Passive House Principles: Passive Elements
There are several ways to gain heat passively. South facing windows can be a huge contributor to heat. However they can also be like a huge hole in the envelope where heat leaks out of the building. Lets put this in perspective. Take a wall 8 ft high and 20 ft long. Without a window with R20 batt insulation the effective R-value is about 17. Insert a large 4ft x 4ft window in the wall which has an R-value of 2. It can be shown that the total R-value of the whole wall assembly becomes R11!!! So the effect of having a R2 window is detrimental to the performance. Even though the window only makes up 10% of the total wall area it effectively halves the R-value and makes a 5% difference to the heat flow out of the building. Now add a couple more windows....and you're living in a green house: Its cold when it's cloudy and sweltering hot when the sun is shining!. Triple glazings should be used for all windows regardless of their orientation on the building. Glass with expensive coatings aren't really required for areas of the home where the sun doesn't shine! However on the south side of the home, glazing with high solar heat gain coefficients are necessary (SHGC). They will ensure maximal transmission of solar energy through the glass. When the energy requirements of a building are as low as a Passive House, the solar gains can make up to 1/3 of your total energy for heating. The key is balancing the size and number of windows so the amount of energy gained is at least equal to or more than the energy lost. This is part of the Passive House design process. Adding thermal mass, such as concrete floor act to buffer over heating of the air inside the home. The concrete will soak up some of that heat which will radiate into the space as the sun goes down. That same thermal mass will help keep the house cool in the summer time.