The insolation for January here in Matsumoto is over four times what it is in Germany, according to the data we have from the Passive House institute. Insolation is the amount of energy you get from the sun. This is pretty important. With about 20 square metres of windows on our south-facing walls, a bit of space to the south before the next house, which is a bungaloo, and having taken care not to design anything that will block the sun out, we stand to gain about 970 kilowatt hours in the month of January. We're using triple windows with special clear glass that will let through more heat.
Usually triple windows let through about 50% of the sun's heat, while double glazing lets through 70%. Of course double glazing will let out more heat than triple glazing because the insulation is much better. In Europe, or at least central Europe, where winters are cold and the sun doesn't make much of an appearance, using triple glass is a no-brainer. Windows are basically a liability when it comes to the thermal performance of the house.
Here, where winters are cold but the sun is above the horizon for much more of the day, and hiding behind clouds much less, south-facing windows will bring heat into the building. They are our heater. They'll bring in over 5,000 kWh in the cold part of the year, and lose less than 1,500. The gain is around 3,750, according to the Passive house software PHPP. As we've put almost half our windows on the south side, it looks like the windows are going to net over 3,000 kWhs of heat in the cold part of the year. That's about 60 gallons of heating oil. Or 290 litres of kerosene, the fuel of choice for heating in Japan for people who have no choice.
Interestingly, if the south-facing windows were double glazed, they would lose a lot more--over 2,000 kWh per year--but they would also gain a lot more--over 6,000. In fact double glazed may even have a slightly higher net gain. There would also be less cost.
Of course curtains aren't so nice when it's snowing outside, and double glazing would allow more of a temperature drop over night, and probably encourage cold drafts and condensation. We would want curtains to improve the insulation of the double glazing. According to the man who makes them, you can sit comfortably up against triple-glazed windows in the middle of winter. This will make the part of the room we can use bigger.
The decision was largely made when we applied for the grant from NEDO, the new energy development organisation.
