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
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
(6,700-degree-day climate, 1,200-sq.-ft. attic)
R-value||Annual fuel cost for attic
heat loss||Annual savings for each
R-10 increment of insulation added||