Question Time: Airtight Homes

ABOVE: Windows are Key In an airtight house, windows are triple glazed, without trickle vents. Ventilation tends to rely on a mechanical system as opposed to opening the windows.

I keep reading about airtightness in new homes. Why should I care?
Insulation is all very well, but if you are really serious about building a genuinely low-energy home, then airtightness is key. You have to control the airflow through a house to really save the heat.

How does airtightness differ from draught-proofing?
In theory, it doesn’t, but in practice, there is a lot more to it. An airtight house has to be designed that way in the first place, and there are lots of little details which you wouldn’t find in a standard British house.

Like what?
You have to design the house with an air barrier, a bit like a cocoon surrounding the living space. Anything which penetrates this air barrier has to very carefully detailed to minimise the air leakage. So a truly airtight house will tend to avoid things like downlighters in the upstairs ceilings, open fires, or trickle vents in the windows.

So, no fires, but how about stoves?
It depends on how strict you are. Many woodburners come with attachments which allow you to draw air directly from the outside, but they are not truly room-sealed.

Can you tell how airtight a house is?
Yes, you can test for it. You remove the front door and replace it – temporarily – with a blower door which pumps air into the house, putting it under a pressure of 50 Pascals. By way of comparison, the cabin pressure in an airplane is routinely set to 75,000 Pascals — you won’t notice a mere 50. But what this pressure will demonstrate is just how ‘leaky’ a house is, by measuring how much air is needed to be pumped in to keep the house at a steady 50 Pascals.

What’s regarded as a good score?
The air leakage measurements are scored in cubic metres of air moving every hour through a square metre of the building envelope under 50 Pascals of pressure — or ‘m3/h/m2 @ 50Pascals’, or simply ‘q50’. It doesn’t relate too well to what happens in practice because homes aren’t under pressure like this, but the figure does provide a useful tool for comparing how leaky a house is.

The Building Regulations in England and Wales currently reckon a pass score is 10 q50. That’s not very difficult to beat, and there are indications that this figure is about to be lowered significantly. By 2013, it looks as though 3 q50 will become the standard. A score below 3 q50 is considered pretty good, and will still allow you the odd luxury like a woodburner. But the gold standard of low-energy housing – the PassivHaus standard – demands a score of 0.6 q50 or less. At this level, even woodburners are a no-no and the house has to have a mechanical ventilation system to maintain good air quality.

We were thinking of having mechanical ventilation with heat recovery. How is this affected by airtightness?
The more airtight your home, the more you need to plan for ventilation. The traditional way of ventilating is with trickle vents in the windows for back ground ventilation, and extract fans for smells and moisture. But in homes built to modern airtight standards (below 3 q50), a wholehouse ventilation system may be required.

How much does a pressure test cost?
Around £300 per house. You need to check with your building inspector that the firm carrying out the test is acceptable.

What if we fail the test?
Often the remedial steps needed to meet a test failure are not difficult to achieve. 10 q50 is not a high standard and can be easily met by careful draught-proofing. The testers themselves are usually very helpful in this. But if you have to meet a score of 3 q50 or lower, then you will need to think closely about the design of the house at the outset.

Is there any alternative to testing?
There is a dispensation for sites with just one or two homes. You can forgo the air pressure test but you have to assume a score of 15 q50, which will upset your Dwelling Emission Rate calculations and require you to install better insulation and other energyefficiency measures to offset total heat losses. It may well be cheaper to have a test done.

Airtight Self-build Homes:

• A Super-E Airtight Kit House
• A SIPs Self-build
• A Low-Impact Eco House

Further reading:

• Green technology: A complete buyer's guide
• How to build a no-heat home
• The end of open windows