How important is air-tightness to energy efficiency?
Posted by Tim Pullen on 24 June 2009 at 11:07 am
A reader recently asked about a seeming conflict between the modern approach to building energy efficiency which, amongst other things, focuses on good air-tightness standards, and combustion appliances, such as wood stoves, most, but not all, of which take their air from inside the building, and so need unobstructed vents.
He asked: "I have an electric Aga which I would like to change for a wood range. I don't see any offerings in the market which have sealed air inlets (ie. connected directly to outside the building). I am not keen on putting in a vent for an unsealed appliance. Your comments on this in particular, and this conflict in general, may be of interest to readers."
The short answer is yes, there is a conflict. In essence, if you want an air-tight house then you can’t have a wood burning appliance.
But before we go there, let’s look at air-tightness – permeability as it is known. The current building regulations call for a maximum permeability of 10m³/hour @ 50 Pascals (that is the rate at which air escapes from the building at a pressure of 50 Pascals). The building regs due out next year are likely to reduce that to 7m³/hr and by 2016 (when all new homes must be zero carbon) it will be below 3m³/hr. A Passivhaus (the most energy efficient building standard we have) allows a permeability of less than 1m³/hr.
Neither of those last 2 standards deliver enough air to maintain a dry, healthy, comfortable living environment. Anything less than 5m³/hour tends to get stuffy and at less than 1m³/hr the CO2 build-up is getting distinctly unhealthy.
As soon as we move towards air-tightness we have to start thinking about mechanical ventilation. Living in an air-tight house means that you can’t open the windows either, but we have to be sure that there is enough, and only enough, fresh air entering the house to stop us from waking up dead. And that fresh air will need to be pre-warmed by exhaust air if we are to maintain the energy efficiency levels.
What this probably tells you is that designing an energy efficient house is a complex business. But before you give up on it think on this: I asked the sales director of Hanse House, one of the major German manufacturers of off-site manufactured houses, including Passivhauses, why we should build to Passivhaus standard. He thought for a moment and said, “My house costs 25 euro per month to run. If I upgraded to Passivhaus it would cost 20 euro.” His shrug was eloquent.
We in the UK do not live in a climate that needs or wants air-tight houses. Ours is a mild, damp climate and we need and enjoy fresh air. The answer to reducing CO2 emissions is not sealed houses, it is CO2-free heating systems.
The reader’s electric Aga will emit tonnes of CO2 every year. Switching to a wood burning Aga will reduce that to zero and that will have more impact on global warming than any amount of air-tightness.
Photo by Popaver
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