Monday, 27 August 2012

Recalculating the windows

Solar gain is a massive part of the passive house philosophy. It is probably no coincidence that the word "passive" is also used in "passive solar" design, whereby the natural radiation of the sun is used to the maximum benefit. Positioning, size and G-value of the windows makes a huge difference to the amount of direct solar gain, as do any obstacles outside.

So it was with some trepidation that I started adjusting the figures in the PHPP file to take account of the houses to the South and South West, and have a more careful look at the numbers in there.
For each window, you start with the dimensions, U Value of frame, U Value of glass, Psi value of frame and Psi value of installation. The U value is a simple, one-dimensional heat loss per unit area per degree of temperature difference between inside and outside. The Psi value is the thermal bridge effect, which is the extra heat loss per unit length along a boundary between two insulators, per degree of temperature difference. For example a square window, one metre on each side, has an area of 1, but there are 4 metres of boundary between the window and frame, and 4 metres of boundary between the window frame and the wall into which it is installed, so there are 8 metres of thermal bridge to take into account.

Those figures are enough to calculate the heat lost through the windows, over the year. As a rough guide, there are 70,000 heating degrees hours in the Matsumoto year, compared with 80,000 in central Europe. That's the total of the temperature differences for each hour over the year, so if it's freezing outside and 20 degrees inside for an hour, that's 20 heating degrees. Just as you can multiply a U value by the temperature difference to work out the instantaneous heat flow, you can multiply the U value by the heating degree hours to get the number of kWh of energy you need to replace the heat.

That's the heat being lost through the window. Window U values are based on the area of the frame and take into account the U value of the glass and the U value of the frame. Solar gain of course will only come through the glass, and not the frame. The heat being gained also depends upon the angle and orientation to the sun, depth of overhang on the top window sill, depth of sides to the left and right, and any obstacles in front. A south-facing window will get the most solar gain, and if the window is tilted from the vertical it will also increase the gain. All obstacles and overhangs will reduce the heat gained.

As far as the PHPP software is concerned, as well as the straightforward orientation and angle, which for most of our windows is due south and vertical, we need to put in the depth and distance of window reveal, depth and distance of overhang, and height and distance of shading object.

We started off with the house to the south as a shading object for all windows, 16 metres away and 5 metres above the ground floor windows, 3 metres above the upstairs windows. This makes a surprising difference even though the sun is never behind them. For the window reveal, which is to the left and right of the window, we put in a depth of 100 mm, and distance of 100 mm. The upstairs overhang we put in as 100 mm deep and 150 mm high, while the downstairs overhang is set as the balcony which is 600 mm deep and 600 mm high.

This gives us a total Passive house score of 12.3 kWh per square metre of house area per year, with a total solar gain of 5,392 kWh per year.

Actually the ground floor window panes are all at a depth of 140 mm. This change of 4 cm on the south facing windows wipes 0.3 off our score and loses us a total of 76 kWh per year. The middle concertina window panes are 120 mm from each side. The window to the West, a double tilt turn, dreh-kipp as they say in German, has panes 100 mm from each side.

Actually, they are not all at a depth of 140 mm. The west side of the kitchen window is fixed into the frame rather than into a leaf inside the frame, and is only 125 mm deep and 65 mm from the side.
It's actually not so simple, as there are wooden pillars supporting the balcony, right next to the reveal, so should we be looking at the reveal for the right side of the middle concertina as 140 deep, 120 mm away, or 300 mm deep, 150 mm away? Or in fact 800 mm deep as there is another pillar further out on the terrace supporting the balcony. I think putting in these details is going to give us a mean score as there is only one figure for reveal, which it presumably puts on both sides of the window pane. For the middle concertina pane, we can put the reveal a metre away, which should help our case.

For the furthest west window, the house next door should probably be treated as the reveal, which would be 10 m deep and 3.4 m away, were it a part of our house, which it most definitely is not, event though it is so close that it seems as if it wants to be. There are also a couple of balcony pillars there, 180 mm away and I suppose 800 mm deep, or 300 mm deep depending on which pillar you take. Perhaps these should be treated as the reveal. There is daylight between them, so it seems mean to treat it as 800 mm deep, but probably little radiation gets through that as there is a solid house beyond. The next house gives a figure 50 kWh per annum worse than the two pillars, which is another 80 kWh/a worse than one pillar.

I wonder whether it takes reflected radiation from the reveals into account. Also there's probably a considerable amount of heat reflected off the stone tiles on the terrace and into the house. It would be great to measure all of this with a solar radiation meter. I don't happen to have one though!
And the balcony is actually 800 mm above and 800 mm away from the top of the window panes, rather than 600 mm. This is the same angle, but the further away overhang leads to less solar gain, presumably because less indirect radiation from the sky above gets through. This is a difference of 47 kWh/a.

More accurately, the upstairs window reveal is 170 deep and 100 distant, taking into account the rails for the shutters, and the overhang is either 125 deep and 75 high, or 350 deep and 350 high depending on whether you take the plaster or the box for the shutters as the cutting edge of the light. That makes a difference of 97 kWh/a.

So having started with a total score of 12.3 kWh per square metre of house area per year, putting all these adjustments in, in the worst case of each scenario, we lose 541 kWh per annum solar gain, and get a score of 14.6, still within the Passive House limit of 15 kWh/m2a. There are plenty of places we can argue the toss, if necessary, especially with our double-leafed windows where the strict conditions have been applied to both sides of the window.