How Much Insulation Is Enough?

Recommended residential insulation levels by climate zone

POSTED ON MAR 1 2012 BY ALEX WILSON

Most leading energy experts today recommend installing a lot more insulation than is common practice. This wall for a Passive House in Seattle will hold about a foot of insulation.

I’m often asked the question, “How much insulation should I install in my house”? It’s a great question. Let me offer some recommendations:

First of all… it depends. It depends to a significant extent on where you live. And it depends on whether we’re talking about a new house or trying to squeeze insulation into an existing house.

To simplify the discussion, let’s assume, for the time being, that we’re talking about new construction

As for location, I’ll provide recommendations for three different climates, based on theĀ U.S. Department of Energy (DOE) and International Energy Conservation Code (IECC) climate zones. These DOE climate zones range from Zone 1 at the extreme southern tip of Florida, to Zone 7, which covers the tip of Maine, stretches across the northern reaches of Michigan, Wisconsin, Minnesota, and North Dakota, and includes a few high-elevation places in the Rockies (see map). In my recommendations, I group these into three larger zones for simplicity.

Hot climates: Zones 1-2
Zones 1-2 include the hottest areas in the U.S., covering most of Florida and a band west to central Texas, as well as southern Arizona and the Imperial Valley of extreme southeastern California.

Here, I recommend a 3-5-10-20-60 rule: R-3 windows, R-5 under slabs and for any below-grade foundation walls, R-10 for above-grade foundation walls and slab perimeter (full foundations are rare in these climates), R-20 for above-ground walls, and R-60 for attics. These recommendations come from an informal conversation with John Straube of Building Science Corporation. Again, these are true R-values (unit values for windows).

It will surprise some to see the recommendation for attics to be the same as in cold climates. This is because of the difference in temperature (delta-T) between the living space and the attic on a hot summer day can be as high as wintertime delta-T in a cold-climate between indoors and outdoors. With windows, I further recommend a solar heat gain coefficient (SHGC) of 0.3 or lower to minimize unwanted solar gain.

What about existing houses?
In new construction, the incremental cost of increasing insulation levels are relatively modest. With existing houses, retrofit insulation costs are usually much higher, so it is usually difficult to justify such high insulation levels. The exception is attics, where adding lots of additional insulation is usually quite affordable.

So, in existing homes, determining reasonable insulation levels is project-specific. In a full gut-rehab (where the house is taken down to the structure, or the frame is opened up on either the interior or exterior), achieving close to the recommended insulation levels for new construction may be possible (though higher costs for extending window and door jambs and, sometimes, roof overhangs also need to be considered).

And with windows, whether to replace or improve existing windows is a key question. Look for recommendations in future blogs.

The United States is divided into seven climate zones by the U.S. Department of Energy and the International Energy Conservation Code.