The extensive amount of glazing on the South face of the house is needed for solar capture, but at night in the winter this is also the source of greatest heat loss. Obviously there are glazing options available that could reduce  this heat loss and it necessary to understand the technical aspects of glazing to come to a decision.

So lets start with U-Factor. This is the primary measure of heat loss. A normal pain of glass has a U-Factor of about 1.2 This means that the glass would lose 1.2 BTUs every hour for every degree Fahrenheit difference across the glass. Lets look at that in a practical example.

  A quick glance at the St John Climate show that on a December night the external temperature drops to  19° F (and this is average it could be even colder) . Suppose the room temperature was 69° F at the same time. The temperature difference across the glass would be 50F°.  So every square foot of glass would lose 50BTU/hour. There are approx. 500 sq ft of windows on the lower floor. So the total heat loss for the lower floor would be 25,000BTU/hour.  That's a lot of heat loss! If we use glass with a U factor of 0.6 the heat loss would be halved. So we should use windows with the highest U-Factor ! !

Unfortunately its not that simple as we must also consider the SHGC (Solar Heat Gain Coefficient). The lowest U-Factor windows often have the lowest SHGC which means while they cut down on heat loss they will also cut down on the heat gain we want during the day.

The window in the illustration above has a U-factor of 0.29 and an SHGC of 71% so 71% of the heat falling on the window enters the room. The window to the right ha a U-Factor of 0.29, better for insulation but only allows 39% of heat into the room.

So we will have to find a balance between high U-Factor and a high SHGC. Later we will be expanding this section to look a specific product.