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What is the air leakage test Confidence Factor?

It’s all down to a change in the way buildings are assessed when they are not tested for air tightness.

Under the previous version of Part L (2006), any houses which weren’t tested were either assumed to have the same result as an identical house, or were simply left with the assumed figure which was used in the calculation before the house was built. It was a simple approach, but not exactly accurate.

This system was scrapped when the 2010 Part L regulations were brought in - new rules were put in place around how air tightness results in As Built SAP assessments should be measured. Firstly, more plots on a site must now be tested compared to a few years ago, results from previous or similar sites cannot be used at all, and any plots which are not tested must have a ‘confidence factor’ included.

So what is this ‘Confidence Factor’?

Let’s say you have a site of five new houses, and when the SAPs were completed at design stage the assessor set all plots to an assumed air test value of 6 in order to show compliance with the Target Emission Rate in SAP.

Once construction is complete, you have air tests completed on three plots. The air test results are 4.5, 4.9 and 5.2.

Those three plots which have been tested will have the actual results used in the final SAP assessment and EPC… and as the results are better than the 6 which was used at design stage, the end results are going to be better.

The other two plots were not air tested, so the Confidence Factor applies. In this case you take the average of all the results for the site ( (4.5+4.9+5.2)/3 ) = 4.87 and then add another two = 6.87.

As the design stage SAPs were based on an air test of 6, and this new result is higher (at 6.87), there’s a chance the emission rate will be increased over the target, and therefore these houses could no longer comply with SAP.

Our developer has two possible ways around this: They can have the other two plots tested (as chances are they’ll perform better than six given the results of the previous three), or they can do some additional sealing work and have plots retested.

For example, if the developer took the two plots which were tested at 4.9 and 5.2, improved the air tightness, and then retested to 3.8 and 3.6, this would improve the average result ( (4.5+3.8+3.6)/3 ) = 3.97. The two plots which aren’t tested can now be based on an assumed air test result of 5.97 which is slightly better than the target of 6, so they’ll comply without modification.

There are a few other points to be aware of here – air testing only applies to new build projects… you’re rarely expected to achieve a compliant air test result on an existing building which you’re renovating.

Also, if you’re working to 2010 Part L and you don’t have an air test completed on site, the SAP assessment is defaulted to a worse case value of 15! This makes it incredibly difficult to get the emission rate to show compliance without adding less desirable items such as renewable and low carbon technologies. If you have a new site of three dwellings or more, you’ll be expected to have at least one of them air tested.

In the commercial sector, Confidence Factors do not apply. It is a requirement to have all new commercial units larger than 500sqm air tested – but we sometimes recommend air tests on smaller units to reduce the overall emission rate of the building.

To summarize: when your development is complete and air tests are being planned, make sure you know what figure was used in the design stage SAP reports. Either have all plots air tested to meet this design stage figure, or have some of the plots tested – but if you’re doing this make sure the average result is better by two points to allow for the Confidence Factor.

If you have any further queries on this, or need another aspect of Part L explained in more detail, let us know by contacting 08458 386 387.  

Jon Ponting

Author: Jon Ponting

This article was published by Jon Ponting on 09.11.2012.