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.
Building Regulations U-Value minimum standards 1970 – present*
See also Appendix 1. Below on ‘Limiting fabric parameters’
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.
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.
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.
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
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.
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
LABC Warranty Survey 2018 ‘What is the average house size in the UK?’
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.
LABC Warranty Survey 2018 ‘What is the average house size in the UK?’