Q&A: Optimal Insulation Thickness

A.Energy and sustainable design consultant Andy Shapiro responds: Every inch of insulation you add to an attic results in less savings than the previous inch you added. That’s just the way the physics works out. The good news is that adding another inch of insulation in an attic costs very little, if any, in added labor, so it’s mostly added material costs. For a house in central New York (6,700 degree days) with typical efficiency oil heat (75%) and oil at $1.25/gallon, the cost for the heat lost through the insulation in a 1,200-sq.-ft. attic for a typical year is summarized in the table below.

Local codes usually dictate the minimum acceptable level of attic insulation. What kind of savings can a homeowner expect when they upgrade from R-30 to R-40? It would be worth another $20 in the first year. If the homeowner expects a simple 10-year payback, then up to $200 could be spent on the upgrade to R-40, and still be worth it. The added cost for R-38 unfaced batts over R-30 (retail at my lumber yard) is $0.12/sq.ft., or about $150 for the attic, so that looks like a good deal for the owner.

How about R-50? To get R-50, you’d probably add R-19 to the R-30 batts, at a cost of $0.24/sq.ft. over the R-38 batts alone, or about $290 for the whole attic. The first-year savings is $12, so with a 10-year payback, it’s worth $120. That doesn’t look like a great deal for the owner. It gets worse for more insulation: it would cost another $280 to go to R-60 over R-50, with a 10-year savings of $80.

This is why R-38 batts are so common in attics in heating climates. It’s usually the right price point for the owner. If you are blowing in cellulose (my favorite, as it fills all the odd-shaped places well and is a recycled material), the incremental costs are a little different. All this said, I’ve never heard of an owner who was sorry that they put in too much insulation.

Fuel Cost vs. Attic Insulation R-Value (6,700-degree-day climate, 1,200-sq.-ft. attic)