How Much Insulation Is Enough?
Originally published February 28, 2012
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. Let's assume, for the time being, that we're talking about new construction to simplify the discussion.
As for location, I'll provide recommendations for three different climates, based on the 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. Cold climates: Zones 5-7Zones 5-7 cover much of the northern half of the U.S., from roughly the Mason-Dixon Line at the East Coast across the northern third of Missouri and the northern edge of eastern Kansas, then dipping south in the higher-elevation Plains States through northern New Mexico, northern Arizona, nearly all of Nevada (except the Las Vegas area), and the northeastern corner of California and the eastern three-quarters of Oregon and Washington.
For these locations, I follow the widely quoted recommendations from Building Science Corporation and aim for the 5-10-20-40-60 rule. These numbers refer to the R-value recommendations for windows, foundation slabs, foundation walls, above-ground walls, and attics (or roofs), respectively.
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These recommendations are for "true" R-values, not the nominal values listed on insulation packaging. For example, if you install R-19 fiberglass batts in 2x6 frame walls, with the studs 16 inches on-center, double top-plates, and other elements of "standard" framing, the actual R-value of the whole wall with the R-19 insulation will be about R-15. The whole-wall R-value is lower because of "thermal bridging" through the wood framing.
To achieve R-40 in the walls requires a lot of insulation--far more than is found in standard construction. This level of insulation, if combined with strategies for minimizing air leakage, will result in a house that will be affordable to heat even if energy prices double or triple. And if combined with some passive solar heating will result in a house that should never come close to freezing in winter, even if the heat is turned off.
With window R-values, the recommendation refers to the "unit R-value," a measure that averages the center-of-glass R-value and the R-value at the window edges--where the heat loss is greater (at least with high-performance windows). These unit R-values are the inverse of the U-factors listed on NFRC (National Fenestration Rating Council) labels found on most new windows: R = 1/U.
Hot climates: Zones 1-2Zones 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.
For these locations, I recommend intermediate insulation values between those for cold climates and hot climates. I suggest a 4-5-10-30-60 rule: R-4 windows, R-5 under slabs, R-10 foundation walls or slab perimeter, R-30 above-grade walls, and R-60 in the attic or roof.
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.
Published June 1, 2018