Friday, 17 February 2012

0.28 = 0.54, 0.26 = 0.38... Confused!

After another two-month wait, I got the results of the airtightness tests in a letter. I sent the insulation people a reminder by email on Friday and the letter came on Monday, with an apology that they had already sent the information to the architect and assumed he had sent it on to me.

The results, from a Passive House perspective, look very good. The standard we were aiming for was air leaking at a rate of 0.6 times the total volume of the house per hour, with a pressure difference of 50 Pascalls between inside and outside. The lower the figure, the less air is leaking and the better the airtightness.

The result back in August was 0.54. Unfortunately we didn't realise at the time that the result was this good. The result in December, after filling the hole under the front door, adding caulking around the windows and getting the big window to shut, was 0.38. This is 35% better. In the Passive House software, this reduces the performance of the house from 13.5 to 12.6 kWh/m2a (kWh of heating energy per square meter of floor space per year). That's a 7% saving on the house's heating requirement. Obviously airtightness affects thermal losses as leaking air will carry heat in or out of the house. 

The Japanese measurement of airtightness, C, the number of square centimetres of gaps per square metre, was 0.28 in August, and 0.26 in December. An improvement of only 6%. Clearly the two figures don't have a linear relationship. We were apparently aiming for a C value of 0.2, although 0.3 seems to have been a closer target.

The C value seems to be based on on the air leakage at a pressure difference of 9.8 Pascals, and then goes through some exponential arithmetic. In reality, the pressure difference between inside and outside is surely going to be a lot less than 50 Pa, and probably even less than 9.8.

According to the results sheets (graphs for August above and December below, pressure on the y-axis, leakage rate along the x-axis), it looks like the leakage was measured at roughly 10 Pa intervals, from 10 to 50, and a line drawn on double logarithmic graph, deducing some exponential relationship between the pressure difference and rate of leakage. The Japanese and European standards are at different ends of the range of measurements, so neither estimate will be completely accurate.  This way of testing should give a very good estimate for the leakage at 30 Pa difference. 

I'm really interested to know how the C value is calculated, and the rationale for this. And how does the Passive House software calculate the effect from the figure for 50 Pa?