Tuesday, 8 May 2012

Getting certification and thinking about solar gain

The passive house lady visited in April and told us what we need to do to get passive house certification.

Basically we need to send in the PHPP excel file, which contains all the calculations, then send any drawings and data we have to justify the numbers we have put in.

We need cross-sections of the house showing where the vapour barriers are, and showing the wall sections to satisfy the insulation calculations and the airtightness requirements.

We need some kind of plan of the ventilation system, and ideally projected and measured values of the flow of air into, out of and through each room in the house.

We talked a bit about solar gain from the windows, which I'll go into more detail on later. When I started playing around with the PHPP file, I noticed very quickly what a massive difference was made by tiny changes around the windows. For example moving the south-facing windows a few centimetres further out could be the difference between meeting or not meeting the passive house criteria. I've tended to think of solar gain in a rather bow-and arrow way, that the sun is directly radiating into the house, in a straight line. In fact in Europe they estimate that 50% of solar radiation will come directly from the sun (in a straight line) and the other half will be reflected, and come from all different directions. In Japan, on the other hand, about 75% of the sun's radiation is coming direct, and 25% is reflected because there is less cloud cover. This means the PHPP calculations could be a bit pessimistic for our South-facing windows, which will actually bring more heat into the house. 

An interesting and counter-intuitive consequence of this notion of reflected radiation is that in the summer we could get more solar gain on cloudy days, when the geometrical shading blocking out the direct sun will be less effective at stopping dispersed and reflected radiation. On sunny days, though likely to be much hotter, we should be able to cut out most of the direct radiation, and very little is going to come from the blue sky. This is the opposite of the way that the most solar gain is coming into the house in the coldest months of the year.