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How Much Insulation Is Enough? 

May 25, 2013 | Posted by Mitt Jones

How much insulation is enough? It’s a good question, because adding insulation doesn’t always help as much as we might like.

Before I attempt to answer the question, here’s a timely disclaimer: If you know you have a comfort or energy problem with your house, call in a home performance contractor—even if the rest of this article tells you that you have enough insulation. Something else may be going on, such as high air leakage or a problem with the way the insulation was installed.

Now back to our answer. Let’s start at the bottom.


If you have a vented crawlspace and there is no insulation between the joists, you need insulation there. If there is insulation installed between the joists and it’s installed well enough that you can’t see the subfloor above it, then you may be okay leaving well enough alone. (Check out Evaluating Crawlspace Insulation if you want to know more.)


Now for walls: If you have insulation in your walls—even 1970’s-era 2-inch batts—you’re probably done unless you live in a very cold climate. If you happen to find yourself tearing drywall off your walls, it’s a great idea to replace the 1970 batts with some nice, new R-15 batts. But trying to blow more insulation into the wall cavities when fiberglass batts are already installed may cause more problems that it’s worth. If you’re really determined to improve the insulation value of the walls, adding injection foam may do the trick, but you’ll spend a good bit of money for what may prove a relatively small improvement.


Finally, the attic: If you can see the top of the joists, you definitely need more insulation. If insulation covers the top of the joists, you may be okay—at least in a climate as mild as the Portland metro area’s. We normally insulate attics to R-38, and that’s a reasonable target for our climate. If you already have R-25 installed, however, the improvement you’d realize from an additional R-13 is incremental in a mild climate. R-25 works out to about 10 inches of blown fiberglass or 7 to 8 inches of cellulose.

The thing to keep in mind as you think about adding insulation is that adding R-13 helps a lot less when you’re starting at R-25 than if you’re starting with no insulation. To learn more, take a look at Why More Insulation Isn’t Always the Answer. 

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