Tag Archives: airtight tape

  • 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

     

     

  • What exactly is a VCL?

    Vapour Control Layer (VCL) Explained

    One of the most commonly used, and widely recognised, acronyms in construction is VCL, which stands for vapour control layer. A VCL is a critical building component designed to protect the building from potential degradation (or poor performance) by managing the passage of water vapour within a building structure. In other words, it is used to manage condensation risk. Condensation is formed when warm moist air condenses into a liquid on contact with a colder surface. A vapour control layer is typically installed on the internal side of the insulation to control the passage of warm moist air (water vapour) entering the structure. However, as a simple acronym, there is a problem because in most applications a specific level or type of performance is required, as a ‘catch all’ acronym VCL is wide open to error.

    Condensation on the internal side of a window, showing that there was plenty of water vapour in the room, which has condensed onto the cold internal window pane. This is a good example of how much water vapour can be available to penetrate and condense within a building structure.

    Unfortunately, and perhaps part of a wider misunderstanding, to many the term VCL is a synonym for polythene sheet, add in only a basic understanding of how condensation forms (as described above) and it is easy to see how the use of the term VCL can cause considerable confusion and anxiety. This is made worse by a large number of alternative terms such as vapour check, vapour permeable membrane, vapour barrier, vapour retarder, ACL, AVCL, vapour diffusion retarder, variable diffusion membrane, monolithic membrane, vapour diffusion barrier, airtight membrane, vapour tight membrane, microporous membrane, breather membrane all used to describe products covered by the acronym VCL …………………frankly it’s no wonder people get confused by all of this. In this article, we will try to bring some clarity to the description and use of internal membranes to help you decide which type to use, where and how. A companion piece is in preparation about external membranes.

    Broadly put there are 3 types of internal membrane.

    Type 1: An impermeable barrier such as polythene, this lets nothing through.  It’s a vapour barrier or a vapour block, has only one function: to stop water, in all forms. An impermeable barrier cannot let water vapour back out of a wall when generated by solar gain - see point below - so this type of membrane has serious limitations in all but a few circumstances. Needless to say, the installation of an impermeable barrier needs to be 100% perfect for it to work.  So, no holes, no gaps at the joints or overlaps, no accidental cuts or nicks and no major scrapes.

    Type 2: A membrane that acts as an impermeable membrane most of the time but has some capacity to allow vapour transfer in certain circumstances.  These are often described as a “vapour retarder” or a “vapour check” and they are designed to work under specific conditions where the inherent properties of the membrane can be relied upon.  Since they only allow the movement of water vapour under these narrow and specific conditions, it is essential to apply them in appropriate situations.

    Type 3: A membrane with variable permeability is often called an intelligent membrane. These are vapour control layers whose ability to allow moisture vapour to pass through depends on circumstances.  Also known as variable diffusion membranes (VDM), most allow vapour movement in both directions, depending upon relative humidity either side of the membrane.

    Some membranes in categories 2 and 3 can be described as breathable, you can read more about breathability here.

    The choice of a membrane can largely be determined by location and build type so a vapour barrier (type 1) can be used under a concrete slab but an intelligent membrane (type 3.) is better to line the inside of a warm roof space.

    The terminology used to describe membranes in buildings is hugely confusing and often ends up being concentrated into the single three letter acronym ‘VCL’ appearing on a drawing, but we know that VCL is a catch-all acronym it means nothing without some context or explanation, for example, we often notice people using the terms VCL and ‘breather membrane’ interchangeably, particularly with regards to pitched roofs. Whilst they have a similar purpose, there are a couple of important differences between the two.

    So why do we need either? Quite simply – the vapour control layer is there to prevent condensation, which can cause a number of problems, including:

    • Structural damage due to rotting timber, whether this be a timber frame, joists or rafters
    • Insulation losing its thermal performance due to having absorbed the moisture
    • Mould, which not only looks unsightly but can also lead to respiratory problems and other health issues

    People generate moisture inside their homes, through breathing, through cooking and particularly by washing themselves and their clothes.  To prevent condensation, we need to eliminate this water vapour from inside the building. We also need to get rid of moisture that is outside the habitable zone but within the building envelope.  This might be water from construction – fresh concrete, for example, takes many months to dry out fully - or perhaps rainwater that seeps through tiled roofs or is wind-driven up under the eaves.

    Traditionally we have eliminated moisture by ventilation; for example, by ventilating the space between the insulation and the slate or tile on a pitched roof. However, studies have shown that ventilation directly above an insulation layer can reduce its thermal efficiency, which means more and more people are opting for an unventilated roof.

    Some Definitions may help

    • Airtight layer- prevents the movement of air which may/ may not act as a Vapour Control layer
    • Vapour Control Layer- a material which can limit both movements of vapour by diffusion, and air movement
    • Breather Membrane- defined as a membrane with a vapour resistance less than 0.6 MNs/g situated on the external side of the insulation acts as a weatherproof layer whilst still allowing water vapour to be passed to the outside.

    Understanding your walls, temperatures and condensation

    With a plethora of membranes on the market, each designed to do a different job and behave in a subtly different way it is easy to be confused about which membranes are required to create a dry and airtight building structure.

    Starting with the basics, when insulating walls you create a temperature gradient across them with the warmest being on the inside and coolest on the outside during the winter months. You can imagine a graph of the temperature showing a fairly steady decrease in temperature as you move closer to the outer surface of the wall.

    When you take warm, moist air and cool it (as it will moving through a building structure) you find moisture condenses at a point known as the ‘dew point’ or ‘condensation point’.  This will typically be the intersection of an impermeable or low permeability surface with the temperature falling low enough for water vapour to become liquid.  This is how damp accumulates inside the fabric of your walls or your insulation, to the severe long-term detriment of your building.

    Why you shouldn't use a ‘vapour barrier’ (Type 1. Membrane) in your walls

    Different construction and insulation materials cope differently with condensation. Some materials, such as masonry, can absorb and release it again once the weather warms without too much damage. However, when using vapour impervious insulation in timber frame construction, any condensation forming in the walls tends to be absorbed by the timber, a process that can cause rot. Additionally, during the winter months when this condensation tends to occur, driving rain may also enter the fabric of the building, further increasing moisture levels in walls. It is therefore very important to prevent this condensation process occurring in the first place, for the longevity of the building.

    One further complication to the above process can be found in the summer months. The temperature gradient is often reversed and the higher temperature is found on the outside of the wall and the lower temperature on the inside. This creates a situation where moisture is driven inwards and condensation can form close to the inner face of the wall instead.

    In the UK until relatively recently an impervious vapour barrier was used on the inner face of a timber frame and was thought to prevent condensation formation by simply blocking the flow of moisture-laden air through the wall. However, it has since been found that not only are vapour barriers regularly full of holes which let moisture through during the winter months, they also cause the accumulation of moisture inside the wall during the summer months. This was caused by the barrier preventing moisture from escaping towards the interior of the building.

    Water vapour that has condensed against a VCL, in this instance a polythene sheet (type 1) used as a VCL over mineral wool and behind plasterboard. Damage can be seen on the timber stud. This occurred within 2 years of installation. Photo courtesy of SkamoWall.

    The high humidity levels and warm temperatures found in these walls combined to form perfect conditions for mould and rot to thrive. This was problematic to both the timber structure, as it rotted, but also to the inhabitants of the building as mould spores are well known to cause respiratory problems and ill health.

    Vapour barriers are still useful though. One of the few places above ground level where a complete vapour barrier should be used is in flat roofing when using foil faced PIR insulation. In this case, you need to lay a vapour barrier on top of your flat roof deck before you lay the insulation and your flat roof covering.

    Using a vapour control layer to control condensation

    The answer to keeping timber-framed walls and roofs dry is to use a layer to restrict the flow of moisture but not to try and stop it. In other words a VCL (or a vapour retarder). A VCL is always used as close to the inner face of a wall as possible and reduces the amount of moisture passing through the layer to low levels, ensuring only insignificant amounts of condensation occurring within the structure. Additionally, this will allow moisture that is driven towards the interior in the summer months to slowly pass back inside the building. This prevents the conditions for mould forming and ensures the longevity of the structure.

    Vapour control can be performed very accurately by the many membranes available but it can also be performed at a basic level by OSB, whose vapour resistance (or vapour permeability) is similar to that of some membranes. The benefit of using OSB as a VCL is that it is far more robust than a 0.2mm membrane and does not require the installation of another layer into your timber frame structure if used internally. However, you will need to test the airtightness of the OSB before using it as there is some variation in air permeability. For guaranteed results either use an airtight VCL membrane, such as the ProClima Intello Plus or Constivap or a board such as Unilin Vapour Block or a liquid applied membrane such as Blowerproof. Blowerproof and Intello Plus are both BBA certified.

    It is also advisable to try and minimise the amount of moisture that enters your building fabric during construction.  Much of our construction timber, sometimes including expensive windows, roof joist assemblies and even SIPs panels, are commonly stored on site with little or no protection against rain, especially driven rain.   Once wet, they can take a significant amount of time to dry out, contributing to the internal moisture load a new dwelling has to deal with.  This can even delay and degrade the final stages of construction: for example, airtightness tapes on OSB have been known to come off during airtightness testing, not adhering properly because the timber is still too wet.

    The latest type of VCL membrane is the 'intelligent' membrane, such as Proclima's Intello Plus membrane. These are very useful products that remain very vapour tight (low vapour permeability) during the winter months when it’s important to try and prevent moisture from entering your structure from the interior. As temperature and humidity in the walls rises the pores in the membrane open and allow moisture to migrate towards the interior of the building. This gives the best of both worlds and ensures your structures remain as dry as it is possible to be.

    Watch the video to see how an intelligent VCL works

    Using a vapour-check or foil backed plasterboard as a VCL

    Vapour control layers are always required whenever you insulate, irrespective of the insulation used. They should be used to form a continuous airtight layer and so all the joints and any penetrations must always be sealed with the appropriate airtightness tapes. Without good levels of airtightness the VCL does not work and moisture levels cannot be controlled inside the structures. Products such as vapour-check, foil backed or insulated plasterboard tend to act as a vapour barrier but with none of the joints or penetrations sealed. These products should not be used instead of a VCL or where a VCL is used.

    IMPORTANT REMINDER

    Always refer to a qualified designer if in doubt or ask the manufacturer for technical and installation advice, we are always happy to answer any questions about airtightness or vapour control or point you in the right direction.

     

     

    Thanks to Paul Jennings and Chris Brookman for their contributions to this article.

    Paul Jennings has over 30 years’ experience of airtightness testing, in the UK and around the world, and has been extensively involved in the delivery of onerous airtightness specifications in Passivhaus and other low-energy projects. He tested the first UK certified domestic and non-domestic Passivhaus buildings, both in Machynlleth, on the same day more than ten years ago, and recently led the team that used 8 sets of test equipment to carry out the most complex airtightness test carried out in the UK to date, on Agar Grove Phase 1, in London. He trains airtightness testers and pioneered the development and delivery of airtightness champions training courses. He has been instrumental in improving our processes and tools for achieving good airtightness, as well as training sealing operatives and delivering numerous CPDs and conference presentations to a wide range of building professionals on different aspects of airtightness.

    Chris Brookman lives in a Passive house of his own design which he built based on his own life principles of low impact, low energy living and human health. Chris runs Back to Earth, is a recognised expert on green building and passive construction, he has written widely on the subject and is a keen blogger on the practical and technical aspects of delivering sustainable construction; Chris also curated the first online Wood Fibre Insulation course

  • Adhesive tapes: how do they work, what should they be able to do, and what can they do?

    Why do adhesives stick?

    Considerable lengths of various adhesive tapes are used when sealing buildings. A typical application is seen in the photo where tapes are being used to seal and connect an internal airtight membrane.

    Adhesive tapes are used as bonding aids in a wide range of applications in the creation of airtight building envelopes. Several hundred metres of tape is often used on a single building! Adhesive tapes have become established as bonding agents for these applications (just as nails are the standard solution for timber structures). They have to fulfil their functions for a number of decades to ensure that the building in question fulfils the standards expected by the energy consultant and by the building client. This article provides an overview of adhesive technology and the key properties of adhesive tapes typically used in construction.

    Aren’t all adhesive tapes the same?

    This illustration shows the various forces that act in an adhesive bonded joint. Cohesion refers to the internal strength of the adhesive. Adhesion refers to the sticking force to the subsurface. As a rule: the higher the adhesion, the lower the cohesion. An optimal balance between cohesion and adhesion is crucial for permanent adhesion. (See information box 1 with regard to adhesive tape tests on construction sites)

    Adhesive tapes might appear similar or even identical at first glance: when you compare different products, they all have a backing material. Depending on the planned application for the adhesive tape in question, this backing material may consist of paper, plastic film or fleece. An adhesive substance has been applied to the backing, and this adhesive substance is covered by a protective sheet or protective paper on the underside of the tape. The various types of backing facilitate different areas of application. For example, a tape that can be used both indoors and outdoors must have a UV-stabilised backing; an adhesive window-sealing tape must have a fleece backing that can be plastered over. The difference is easy to recognise. However, if you consider the adhesive substance itself, the difference is not so easy to identify. A review of data sheets is often of little help in this regard, as they generally only specify limited technical data – and this data is also difficult to compare.

    Adhesive tapes for the creation of air-tightness are generally manufactured using two main production methods. The majority (around 80 – 85%) are produced as dispersion adhesives. In this process, acrylates dissolved in water are applied to the backing material in a liquid state. Emulsifiers are added to the dispersion to ensure that the dispersion remains homogeneous and that the acrylates dissolve in water in the first place. The function of these emulsifiers is to attract water. The water is then evaporated in long drying tunnels later in the production process. The dissolved acrylates bond with one another, form long chains of molecules and become »sticky« as a result. The emulsifiers remain in the adhesive film, but no longer serve any purpose.

    A more exclusive group of adhesive tapes is manufactured using a solids-based adhesive containing pure acrylate. This production technology is relatively new and more laborious from an engineering viewpoint compared to the process used for adhesive tapes with acrylate dispersions. The adhesive is applied to the backing material in the form of a viscous mass and the individual acrylate molecules are cross-linked by the controlled addition of energy in such a way that the desired adhesive properties are created.

    Honey and stone, or adhesion and cohesion

    Honey has high adhesion – it sticks immediately to every surface. However, its cohesion is low, which means that honey drops off the surface under the action of its own weight. Stone is the exact opposite: it has high inner strength, i.e. cohesion, but has no adhesion and therefore does not stick to surfaces.

    Adhesion and cohesion can be demonstrated very well by comparing runny honey with a stone. Honey exhibits good adhesion and sticks to surfaces very well as a result. However, its inner strength (cohesion) is so bad that it runs off in drops under the action of its own weight. A stone has high inner strength, i.e. cohesion, but very low adhesion. Good adhesion is generally associated with poor cohesion and vice versa. A good adhesive tape results from an ideal balance between cohesion and adhesion.

    Why do adhesives stick? Sloths, squirrels and geckos

    These photos show how strength builds up over the course of contact time. An adhesive tape was employed here that can be used for interior air sealing and exterior wind sealing. The initial adhesion – after 20 minutes – can be seen on the left; the significantly stronger adhesive bond after 24 hours can be seen on the right.

    Let us consider the interesting question of how and why an adhesive tape is able to stick things together. The bond with the substrate is achieved using various mechanisms. Sheeting or a pane of glass may appear smooth at first glance, but their surfaces actually look very different – with hills and valleys – when viewed under magnification. The adhesive flows around these structures and claws to the surface like a squirrel on a tree or grips the surface like a sloth wrapped around a branch.

    If the adhesive is in direct contact with the surface, attractive forces – so-called Van der Waals forces – will result between the two elements at a molecular level. The closer the adhesive comes to the surface, the more these forces will come into play and increase the strength of adhesion to the substrate. A similar principle applies with the gecko, which is able to walk upside-down on smooth surfaces such as panes of glass. This is made possible by a large number of very fine setae (hairs) on the feet of geckos, which increase the contact surface and thus facilitate sufficiently strong adhesive forces.

    Take your time: the build-up of adhesive force

    It can take some time before an adhesive has flowed into a subsurface fully and established a strong bond with it. Adhesive strength is generally built up over a period of hours. The reason that all manufacturers recommend that their adhesive tapes should be pressed into place can be explained by the mechanisms described above: an adhesive must be brought into close contact with a subsurface to be able to flow around and surround it.

    A drop of water brings clarity – The influence of surface tension

    There is a commonly held myth that an adhesive should be able to stick to every surface. And if an adhesive bond doesn’t hold as desired, then it’s always the adhesive agent’s fault! However, this assumption is false. Nobody would think of taking two pieces of sawn timber, applying wood glue to them, pressing them together briefly and then pulling them apart again immediately, and then saying that the glue was responsible for the fact that the bond didn’t hold.

    Surface tension of foils: the low-energy surface has few attachment points and a low surface tension. It is not able to pull the water drop out of its shape. The more attachment points there are, the more energy the surface has and the more the water drop will be pulled out of its round shape as a result. High-energy surface: The liquid spreads across the material.

    The quality of a given bond is always dependent on the bonding agent, the subsurface and the method of applying the bond. The release films used are evidence that not all foils are suitable for adhesion: some adhesive tapes can be easily removed from their release films. On the other hand, there are films that tapes bond well too, but which then become detached under tension. Finally, there are also films that adhesive tapes cannot be removed from at all. The surface tension of membranes is responsible for all of this. This tension describes how well a given membrane can be ‘wetted’ by an adhesive – in other words, how well the adhesive can get close to the surface of the membrane to be stuck. The surface tension of a membrane cannot be seen, and this value is specified in data sheets by a few limited number of manufacturers.

    Water drop test

    Silicone paper: Surface tension: < 30 N/mm. Very poor surface wetting. Very hard to stick for this reason.

    Weak wetting and a poor adhesive ability for this PE airtightness membrane: approx. 35 N/mm

    Double-layered airtightness membrane: Very good wetting and good adhesive ability, as the surface tension is greater than 45 N/mm.

    How can one estimate surface tension on a construction site? One possible method here is the water drop test: a drop of water is placed on the surface of the membrane and it is observed how well the drop of water spontaneously wets the surface. The greater the surface tension (surface energy) of the membrane, the greater the likelihood that the water drop will be pulled out of its “drop” shape. This indicates a stronger and more reliable adhesive bond with an airtight membrane.

    Of course, this test does not provide precise information, but it has proven useful in practice over a long period. Membranes with a surface tension of > 40 N/mm are recommended for permanent airtight adhesive bonds. Membranes with surface tensions significantly below this value are often used in building practice. In order to supply the market with adhesive tapes that can still stick to these lower-quality surfaces, large quantities of resins are added to acrylate dispersion adhesive tapes, in particular. These resins stick aggressively to poor surfaces. However, the problem here is that resins can oxidise with oxygen, become brittle over their service lives and lose their adhesive strength. To prevent this from happening, it is recommended to ensure that adhesive tapes that only contain pure acrylates are selected.

    As well as being used for adhesive bonds for membrane overlaps, acrylate adhesive tapes can also be used on joints to adjacent building components consisting of timber, stone, wood fibreboards, plaster and concrete. This is possible as long as the surface is generally even, free of dust and resistant to abrasion. If all three of these prerequisites are not fulfilled by a given surface, it can be pre-treated with a primer. Primers for acrylate tapes are applied in liquid form and differ from undercoats in terms of their mechanism. An undercoat penetrates deep into the surface and strengthens it. A primer for an acrylate adhesive tape is designed to penetrate into the subsurface and also to form a film on the surface that levels out any unevenness. These primers have proven themselves in practice. It is critical that the primer is suitable for the adhesive tape: i.e. one should always think in terms of overall systems.

    Resistance to moisture – why are there differences?

    Adhesive tape after storage in water for 24 hours: Top: a conventional acrylate dispersion adhesive tape, re-emulsified with water; the adhesive has lost its strength. Bottom: pure acrylate on a solid basis is absolutely water-resistant.

    Nobody wants moisture on a building site, but regrettably the reality often very different! Adhesive tapes have to be able to reliably withstand the challenges of moisture after they are installed. The first protective layer is the backing material that is used. A film is clearly more resistant to water than paper. However, moisture does not always come only from the outside, but often from the subsurface too. In this case, the advantage of the external protective effect of the film is reversed, as the moisture cannot escape through the film and builds up instead between the adhesive and the film.

    As already described, acrylate dispersion adhesives contain emulsifiers in their adhesive film after production. A characteristic of emulsifiers is that they store water, and they are still capable of doing this years later. If an acrylate dispersion adhesive comes into contact with water again, the adhesive re-emulsifies often assumes a white colouring and can lose adhesive strength. Pure acrylates are fully water-resistant, as they do not react with water – in this way, their adhesive strength is preserved.

    See yee, who join in endless union – Durability: experience and laboratory tests

    Cohesion adhesion test with 47 adhesive tapes: 40 adhesive tapes failed within two years in a long-term test with low loading.

    Reference is often made to the positive experience observed over the last 20 years with regard to the durability of adhesive tapes. When we plan and build a house nowadays, clients expect the built structures and the materials used to have a service life of 50 years or longer. As a result, it is even more important when selecting adhesive tapes to take into account long experience in the marketplace alongside ageing tests that confirm the high durability of bonding agents.

    Consistent rules are coming: a new standard for bonding agents will create a basis for comparison

    The forthcoming standard DIN 4108-11 will specify laboratory tests that have to be carried out for all adhesive tapes. This will create consistent quality standards and provide a basis for users to compare products.

    Presently, adhesive tapes are not regulated by standards and there are no uniform minimum requirements that have to be fulfilled by-products. The draft of DIN 4108 Part 11 that is soon to be published will fill this gap and specify uniform and comparable minimum requirements for adhesive tapes. This standard contains various tensile strength tests on standardised subsurfaces such as wood and membranes, as well as the possibility of having systems (membranes and adhesive tape) tested by manufacturers.

    Many of the requirements demanded from adhesive tapes described above are formulated in this standard. For example, the tapes are pressed into place in a defined manner before conducting a pull-off test, and the test is carried out with a low pull-off speed so as to simulate the long-acting, low tensile stresses that occur in real applications in this test. Ageing will also form part of the scope of the standard. It is not yet possible to state exactly whether and when the standard will be introduced and become part of construction law. However, the standard will form a good basis for comparing adhesive tapes with one another and will help installers and project planners to make informed decisions.

    Summary: permanent adhesive joints are only possible with good systems and the right handling

    Soft adhesives perform better in the ‘finger adhesion’ test, as they are better able to wet the surface of the thumb. This can lead to problems in practical construction applications, as soft adhesives generally have low cohesion forces.

    Actual loading in practice on site: the adhesive joint is subjected to low forces over a period
    of years, so sufficient cohesive strength is important.

    Permanent adhesive joints on construction projects are feasible and can achieve reliable performance; nonetheless, damage to structures often occurs when joints become detached. Knowledge about the fundamentals of adhesion technology and about the loads that will actually be acting in practice is crucial in order to be able to carry out reliable project planning and testing too. An optimal end result can only be achieved with good handling, a high-quality subsurface and a suitable adhesive tape. All three of these criteria should be carefully considered by the specifier and the energy consultant on site. Manufacturers who make statements about the surface quality of their membranes and about the production technology used in their adhesive tapes (solid acrylate or acrylate dispersion) and who offer long market experience, 3 rd party accreditation by reputable bodies (i.e. PHI, BBA, NSAI, BRANZ etc.) appropriate ageing testing and engineering support should be preferred over suppliers who provide little or no information about their products.

    See the Pro Clima range of airtight tapes here

     

    This article was written by Jens Lüder Herms, Dipl.-Ing. (FH), Jens trained as a carpenter and then studied construction engineering. He develops practical solutions for sealing buildings as part of research and development at Pro Clima.

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