Design Air Permeabilty explained

During your trek for an air tightness test, you will be asked what your design air permeability is.  This value comes from your SAP in section 4.5 of the L1A checklist and will be a number equal to or less than 10.  This is the 'score' that you have to obtain to get a valid pass on your property. (If yours says a 15, you know why and I am speaking to the choir ... )

How did this number come about for your SAP?  It's based on the allowances made for the design elements on your build. If you have opted for a well insulated house with an A rated boiler using thermostats around the house, you will most likely have a 10 in here.  If your choice of boiler was onthe cheaper side and you opted for a B rated boiler .. or you wanted to save money on insulation and use 75 mm of insulation in the cavity when 100 mm would have been better, then you may need to get an 8 to pass. And so on it goes down the scale ...

But Why? Well, it all boils down to efficiency. If you are using an efficient boiler to warm your drafty house, you are probably going to use the same amount of energy as a well insulated house using electric radiators so the impact on the environment is the same. 

So, if I need to get a 10 and I get an 11 or higher what happens?  You fail. But, it is not as desperate as you may think.  You will have to get your house down to a a 9 and change (or less) and it is usually doable on the day of the test if you have a reasonable tester.  I am a reasonable tester, and realise that you ned to get a valid test onthe day and would like ot have your report and certificate there and then. If you have only built the one house, then we go down one route, if you have a development of houses we go down a different route. Single househould builders, you have to get a pass and there are no other houses to retest to confirm the results.  Developers, you may have to have an additional house tested to comply with regulations and to ensure that the learnings are passed on to the rest of the builds.

Getting a failing house to pass is another topic, but, let's just say that you will need to take some action, and it will involve expanding foam, silicone, mastic, decorators caulk, and the like. 

For more information or to book a test, call me on 01484 686649 or visit my website on http://www.hometestingyorkshire.co.uk/

I hope this helps

Matt