The one-room addition on Emerson W’s home is not what anyone would realistically consider over-insulated: R-11 batts in the walls and R-19 at most in the ceiling. But the immediate issue is the floor. There’s no insulation at all there, and because the addition sits on concrete piers, there’s nothing to stop the wind from blowing freely below.
“As you can see, outdoor air can simply pass through the open space under the floor,” Emerson writes in a post in the Q&A Forum. “When the flat EPDM roof needs to be replaced in 5 to 10 years, we will install exterior foam board outside the roof deck. Heating is primarily warmed air from rest of house or from the propane fireplace. Cooling is AC from rest of house.”
Emerson is weighing a plan to excavate below the room to a depth of 2 feet before installing batt insulation between the joists, adding 1 or 2 inches of rigid polyisocyanurate insulation over the joists, and covering the assembly with OSB or plywood.
“What approach would you take?” Emerson asks. That’s today’s Q&A Spotlight.
Digging out is a good idea
GBA Editor Martin Holladay has some suggested reading for Emerson (see the “Related Articles” sidebar below), but in general suggests that digging out the soil from beneath the room is a good first step.
“Excavating the soil under this addition is obviously a good idea,” Holladay writes. “So is adjusting the grade nearby, if possible. If you can afford to put in a real crawl space foundation (with concrete walls), all the better.”
If Emerson keeps the pier foundation, opting not to spend the money on a full crawl space foundation, there are some sound strategies for insulating the floor. But a key consideration in all of this…