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What is the Highest R-Value Insulation?

R-Value is the measurement of an insulation material’s thermal resistance.

In layman’s terms, it measures an insulation’s ability to resist heat transfer through contact, or conduction. 


Now at a glance, the insulation ratings above could be a little confusing, so keep in mind those are the per inch R-Values. The U.S. Department of Energy uses an uninsulated attic as an example and it is a great place to start. States like Florida and parts of Texas would only require an R-30 to R-49 in an uninsulated attic.

Traditional insulations, like fiberglass and cellulose, can achieve a higher R-Value because you can essentially double and triple up the material and composition to hit the numbers you need. When you’re talking about spray foam, it’s not that simple.

R-Value isn’t set in stone, meaning there are things that can impact that number not only lowering it but essentially making your insulation ineffective.

When you add insulation, you don’t expect cold temperatures to cause it to lose R-Value, but that is exactly what happens to fiberglass. Studies have found that as the temperature dips, fiberglass begins to lose R-Value which can be a real problem when it comes to energy efficiency and comfort. Another thing that can lower the R-Value of fiberglass, as well as cellulose is if either of the materials gets wet. Both materials retain that moisture, which not only lowers the ability to insulate but also leads to the growth of mold.

Does Highest R-Value Mean the Best Insulation? There is more to consider when buying new insulation and it comes down to R-Value versus creating an air seal. Sure, code might call for R-49 in your attic and spray foam doesn’t hit that number at recommended depths. But what it can do, whether it’s open cell or closed cell, is create an air seal. Foam is the only insulation option that creates that air seal, but now you’re probably wondering about code and that’s good.

You see, you don’t want to try and double and triple the amount of foam you install because you only need about 2-inches of closed cell or 3-inches of open cell depending on the product to create that air seal. I know what you’re thinking, and yes, I said we recommend more than this on the attic roof deck, but that’s to also create a stronger thermal barrier. Heat rises and you want to make sure that the thermal barrier is in place even with the air seal. Piling on inches of spray foam is a waste of money as it diminishes your return. Once you create the air seal, you won’t have anymore return on investment by adding more foam. This chart does a better job showing this than I can explain it.


So, when it comes down to it R-Value is important if you plan to use materials like fiberglass, foam board, or cellulose, but if you’re looking to create an air seal, then those numbers don’t mean as much. The best and most efficient insulation may not in fact have the highest R-Value per inch or per wall cavity. In this case, it comes down again to what the material can do and how it performs.

Foam insulation has a lot of benefits aside from creating that air seal.

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