How many readers of this blog have pull-down stairs to their attic or know someone who does? You are probably not going to believe how much energy is wasted when the stairs are not insulated to match the rest of your attic.
For illustration purposes, let’s use an attic with 1000 square feet of ceiling area. There is R-38 insulation everywhere except the 10 square feet of the attic pull-down stairs. Maybe you have some of the fiberglass batts shoved between the stairs – we see that in about half the houses we assess when conducting home energy audits. The other half of the pull-down stairs have no insulation. With the latter situation being quite common, let’s figure out the likely R-value for the 1000 SF attic. With 990 SF at R-38 and 10 SF at a generous R-1 value, the whole attic R-value drops to R-28.
Because of the 1% of attic that is not insulated, the average R-value for the attic is decreased by a whopping 27%.
Why such a huge drop, you ask? Although the attic pull-down stairs account for only 1% of the total area, the rate heat passes through the stairs is 38 times the rate it passes through the rest of the ceiling. Therefore, it’s like having 380 SF of uninsulated ceiling when 10 SF of attic stair is not insulated.
I need to mention that we are only addressing how heat passes through the attic stair surface, not the additional heat that passes through gaps around the edge of the stairs. A building envelope has both insulation (to limit heat flow by conduction through building materials) and an air barrier to stop leaks.
The learning lesson here is to find a way to insulate your attic stairs. It’s as easy as constructing or buying an attic stair cover or placing rigid foam insulation on the wood surface. Just be sure to cover the entire surface area of the wood.