Thursday, 11 July 2019

Surprising results of air tightness tests in new homes

No-one wants to live in a draughty home. They are uncomfortable to sit in and have high energy bills. Building regulations have stringent rules about ventilation levels and air tightness - but there is strong evidence to suggest that they are not working very well. New homes are often much more draughty than they should be, and adding an extension to an existing home can introduce draughts in the main part of the house, if not done properly. This post draws on evidence from a study by UCL researchers of 144,000 air tightness certificates lodged with the ATTMA database (Air Tightness Testing and Measurement Association) [1]. Also I have included anecdotal evidence reported to me by Paul Buckingham of Sustainable Lifestyles Ltd and others.

Air tightness matters for heat loss.
Poor air tightness makes homes hard to heat and uncomfortable to live in. With reasonable levels of air tightness and ventilation, air leakage accounts for about a third of heat loss. Obviously you do need some ventilation, but ideally it should be controllable. When new homes are tested, vents that are meant to be there, such as trickle vents in windows, are normally stopped up. The tests are intended to check for unintended leaks.

Air tightness matters for fire safety too.
Paul Buckingham gave a talk for Transition Letchworth that I attended where he showed a video of a smoke test in a leaky house. The owners of this house had asked him to do a survey because they were very concerned about heat loss but they had not even considered fire safety. The smoke test showed 'smoke' passing from one room to another through leakage paths in the walls and floors. If there was a real fire, those paths could carry hot smoke and flames. These days we are supposed to put intumescent strips in door seals so that they seal up in case of fire and stop the flames spreading - but if there are other pathways this is hardly worthwhile. You can see the video here.

Most new homes have an air tightness test.
The regulations require all new homes to be tested except that for larger developments you can get away with just a sample. However untested houses are assumed to be less air tight than the tested ones, so the sample has to be even better sealed in order to pass the standard. The UCL study found that in practice three quarters of homes are actually tested. The standard required depends on the design: there is an absolute requirement for all homes, but if the design is relying on air tightness to meet the overall energy efficiency requirements that target must also be met. Air tightness is reported as air leakage per hour, divided by the area of the building envelope, at 50 Pascals air pressure. The absolute requirement is at most 10 m3/m2/hr but for most homes built in 2016 the target was considerably tighter.

Heat leaks are common at joints and where service pipes come into the house.
Good air tightness requires careful workmanship. The problems are usually at joints or where there are holes for gas or water pipes to come through. For example leakage is common where walls connect with the floor, or ceiling or windows. This thermal image shows heat leaking out from a house at various places - the red areas are hot which shows that warmth is escaping. This is not necessarily air leakage - you can also get thermal bridging problems where the insulation is not complete - but either way these leaks indicate serious heat loss.
Thermal image of the front of a house. (Thanks to Paul Buckingham, Sustainable Lifestyles Ltd for permission to use his image). Red areas show heat leaking from the house, not necessarily due to are leakage but some of these are very likely, especially around the door; and in the corner where the gable joins the rest of the house; and at the bottom and sides of the bay.

Air tightness tests show a remarkable number of cases that exactly hit the target.
It is very difficult to get a good seal everywhere so all houses have some leakage. To reliably pass the test first time builders would have to aim somewhat better than the target. So you would expect a range of results, with a peak below the target level. This is not what we see in the UCL study. The top left chart in the extract below shows results from all the tests with the most common target: 5 m3/m2/hr. You can see that an unreasonably number just meet the target.

Results from the UCL study [1]. All the test results have the same target of 5 m3/m2/hr. The bottom left chart shows a spread of results in first tests that failed, which is what we would expect. the top left chart shows an unreasonable number of 'first' tests that exactly hit the target.
The likely explanation: tests are repeated with extra sealing until the dwelling passes.
Anecdotal evidence supports the obvious conclusion. Most houses fail first time, at which point someone comes in with a sealant gun and tape and fills in enough holes such that the test is passed. Maybe the testers are also a little generous in their readings too. We can never know.

This sealant does not last long - sometimes only days.
This extra sealant would not matter if it stayed in place, but in practice it often does not. For example carpet fitters do not like to work around tape sealing the edge of the floor boards with the wall - often they just rip it off. Sometimes sealing tape is used in places that are clearly visible - it is ugly and is removed almost immediately. Even if it is not, the tape or sealant is not as durable as the structure that was supposed to do the same job. It is just a matter of time before it fails and the leaks start. The trouble is, by the time you get to the test stage it is impossible to fix the defect where it originates.

Dry lining without sealing the edges is one common source of failure.
Paul blames poorly fitted dry lining for a lot of the failures. Dry lining should be effective insulation because it trap a layer of air between the plasterboard liner and the wall. To do this it needs to be sealed around the edges:
Direct bond lining systems are often used to dry line masonry walls. Plasterboard is fixed to masonry walls using a gypsum based adhesive to provide a smooth, level lining surface – often referred to as ‘dot and dab’. It is important to seal external wall perimeters and around any openings with gypsum adhesive to prevent heat loss resulting from thermal movement (‘flue effect’). (my italics) [2]
In practice, this is often forgotten. For example, this advice from B&Q does not even mention the edges - it just says you need dabs of adhesive 400mm apart. Sealing the edges takes time so even if you know you are supposed to, you might not do it unless you expect your handiwork to be checked. Thanks again to Paul for this image of cold air leaking in at the wall/ceiling junction and dropping down the walls between dabs behind the plasterboard.
Thermal image of cold air dropping down behind the dry lining from the wall/ceiling junction. Air travels around the dabs of glue holding the plasterboard in place. (Thanks to Paul Buckingham, Sustainable Lifestyles Ltd for permission to use his image).

Air tightness is not the only quality issue in the building industry.
Clearly something is going wrong here. Builders can get away with poor practice leaving householders to suffer the consequences. Air tightness is not the only issue and there are many reports of residents having difficulty with defects in their new homes not being fixed. There has been concern in parliament over this, by the All Party Parliamentary Group (APPG) for Excellence in the Built Environment. Last year they published a briefing pointing out the inadequacy of the industry response so far. Here are some extracts:
The APPG’s inquiry concluded that, in relation to disputes between house builders and buyers “the balance has been tipped too far in favour of housebuilders.” There is reference to a “cavalier” attitude towards customers by house builders. [3]
This is not the first time that the standard of newly built housing has come under scrutiny. The Callcutt Review of Housebuilding Delivery (2007) noted concerns around caveats included within warranties provided on new homes. It was felt that they may not offer adequate protection for consumers. The Office of Fair Trading’s 2008 study of the homebuilding market also considered the effectiveness of warranties. One response was to recommend the introduction of a code of conduct to meet consumer protection concerns. The industry responded with a Consumer Code for Homebuilders, now in its fourth edition. However, the APPG concluded that the Code “does not appear to give homebuyers the safeguards we think they should expect.” [3]
If you have work done on your home, make sure you have an independent supervisor.
What can we do? Frankly I have no idea. You could commission an air tightness test on your new home as soon as you move in; but if it fails you would still have to show that the test done before was invalid. If you have work done in your home, I recommend you get an independent supervisor too - even though it will cost you a little more.

[1] 'Hitting the target and missing the point': Analysis of air permeability data for new UK dwellings and what it reveals about the testing procedure
J. Love, J. Wingfield, A.Z.P. Smith, P. Biddulph, T. Oreszczyn, R. Lowe, C.A. Elwell (2017) Energy and Buildings.
[2] Site Guide: Dry Lining (Association of Interior Specialists)
[3] New-build housing: construction defects - issues and solutions (England) By Wendy Wilson and Christopher Rhodes (House of Commons Library) 2018

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