In 1001 Energy Tips I have several sections on “moveable insulation.” These are window-sized blocks of insulation (rigid foam works best, wrapped in burlap) that I insert in half the windows of the house to save on heating and cooling bills. As such, the R-value of a double pane window is roughly R2; the R-value of a well-insulated wall is R20. Since the heat transfer is directly proportional to the R-value, a window allows ten times as much energy the flow. (The R-value of a single pane window is R1, so it allows twenty times as much heat to flow!)
Heat transfer through the building envelope is also directly proportional to the temperature difference across the wall. A typical delta-T (delta is the Greek symbol used to represent difference) is 10 F. A typical delta-T in the winter might be 40 F. Hence, all things being equal, a given wall section/window/door transmits roughly four times as much heat in the winter as in the summer.
Knowing these relationships, when is the best time of the year (the hottest summer month or the coldest winter month) to pull my moveable insulation batts out and stuff them in their designated windows? Winter!
__________
Excerpted from Going Green, available as an eBook at this list of eBook retailers online.
