When the time comes to choose windows for any project, I really can't stress enough the enormous role windows will play in your overall home performance. Better windows may pay for themselves in a few years of heat savings, while also improving occupant comfort and the overall user experience. So shop around and get all the facts. If in doubt, consider hiring a professional to help you select what is right for your specific project.
1. Remember that the product should be very airtight. Otherwise, the energy saved in heat transfer will be lost through air leakage.
2.When you start shopping in the area of high performing windows, you will be entering the realm of Low E coatings (Low Emissivity). This is a metallic film that is applied to windows which further determines how much heat comes in and how much goes out. Low E coatings on windows will reduce heat loss by reflecting infrared heat back into your home in winter while also reflecting heat back out in summer. On the down side, they can also reduce heat gain when you want it, during the heating season. So a little thought should go into choosing the right glazing characteristics for each side of the house.
3.You won't be getting any solar heat gain on the north facade, so go with the highest R-value, period. On the east and west facades, a combination of a high R-value and low solar heat gain coefficient [SHGC¹] is best to prevent overheating in summer, unless the windows are well-shaded from direct sunlight, in which case treat them as you would on the north.
4.On the south side of the house is where you can really come out ahead. With the right windows, you will gain more heat in the day than you will lose at night, by a factor of as much as 50%. It is actually in your best interest to sacrifice some R-value by way of Low E coatings to increase the solar heat gain from the low winter sun; therefore, when choosing windows for the south facade, look for a high SHGC with the highest possible R-value [or its opposite: the lowest possible U value, since this is the value usually provided for windows and doors].
5.And remember that the frame of your window can actually represent up to 25% of the total window area, so the same logic applies here: a low frame R-value will result in poor overall window performance whatever the quality of the glazing.
-When windows perform better, the overall R-value of the wall system is greatly improved.