Living lightly

Recently, the low outside temperature was 12 degrees and the high temperature for several days was in the 20s. Prompted by these frigid temperatures, I looked at the variation of inside temperature throughout the day. The heating control system in our house allows us to create graphs of zone temperature values, so examined these to see what I could learn.

 

I found that the average temperature decrease over the day when we are out of the house is less than three tenths of a degree per hour with the outside temperature at 40 degrees F. The temperature range is within 64-68 degrees over the ten hours we are at work. Even in the coldest days, the heat loss in a heating zone was less than one-half degree per hour.

The major factor in this is the house's SIP construction (structural insulated panels). Floor, walls and roof consist of panels of encapsulated polystyrene between OSB (a sort of plywood). A panel is around 4 feet wide and up to 22 feet long. Roof and floor are 10 inches thick, walls 6 1/2 inches. Seams between panels are sealed with mastic or spray-on insulation, and all interior seams are covered with a heavy tape to ensure a vapor barrier throughout the house. A second factor is that there are relatively few windows (less than 90 square feet of window and door glazing) and the windows are double-pane with “low-E” glass. That means the outside pane has a coating that prevents radiant heat from passing to the outside, thereby keeping that type of heat in the house.

Installation of these panels is somewhat like building with Legos. Here's an (exploded view of a typical house).