Our Welsh Passivhaus social housing prototype today achieved what is probably the best result so far for a UK pressurised air test for an above-ground, detached house. As predicted, the second airtest which was carried out today on our 3 bed social housing passivhaus prototype (after completion of M&E installation including services penetrations) achieved an even better result than the first test. On decompression at 50 pascals the result was 0.17 air changes per hour, but on average of 50pa decompression and compression, 0.197 air changes per hour was the final result as measured and calculated by Paul Jennings, probably the UK’s most respected air testing specialist.

This result is over three times better than the minimum required by the Passivhaus Institute and right up there with the very best German results. This is all the more remarkable as this is the first time that Holbrook Timber Frame and Pendragon Contractors have ever attempted to achieve the Passivhaus standard. The third air test in July could improve on this figure still further, as the walls will by then be lined and plastered.

[See earlier blog entry, May 2nd, for the lessons learnt and techniques used to achieve this result.]

Question: “what is the significance of the pressurised air test?” Answer: This test is the best indicator of how little warmth will be lost in Winter by cold draughts when the windows are closed. In a properly designed and certified super-insulated Passivhaus, most people will find that little or no supplementary heat will be required. An important factor in achieving this is building weathertightness - enough to ensure that tiny cold draughts don’t take away the build-up of warmth from what may be tiny internal and external heat gains (like the heat from a refrigerator or the heat from the sky on a bright overcast winter day).

Question: where do I get the fresh air for living in such a draught free house? Answer: In winter by means of a silent, very low energy passivhaus certified heat recovery unit that extracts air from bathrooms and kitchens and brings lots of fresh air into bedrooms and living spaces whilst saving around 94% of the heat in the extracted air; putting that heat back into the fresh air. The best of these simple, reliable, automatic units use about 15watts of power on extract (all year round) and 15 watts of power on supply (winter only as the rest of the year windows can be opened without wasting energy).

Testing across Europe indicates that the air quality in Winter inside a low energy passivhaus is usually better than the European standard (never exceeding 1500ppm CO2) whereas most naturally ventilated low energy buildings have big air quality problems with CO2 levels often rising in spikes significantly above the European standard maximum of 1500ppm before the windows are opened to get fresh air. As the cold fresh air comes into the building, the stale air is allowed to escape with lots of heat! Significant amounts of heat are usually required after ventilating, in order to re-heat naturally ventilated buildings. The heat lost is much, much more than the tiny amount of energy going into the heat recovery fans of a Passivhaus!