ATTMA, the Air Tightness Testing & Measurement Association, has been analysing improvements in the new-build dwellings over the last 6 years using its specially designed Lodgement system.

ATTMA members are required to record information from each test by uploading the raw data files into ATTMA Lodgement which takes, analysis and issues certificates for each plot once the necessary checks are conducted. This allows ATTMA to look at not only the average air tightness results, but significantly more data.

So far, over 1 million lodgements have been recorded with significant amounts of data per lodgement. Typically, 3500 tests per week are recorded of which around 15% make up ‘retests’, where a test is repeated after initially failing to meet the maximum leakage rate.

AP50 Average

The UK uses AP50 (m³.h^-1.m^-2@50Pa) as its performance metric. Interestingly, the average air permeability rate has consistently fallen every year, as shown in image 1. However, this fall has been around 3% per year which at the current rate, would take over 20 years to reach Passivhaus levels.

The average result in 2021 is 4.31 which is the same as 2020, showing that 2021 is the first year not to have the 3% reduction. Our theory is that, without change in legislation, the market has little or no requirement to build tighter. The last change in regulation was in 2010, which will have taken until 2012 to filter through to sites. Our theory is that it has taken a couple of attempts at different strategies before the industry has now settled on its requirements.

Construction Type

Most dwellings in the UK (around 70%) are built using block and brick construction, with concrete frame making up 14% and timber making up 8%. ATTMA can look at the average air permeability results from different construction types.

Construction Type	Percentage Built	Average Result (m3.h-1.m-2@50Pa)
Block and Brick	70%	4.64
Concrete Frame	14%	3.27
Timber Frame	8%	4.40
Steel Frame	5%	4.02
Internal Concrete Frame	<1%	3.63
Structurally Insulated Panels	<1%	3.54
Other	1%	3.93

Some interesting conclusions can be drawn from the construction types, perhaps most notably that timber frame construction, which should in theory be utilising its internal airtight membrane, is not outperforming block and brick as much as expected. We also note that concrete frame buildings are achieving far above the national average, perhaps due to a significant proportion of the envelope being made from concrete frame.

Ventilation

The ATTMA Database records the type of ventilation used in new-build homes. This allows us to assess the ventilation strategies and analyse changes. Interestingly, the ventilation strategy is changing with a 10% reduction in the number of homes using ‘natural’ ventilation (which consists of openable windows and a timed mechanical ventilator in the bathroom) and a rapid growth in System 3 (Continuous Mechanical Extract Ventilation) and System 4 (Continuous mechanical ventilation with heat recovery)

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Article by Barry Cope, Group Managing Director, Building Compliance Testers’ Association (BCTA)

If you would like to gain further information on ATTMA’s airtightness database, click HERE to view the recorded presentation and HERE to download the slides of the TightVent-AIVC webinar “Building airtightness improvements of the building stock- Analysis of European databases” held on 19 January 2021.