The 2002 edition of the Approved Document Part L1 (ADL1) came into effect in April 2002 (DTLR, 2001) and incorporated, for the first time, an explicit air leakage target. Compliance could be achieved by:
Adopting the guidance given in the report on Robust Construction Details (see DEFRA, 2001).
Pressure testing the building following the method outlined in CIBSE Technical Memorandum Tm23 (CIBSE, 2000). The air permeability of the dwelling must not exceed 10 m3/(h.m2) @ 50Pa.
Although two separate methods of compliance were identified within ADL1 2002, in the vast majority of cases Robust Construction Details were been used as the basis for regulatory approval as obligatory pressurisation testing was not required.
In 2005, a major review of Part L1 was undertaken following the publication of a consultation document in July 2004 (ODPM, 2004). The outcome of this review was the publication of two revised versions of the Approved Document Part L1; L1A Work in New Dwellings and L1B Work in Existing Buildings.
ADL1A 2006 requires that the building fabric should be constructed to a reasonable quality of construction so that the air permeability is within reasonable limits (ODPM, 2006). Guidance on a reasonable limit for the design air permeability is given as 10 m3/(h.m2) @ 50Pa. Design air permeability is defined in ADL1A 2006 as the value of air permeability that is selected by the designer for use in the calculation of the DER. In the majority of cases, checking compliance with the regulation will require some degree of compulsory pressure testing. The exception to this concerns small developments of no more than two dwellings. Details of the pressure testing requirements are contained within Regulation 20B of The Building Act 1984 (ODPM, 2006b).
On developments of more than two dwellings, the degree of pressure testing varies depending upon whether or not the dwellings have adopted accredited construction details.
Details of the testing regime associated with each method of compliance can be found within ADL1A 2006. The pressure testing regime is more onerous if accredited construction details have not been adopted.
Alternatively, the developer can avoid the need for pressure testing altogether by using a value of 15 m3/(h.m2) @ 50Pa for the air permeability when calculating the DER. However, in order to achieve the CO2 Target Emission Rate (TER) the high permeability would have to be offset against substantial improvements in energy efficiency elsewhere in the dwelling(s). The TER is the minimum energy performance requirement for new dwellings approved by the Secretary for State. It is expressed in terms of kgCO2/m2 per annum emitted as a result of the provision of heating, hot water, ventilation and internal fixed lighting for a standardised household when assessed using approved calculation tools.
For those dwellings that are tested and fail to achieve the required design air permeability, remedial works should be undertaken on the dwelling and the dwelling re-tested. The remedial works should ensure that when the dwelling is re-tested the measured air permeability is less than 10 m3/(h.m2) @ 50Pa and the DER calculated using the measured air permeability is no less than the TER. In addition to undertaking the remedial works, another additional dwelling of the same type is also required to be tested, resulting in an increase in the testing sample size.
Compliance with ADL1A 2006 also requires that the pressure tests are undertaken by a person registered by the British Institute of Non-Destructive Testing in respect of pressure testing for the air tightness of buildings. In addition, the tests should be performed using the procedure set out in ATTMA Technical Standard (ATTMA, 2007).
Approved Document Part L1A 2006 is currently under review. The main proposals contained within the consultation document involve the consultation document involve the introduction of an air permeability target for thos dwellings that are not tested. In such cases, the assessed air permeability of the non-tested dwellings is the average air permeability obtained from other tested dwellings of the same type increased by 2m3/((h.m2) @ 50Pa, to account for the likely variability of air leakage that would be achieved by on-site testing.
AIRTIGHTNESS OF EXISTING UK HOUSING
The largest and most comprehensive source of information on the airtightness of UK dwellings is the Building Research Establishment’s (BRE’s) database of air leakage (see Stephen, 1998 & 2000). This database contains information on 471 dwellings of different age, size, type and construction. However, despite its size, this database is not the result of random sampling and cannot claim to be unequivocally representative of the UK housing stock.
The information contained within the BRE’s database indicates that a very wide range of air permeability exists within the UK housing stock, ranging from 2 to 29 m3/(h.m2) @ 50Pa. In addition, the least airtight dwellings are over 10 times as leaky as the most airtight dwellings.
There is a commonly held perception that new dwellings in the UK are built to a high standard of airtightness (Olivier, 1999). This is not generally found to be the case. Cohort data contained within the BRE’s air leakage database suggests that dwellings built between 1980 and 1994 are, on average, only as airtight as those built at the beginning of the 20th century.
Distribution of air permeability of UK dwellings. After Stephen (2000)
Relationship between dwelling age and air leakage. After Stephen (2000).
Whilst the air leakage data for the older dwellings is not likely to be representative of their airtightness when first built, the data suggests that the airtightness of new dwellings has not improved significantly over the last century.
This contrasts with the average heat loss of GB dwellings, which is estimated to have reduced by over 30% between 1970 and 2004, due to improvements in insulation standards and a change in the mix of dwellings (Utley & Shorrock, 2006).
AIRTIGHTNESS OF NEW UK HOUSING
Air leakage data on recently constructed dwellings is limited. In recent years, measurements have been undertaken by the BRE on a non-random sample of 99 dwellings that had been built to Part L1 2002 (see Grigg, 2004). The results were as follows:
A relatively wide range of air tightness was observed within the sample – 3.2 to 16.9 m3/(h.m2) @ 50Pa, with a mean of 9.2 m3/(h.m2) @ 50Pa.
Approximately two thirds of the sample (68%) achieved an air permeability that was lower than or equal to the maximum specified level of 10 m3/(h.m2) @ 50Pa set in the 2002 Edition of the Approved Document L1 (DTLR, 2001).
Mean air permeability of dwellings built to Part L1 2002. After Grigg (2004).
The results for the apartments and the other dwelling types in the BRE sample were also analysed separately. The reason for this is that apartments tend to be more airtight than other dwelling forms of equivalent area, as they are more likely to have solid intermediate floors, fewer external door and window openings and fewer service penetrations. The analysis indicated that:
Air permeability of the apartments ranged from 3.2 m3/(h.m2) @ 50Pa to 12.4 m3/(h.m2) @ 50Pa, with a mean of 8.0 m3/(h.m2) @ 50Pa.
Air permeability of the other dwelling forms ranged from 5.6 m3/(h.m2) @ 50Pa to 16.7 m3/(h.m2) @ 50Pa, with a mean of 9.8 m3/(h.m2) @ 50Pa.
83% of the apartments achieved 10 m3/(h.m2) @ 50Pa or better compared to 57% of the other dwelling forms.
This analysis suggests that the high rate of compliance for the tested dwellings (68%) is likely to be due, in part, to the number of apartments in the sample.
However, the overall rate of compliance in new dwellings may be slightly higher than that indicated by Grigg since the proportion of flats within the annual new-build total is some 5% higher than the 36% in the Grigg sample. Recent housebuilding statistics from the DCLG indicate that flats represented 41% of all new dwellings completed in England in the financial year 2004/5.
Mean air permeability of flats built to Part L1 2002. After Grigg (2004).
Mean air permeability of other dwelling types built to Part L1 2002. After Grigg (2004).
Recent work undertaken by Leeds Met on behalf of the ODPM Buildings Regulation Division has also looked at dwellings built to conform to ADL1 2002. The results of this work are illustrated in the two graphs below.
Mean Air Permeability of the Tested Dwellings
Mean Air Permeability of the Tested Dwellings by Construction Type
Of the 25 dwellings tested:
A relatively wide range of air tightness was measured, ranging from 4.0 to 16.5 m3/h.m2 @ 50Pa, with a mean of 11.1 m3/h/m2 @ 50Pa.
The mean of all 25 results(11.1 m3/(h.m2) @ 50Pa) suggests that these dwellings are broadly in line with existing data on the UK stock as a whole and that, at least in these cases, the impact of the 2002 edition of ADL1 has been minimal.
Given the qualitative nature of the project it is not possible to extrapolate to the post 2002 new build stock with any confidence but the Grigg (2004) data would suggest that the results obtained are not untypical.
Although the small sample size precludes certainty, the airtightness results do appear to show a difference in permeability between the different types of construction method used. The tightness dwellings were those of mechanically/manually wet plastered construction (since these were flats, their performance could also be a function of form as well as construction), whilst the leakiest were those using light steel frame.
Results suggested that simply adopting Robust Construction Details (precursor to Accredited Construction details), provides no guarantee that the maximum specified level of 10 m3/(h.m2) @ 50Pa set out in the 2002 Edition of the Approved Document Part L1 will be achieved with any degree of consistency.
There is a limited amount of published data available on the air leakage of dwellings that have been built to comply with Part L1A 2006. Measurements undertaken by the NHBC on 1293 dwellings of different size, type, and construction indicate that although the majority of the dwellings (>95%) achieved an air permeability below the regulatory standard of 10 m3/h.m2 @ 50Pa first time, a wide range of airtightness was still observed (see graph below). The mean of the samplewas approximately 6 m3/h.m2 @ 50Pa. The results are broadly consistent with those obtained by Building Sciences Limited on a separate group of 750 dwellings.y
Air permeability of 1293 dwellings built to ADL1A 2006. After NHBC
A more detailed analysis of the results obtained for masonry dwellings within both datasets suggests that, on average, dwellings with a wet plastered finish are more airtight than dry-lined dwellings.
COMPARISON WITH OTHER COUNTRIES
The UK is not the only country to have whole building airtightness requirements. Currently, Belgium, France, the Netherlands, Norway, Sweden, Switzerland, and the USA have criteria to limit whole building air leakage from dwellings (Limb, 2001). However, different countries express the air leakage criterion in different ways, making cross-country comparisons difficult. For instance: Belgium, the Netherlands, Norway and the USA express the criterion in terms of ach at a specific reference pressure (4, 10 or 50Pa); France, Switzerland and the UK express the criterion in terms of m3/h/m2 at a given pressure difference (4 or 50Pa); whilst Sweden uses l/s/m2. Nevertheless, a simple and relatively crude comparison can be undertaken if assumptions are made about the volume and surface area of a typical building, and by normalising the air leakage criteria to a standard pressure differential. Such an approach was adopted by Limb (2001), who assumed an internal building volume of 300m3, a surface area of 250m2 and normalised the figures to a pressure differential of 50Pa. The Table and Figure below illustrate the results of such a comparison, for all of the countries that have air leakage criteria for dwellings.
Whole building requirement
ach @ 50Pa
<3 ach @ 50Pa for dwellings with balanced mechanical ventilation.
<1 ach @ 50Pa when heat recovery devices are used.
0.8 to 2.5m3/h/m2 @ 4Pa
Class 1 ventilation system – Min 0.4 to 0.72 ach @10Pa and max of 1.4 to 2.24 ach @ 10Pa.
Class 2 ventilation system – Max 0.72 to 1.15 ach @ 10Pa.
Detached and undetached houses – 4 ach @: 50Pa.
Other buildings two storeys high or less – 3 ach @ 50Pa.
Other buildings more than two storeys high – 1.5 ach @ 50Pa.
Envelope should be so airtight that the average air leakage rate at 50Pa does not exceed 0.8 l/s/m2.
Max 1.6 ach @ 4Pa. Requires no part of the US to be tighter than 0.28 ach @ 4Pa.
Maximum whole building airtightness requirements for dwellings. Adapted from Limb (2001).
Comparison of maximum normalised air leakage criteria for dwellings
The comparison highlights the wide range of normalised air leakage criteria that exists. The most stringent criteria tend to be found in countries with severe climatic conditions, such as Sweden (2.88 ach @ 50Pa), whilst countries with more temperate climates tend to have less stringent criteria, for instance France (11 ach @ 50Pa).Part of the reason for this is likely to be the fact that in countries that experience severe climatic conditions, leaky buildings can result in extreme user discomfort. The above Figure also illustrates the considerable gap that exists between the UK and countries such as Norway, Switzerland, Belgium and Sweden. In fact, the data would suggest that there are very few tested dwellings in the UK that would satisfy the air leakage requirements of these countries.
A number of studies have also compared the airtightness of existing UK dwellings with those in other countries, for instance Olivier (1999) and Orme, Liddament & Wilson (1998).Although the size, structure and non-random nature of the dwellings included within these studies may preclude certainty, the results suggest that existing UK dwellings tend to be comparatively very leaky. In terms of new UK dwellings, experience suggests that the current levels of airtightness achieved in the UK are somewhere in the region of around 3 to 15 ach @ 50Pa.These levels of airtightness were achieved in countries such as Canada, Sweden and Switzerland some years ago. Even dwellings that are classed as very airtight by UK standards, such as Stamford Brook (mean of 4.3 ach @ 50Pa), the Lower Watts House (3.6 ach @ 50Pa) and the Longwood House (3 ach @ 50Pa), tend to be normal practice in countries such as Canada, Sweden, Switzerland and Norway.Furthermore, even the one of best performing developments in the UK, the Hockerton Houses (mean of 1.25 ach @ 50Pa for 3 of the dwellings), falls short of some of the best performing dwellings overseas (Passive houses at Kronsberg with a mean of 0.29 ach @ 50Pa).