One crucial mistake was not paying enough attention to the coatings on the double-pane windows. The windows were purchased new at a reuse center but no longer had the National Fenestration Rating Council (NFRC) sticker on them, leaving us “in the dark” about their transmittance characteristics. Once I stood inside, in front of these windows on a sunny fall day, the lack of warmth coming through clued me in that the low-e coating was hobbling the passive solar gain of the small cabin.
While the south-facing wall had almost 43 square feet of solar collection area for the interior space of 120 square feet (or roughly 35%; well above a 7% to 10% ratio of collection area to living area), most of the window area was barely adding to the heat gain. Given that the wall space occupied by the windows was of a substantially lesser R-value than the surrounding walls and thereby losing much more heat, the total contribution of the windows to heat gain was probably a wash.
Passive solar heating won’t work if window coatings are keeping the sun out. So be sure to check out the NFRC label carefully. Any window being installed for passive solar must meet the following criteria:
- Have a solar heat gain coefficient (SHGC) greater than 0.60
- Have a U-factor of 0.35 or less
- Have a visible transmittance (VT) greater than 0.65
- Be certified by the NFRC
