In a residential build, the test result will be passed to the DEAP assessor who will then update the calculations
Air Permeability Testing is a crucial phase of any build as the result will significantly effect the energy performance of the building.
Here we are going to give you our top tips for ensuring your building is as air tight as possible.
But first some background for the uninitiated…
Air Permeability Testing – What you need to know
The building regs require that all new dwellings achieve an air leakage of less than 7m3/hm2. That is the air leakage rate per hour per square meter of envelope area. If we are being precise, there are a few caveats around DEAP BER Calculations and dwelling types which can affect this, but broadly this is the rule. The m3/hm2 figure is the headline ‘pass or fail’ number produced by the air testing engineer and demonstrates how much air is being sucked into the building through ‘leakage’ when the fan is operating at 50 Pascals.
Uncontrolled Air Leakage
The key is that this test measures uncontrolled air leakage. It is not concerned with trickle vents, extract fans or ventilation systems. Controlled ventilation sources will simply be taped up or sealed prior to testing.
Therefore air permeability testing is looking for gaps and cracks in the fabric of the building.
Some certification schemes look to go further than building regs standards – for example the Passivhaus standard requires all new builds to reach a maximum of just 0.6 air changes per hour.Whilst this is a huge improvement against a ’10′, it should be noted that there are slight variations in the testing metrics and procedures.
Whilst testing may take place at any time in the build process, it is common for air testing to take place in the final throes of a project prior to issuing of final DEAP Calculations and an BER.
In a residential build, the test result will be passed to the DEAP assessor who will then update the calculations, establish that a pass has been achieved, and issue final reports and an BER.
Effects on BER
Importantly, the DEAP BER rating will be significantly affected by the air leakage rate, as high levels of uncontrolled air leakage will reduce the energy performance of the building.A BER Assessor will generally set a design air permeability target of between 5-10m3/hm2. This is a reasonably achievable performance although in some cases this may need to be set lower.
Well the dwelling may be struggling to meet its emissions targets and a low air leakage may compensate for other areas, either arising from poor design, or factors beyond the developers control. Whilst it would be great for our heating bills and DEAP Calculations if we produced super airtight houses every time, this obviously can cause issues when a dwelling is not designed for very low levels of ventilation.
Ventilation specialists will generally quote an air tightness of ’3′ as a healthy rate for a naturally ventilated house. That is, ventilated only with extract fans, trickle vents and windows. Anything tighter and some form of forced ventilation will be required, i.e a mechanical ventilation system. Some mechanical vent systems have a heat recovery function (MVHR) which draws heat from wet rooms and recycles it for use in the rest of the house.
Our Top Tips
During the course of testing hundreds of different buildings, we’ve learnt a few things.
Design it out
“These are very much superficial fixes – we can quite easily identify these problems in a pre test check without even running a fan – and pretty much guarantee a pass against building regs standards. However, there is no substitute for good air tightness design right from the architects drawing table. Passivhaus builds and the like are different beasts and will need a lot more thought around air tightness barriers in the construction itself. The most important point on any build is to get all the trades onside and get them thinking about air tightness as they go. If everyone buys into the air tight theme, with good practices from the start, you can avoid the problems later on.”
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