal College of Physicians, Every Breath We Take, indicating that air pollution is leading to an estimated 9,500 annual premature deaths in London alone. The report authors recognise the current lack of focus on indoor air. Nonetheless, clients and designers can have significant influence over VOC and particulate levels by selecting low or zero emission products and systems.

    A quick look at the issue of fire

    All insulations will meet fire safety standards, but this is a minimum rating. Fire protection is a challenging topic and it combines materials (including fire testing and certification) and design in an effort to minimise risk, in all cases it is the mix of these two elements that will determine regulatory and performance compliance. There are also issues to consider in the use of fire retardants which evidence shows can be detrimental to human health[iii] However, there are some inherent properties of natural insulation that should be considered. Most natural insulation materials either resist combustion such as sheep's wool or 'char' quickly creating a carbon layer that helps resist the spread of fire such as wood fibre, additionally when burnt they will not give off toxic fumes such as cyanide as polyisocyanurate (PIR) or petrochemical insulation materials do.

    Will the house be standing in 100 years?

    Condensation is one of the costliest risks to buildings causing huge maintenance repairs and structural damage. Natural materials are better able to absorb and release water meaning it is better able to protect from and buffer moisture thereby becoming a key part of healthy living and building durability.

    Comfort for occupants

    Insulation improves comfort by moderating external effects and smoothing out variations. This applies to cold, heat and sound. One of the overlooked benefits of natural insulation is delivered by increased mass. This means it is better at reducing both overheating and noise pollution than synthetic insulation.

    Improved well-being

    Evenly warm walls deliver more radiant heat. Because people find radiant heat particularly pleasant, it is frequently possible to lower the actual ambient temperature without reducing the internal comfort. This leads to the positive side effect that reducing the ambient temperature by one degree means approximately a 5% energy cost saving.

    Effective protection against mould

    The humidity in the air will only condense on a cold wall, by creating warm walls, this condensation is eliminated. Without damp patches, mould is unable to grow. Mould is avoided from the outset.

    Reduced air movement

    Draughts caused by convection can be unpleasant. At uninsulated, external walls the air cools down, falls to the ground and flows to the centre of the room where it warms up and ascends again. This does not occur with well-insulated buildings. The cooling effect is reduced or eliminated. If the air is still, we do not feel these draughts and less dust is disturbed, providing positive side effects, particularly for allergy sufferers.

    Cancelling out the noise for a peaceful night’s sleep

    The higher density of natural insulations - such as wood fibre - makes them better at reducing noise. Sounds external to the building, such as traffic or music, as well as those from within the building, through walls and ceilings are attenuated better by wood fibre than synthetic equivalents. In providing better protection from acoustic pollutants, occupants often report a building as being more restful and relaxing thereby encouraging better mental health.

    When a building is well-designed and well-built, occupants should be at their peak comfort. With the average person spending approximately 80% of their lives in enclosed rooms, an occupant’s well-being is imperative. Therefore, the products used to achieve this should cover all the issues affecting a building’s construction, its impact on both its occupants and nature.

    Buildability

    During construction, the great British weather inevitably gets a building’s shell thoroughly soaked before the roof goes on and it can begin to dry out. Using insulations that trap moisture and do not allow it to easily escape can cause damage to timber frame buildings and roof structures. Wood fibre sheathing and sarking boards are designed to be exposed to the elements during construction by adding in paraffin wax (candle wax) to the mix during manufacture. This means rain will repel, even when a board is cut, as the wax is ‘through and through’. Additional protection should be considered if the wood fibre is too exposed to prolonged bouts of heavy rain.

    Whilst wood fibre insulation can appear to absorb rainwater it dries very quickly afterwards without any detriment to the insulation material itself. Materials such as glass or mineral wool take up water in a similar way but are not able to dry quickly and should be removed if soaked to avoid damage to timbers.

    Obviously, you should try and avoid soaking your insulation materials but if the worst happens you know that wood fibre will have no issues.

    Wood fibre is clean and easy to use, there’s no chance of toxic fibres or dust, in short, it’s easier to handle and fit meaning that the installer tends to achieve a higher quality job. The snug fitting batts leave no gaps and the tongue and groove profile for the rigid boards ensures a tight secure fit. Disposal costs are less as natural insulation requires no specialist waste facilities.

    Conclusions

    Buildings should be considered not as standalone discrete entities, but as part of a system in constant and dynamic interaction with people and the environment. This interconnectedness means benefits, problems, solutions and consequences cannot be effectively addressed in isolation. If we adopt this broad and holistic approach, the benefits of natural insulation products and systems will come to the fore, and we should then expect the rate of market uptake to accelerate dramatically.

     

    Authors note:

    Thanks to the following for contributions to this article

    The Alliance for Sustainable Building Products

    The Natural Fibre Insulation Group

    The UK Centre for Moisture in Buildings

     References

    [i] ASBP-Briefing-Paper-The-health-and-wellbeing-benefits-of-natural-insulation-products-and-systems

    [ii] Eden Renewables ASBP Presentation Healthy Buildings Conference and Expo 2017, February 2017

    [iii] greensciencepolicy.org Flame retardant chemicals in building insulation

     

  • Design elements that help prevent overheating

    The current prolonged spell of hot weather will have highlighted to many of us the susceptibility of our buildings to overheating.

    The issue of overheating in buildings is a serious health risk, figures published on mortality rates during heatwaves make grim reading but still, our construction methods and building regulations fail to deliver buildings resilient to overheating.

    In this article, we look at why this is important, how we can design for cooler more comfortable buildings and what to look out for when planning building work.

    What is a heatwave?

    The World Meteorological Organisation definition of a heatwave is "when the daily maximum temperature of more than five consecutive days exceeds the average maximum temperature by 5oC, the normal period being 1961-1990". They are most common in summer when high pressure develops across an area. High-pressure systems are slow moving and can persist over an area for a prolonged period of time such as days or weeks.

    They can occur in the UK due to the location of the jet stream, which is usually to the north of the UK in the summer. This can allow high pressure to develop over the UK resulting in persistent dry and settled weather.

    When was the hottest heatwave in UK history?

    The scorching summer of 1976 was the hottest summer since records began. It led to a severe drought owing to the exceptionally dry conditions, although it is thought that 1995 was drier. In the summer of 1976, Heathrow had 16 consecutive days over 30oC from June 23rd to July 8th, and for 15th consecutive days from June 23rd to July 7th temperatures reached 32.2oC somewhere in England. But the single hottest temperature of 38.5oC was set on August 10th, 2003, the heatwave of 2003 led to the government creating a Heatwave Plan for England in response to the increase of deaths directly attributed to the heatwave.

    You can read the Heatwave Plan here https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/711503/Heatwave_plan_for_England_2018.pdf

    The current 2018 summer heatwave has seen the hottest day of the year so far recorded on Sunday, July 8th  a temperature of 32.4oC (90F) recorded in Gosport, Hampshire.

    Outdoor Temperature Thresholds – the effects on health

    Statistical analysis shows that maximum daytime outdoor temperatures are a predictor of heat-related mortality.

    In London, mortality starts to rise when the maximum daily external air temperature goes above 24.7ºC and has been estimated to rise by approximately 3% for every further 1ºC increase in external temperature. In other regions, the thresholds at which mortality starts to rise are lower. For example, the threshold for the North East of England is 20.9ºC.

    Heatwave Plan Regional Threshold Temperatures for Heat-Health Watch Alert Levels 2-4.

    Design considerations to prevent overheating - decrement delay and thermal buffering.

    Anyone familiar with spending a hot summer's day in a caravan and then another in a stone house with closed shutters will appreciate the meaning of ‘Decrement delay’. The inside of the caravan closely maps the rise and fall in external temperature to provide the familiar stifling effect on the occupants. In contrast, the internal air temperature of the stone house stays well below the midday heat, barely varies throughout the day and so provides relative comfort to those sheltering from the sun.

    In the caravan, as soon as the outside cladding starts to heat up, output is detected within minutes on the inside face as most of the heat quickly transfers through the aluminium / lightweight insulation composite; whereas as the face of the stone wall heats up, the heat is absorbed by the stone and progresses slowly from the outside inwards. Hours later, some of that heat has arrived on the inside face of the wall whilst the remainder is released back into the cooler evening air.

    The interesting, and often baffling, aspect of this phenomena is that the two materials can have very similar u-values - so that in steady-state conditions where heat applied at a constant rate over a period of time to the external face of both materials, there is an equally constant flow of (diminished) heat from the inside surfaces. Crucially though, for the purposes of thermal design, one material will start delivering heat to the inside before the other.

    Heat transfer factors: Conductivity, Density and Specific Heat Capacity

    But let’s start with understanding how different materials cause different heat flow rates.  What’s actually happening within the materials? The answer relates to the dynamics acting between three variable characteristics whose values are unique to each material. The rate of heat transfer is determined by:

    And of course, a further factor is the quantity or thickness of the material the heat is transferred through.

    Thermal Diffusivity

    Thermal Diffusivity ties the above factors together into an equation that measures the ability of a material to conduct thermal energy relative to its ability to store thermal energy.  In effect, it is a measure of thermal inertia or ‘buffering’.

    The equation is:

    Thermal diffusivity = thermal conductivity / specific heat capacity x density

    Examples:

    • Rigid polyurethane insulation has a thermal diffusivity of approximately 4.46 x 10-7 m2/s
    • Timber fibre insulation has a thermal diffusivity of approximately 1.07 x 10-7 m2/s
    • Copper has a thermal diffusivity of around 1.11 × 10−4 m2/s

    In a material with high thermal diffusivity, heat moves rapidly through it because the substance conducts heat quickly relative to its volumetric heat capacity or 'thermal bulk'.

    In the above three examples, we can see that heat races through copper while it moves more rapidly through rigid polyurethane than it does through timber fibre board.

    Introducing the variable heat source - Periodic heat flow

    ‘Thermal diffusivity’ then accounts for the different rates of heat transfer through the variety of materials from a constant heat source.

    However, when we look at the transference of the heat from the sun striking real-world roofs and walls, the heat source is not constant. The variability caused by such as the sun’s passage through the sky is known as ‘periodic heat flow’ and, because the sun behaves, in the same way, every day (‘diurnal’), its effects can be designed for.

    Decrement delay

    So, what is ‘Decrement delay’ and what can it do for us? In discussing its use we’re looking to take advantage of the fact that some materials are slower at transferring heat than others. In buildings, the benefits of decrement delay are only realised where the outside temperature fluctuates significantly higher and lower than the inside temperature. So, ideally, if the maximum heat delivery to the wall or roof is at around midday, and because we can achieve a delay in heat transfer, it should be possible for that heat to finally penetrate the wall or roof to the interior of the building some time later when the inside of the building is relatively cool and the outside temperature has fallen as the sun goes down. At this point, the heat ‘stored’ in the wall can be released from the wall in both directions without overheating the inside.

    The time it takes the peak temperature in the middle of the day on the outside of a wall or roof, to make its way to a peak temperature on the inside face, is called 'time lag'’ phase shift or, more commonly, 'Decrement delay'.

    By controlling decrement delay it is possible to control and prevent the overheating of a building.

    A delay of between 8 and 12 hours might be considered optimum in normal conditions during extended periods of hot weather and cloudless skies up to 16 hours may be required.

    It’s worth noting too at this point that in a construction element containing several layers, the sequence of the material layers that heat passes through is also a factor in determining decrement delay. For example, in masonry construction where insulation forms one layer, locating it on the exterior of the masonry can significantly enhance the decrement delay effect.

    The image below shows a roof make up with a variation in decrement delay (phase shift) achieved only by changing the insulation type, the overall section U value remains the same but the difference in phase shift is 8.8hours between the maximum at 16 hours and the minimum at 7.2 hours.

    Image courtesy of Steico


    How Decrement delay / Thermal buffering works on a rendered stone wall



     

    Amplitude damping and the ‘Decrement factor’

    A stable indoor temperature is an aspect of thermal comfort. In conditions where the outside temperature fluctuates relatively widely, a constant indoor temperature is desirable. For example, an outside temperature might vary between 10ºC and 30ºC while internally it might vary just 1ºC above or below 20ºC. The fabric of the building envelope has effectively dampened the degree of oscillation.

    The ability to attenuate the amplitude of the outside temperature to that of the inside is known as the 'decrement factor'.

    It is calculated as f (the decrement factor) = Ti (the maximum swing from the ambient temperature on the inside) / Te (the swing in external temperature)

    Example: From above, the peak to peak amplitude of the outside temperature is 20 degrees, and for the inside temperature it is 4 degrees.

    f = Ti / Te = 1 / 10 = 0.1

    Hence the closer the decrement factor approaches zero, the greater the effect the construction element has on attenuation. I.e. the smaller the decrement factor, the more effective the wall / roof at suppressing temperature swings.

    The decrement factor is determined by the type and thickness of the materials that make up the wall or roof that the heat passes through.

    (Confusingly, the same effect or 'amplitude suppression' is often expressed as a straight ratio. Taking the example above, the ratio would be 10:1 or 5:1 or more often just '10')

    For a construction element containing several layers, the sequence of the material layers that heat passes through is a factor in determining decrement delay. For example, in masonry construction where insulation forms one layer, locating it on the exterior of the masonry can significantly enhance the decrement delay effect.

    Calculating decrement delay

    The response of construction elements to periodic cycles in temperature and heat gain can be quantified by using the thermal admittance framework as described in EN ISO 13786:2007. The framework also provides the basis for the CIBSE 'Simple Dynamic Model' for calculating cooling loads and summertime space temperatures (CIBSE (2005) Guide A: Environmental design).

    Manually calculating thermal response simulations is not for the faint-hearted, but a number of programs are available to take the load - notably the freely available, Excel spreadsheet based 'Dynamic Thermal Properties Calculator' developed by Arup and distributed by the Concrete Centre.  https://www.concretecentre.com/Publications-Software/Publications/Dynamic-Thermal-Properties-Calculator.aspx

    The importance of decrement delay

    Decrement delay will nearly always be more important in hotter climes than in the UK. An exception though is in timber/steel frame construction. One of the more common criticisms directed at lightweight construction is the lack of thermal mass - which can lead to the familiar 'caravan effect' above. Whereas masonry construction has the obvious benefit of 'heavy' materials such as brick and block, framed structures are typified by combinations of a cavity and lightweight insulation - leading to low thermal diffusivity and so little in the way of decrement delay.


    Steico Special Dry Wood fibre insulation boards used on a pitched roof. Picture Courtesy Kithurst Builders Ltd

    Until recently, insulation products were chosen mostly on the basis of a combination of their u-value and their thickness. Since the most commonly used insulation materials such as polystyrene, polyurethane and mineral wool had broadly similar densities and heat capacities, their decrement capabilities were relatively insignificant. With the increasing availability of wood fibreboard insulation materials in the UK, boasting comparatively high levels of diffusivity, designers can look forward to realising thermal performances more closely mapped to traditional masonry construction.

    Some examples of materials

    Don’t building regulations already set a standard?

    Approved Document Part L1A is designed to drive the conservation of fuel and power, rather than set thermal comfort standards. It requires housebuilders to make “reasonable provision to limit heat gains” in dwellings in order to reduce the need for mechanical cooling. Specific criteria or thresholds are not specified. The overheating ‘check’ in SAP Appendix P provides a means of demonstrating that reasonable provision has been made, but the calculation is not integral to the SAP rating and it is unclear what happens if a development fails the test.

    The dwelling should have appropriate passive control measures to limit the effect of heat gains on indoor temperatures in summer, irrespective of whether the dwelling has mechanical cooling. The guidance given in paragraphs 2.38 to 2.42 of this approved document provides a way of demonstrating reasonable provision.” Criterion 3, Approved Document Part L1a

    The Standard Assessment Procedure (SAP) is the Government’s procedure for rating the energy performance of homes.

    Designers and developers in the UK need to show compliance with SAP for each of the domestic units they are designing. It is not a design tool, but rather a compliance tool intended to produce an energy rating.

    SAP Appendix P is a simplified check of whether the home could have an overheating problem. It uses regional average external air temperatures for the months of June, July and August, heat gains and fabric characteristics of the building in order to calculate monthly mean summer internal air temperatures. These are then compared to a table of threshold temperatures. For monthly mean internal temperatures below 20.5°C, overheating risk is predicted to be ‘not significant’, whereas for temperatures of 23.5°C and above, the risk is ‘high’.

    Issues with SAP

    The temperature inside a home depends on many variables and changes throughout the day. Overheating risk during severe hot weather events cannot be calculated using monthly average temperatures. Neither can the impact of the Urban Heat Island effect or future changes in climate.

    Housing Providers and experts consulted by the Zero Carbon Hub raised many concerns with SAP Appendix P. For example, it allows unrealistic assumptions to be included, such as that windows are constantly open, which make it too easy to pass.

    SAP 2012 (Appendix P): Levels of threshold temperature corresponding to a likelihood of high internal temperature during hot weather.

    To sum up

    We have to acknowledge that overheating is a problem to be avoided and that we don’t need a heatwave to prod us into action, buildings can overheat for a multitude of reasons but the use of materials that help buffer heat preventing rapid transfer through the building can be used to substantially mitigate the risk. Typical areas that will benefit from such design are timber frame and lightweight structures, on a brick or masonry house this will usually be the roof. This is an important area to factor in heat buffering as it combines the warmest part of the house (hot air rises) with the largest solar collector on the building – the roof itself. Wherever possible design with overheating in mind, the Building Regulations do not require minimum standards for decrement delay and SAP is currently under review regarding overheating, so the choice to design and build to prevent overheating is one that rests solely with you.

     Further reading

     BRE Overheating in Dwellings

    Tackling Overheating in Buildings

    Overheating - a growing threat that mustn't be ignored

     

     Our thanks to Sandy Patience Dip Arch RIBA editor of www.greenspec.co.uk for material used in this article

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