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What is R-Value? (Definition/Meaning/Importance)
Amanda Ringler

By: Amanda Ringler on December 25th, 2019

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What is R-Value? (Definition/Meaning/Importance)

air seal  |  R-Value  |  michigan insulation code  |  conduction  |  convection

One of the greatest mysteries in home insulation is what does R-Value mean? 

That might be a little dramatic but think about everything you’re reading while researching home insulation. You’re likely getting beat over the head with numbers that don’t mean anything to you because they don’t explain what they mean as it pertains to your home insulation.

Honestly, if I didn’t work with an insulation company, I would have no idea what the heck R-Value was because at times it can be hard to understand. Luckily for the purpose of this article, I was able to partner with RetroFoam of Michigan’s general manager, Eric Garcia to find the best way to answer insulation R-Value meaning in a way that makes sense to the average Joe.

We are on a mission to educate homeowners, so with that being said, I’m going to explain insulation R-Value in a way that makes sense to everyone.

What Does R-Value Mean in Insulation?

The quick answer to the meaning of R-Value is that it is a measurement, or rating, which works in conjunction with a material’s thermal resistance, or its ability to resist heat transfer through conduction.

That answer wasn’t quick at all and there are some pieces that need to be better explained before we move forward.

Thermal resistance is the ability to resist heat transfer or loss through conduction, which means through coming into contact with a warm or hot material. 

Think of it as sitting on a cold bench. Basically, while the cold on your bottom might take years off your life, the heat of your body transfers to the cold metal. This is conduction in a nutshell.

Now back to R-Value.

When it comes to heat transfer – the higher the R-Value the greater resistance the material will have to heat transfer.

What Materials Have R-Value?

All insulation materials have R-Value, but how much really depends on several factors.

Insulation R-Value basically means the thicker the insulation, the more efficiently it will keep your home comfortable, in the case of traditional insulation like fiberglass, cellulose, foam board, and a few other options. There are materials, like foam insulation, where R-Value isn’t as much of a factor because it creates an air seal so there is no air movement through it.

Many homeowners use R-Value to compare insulation, but this can be problematic because of what R-Value doesn't take into account.

But I digress, we’ll have more on that in a moment.

Now do some math and figure out how R-Value is calculated.

How is R-Value Calculated?

When it comes to determining a material’s R-Value there is a lot of math and science involved, and I’ll leave that to the professionals. The way they calculate R-Value is:

                                                                          Hours x square footage x degrees Fahrenheit

________________________________________________________ 

                                                                                                                BTU                                                  = Thermal resistance

Another good analogy used by the Building Performance Institute comes right out of the kitchen. You wouldn’t grab a hot pan out of the over without a mitt, because that scorching of your bare hand is conduction. Using the oven mitt resists that conductive heat transfer, but the longer you hold it the more you start to realize that resistance will only last for so long. 

The rate at which that hot pan starts to get hotter and hotter in your covered hand depends on the material of the oven mitt, which is its R-Value.

This my friends is how you figure out R-Value.

But now let’s talk about what it doesn’t measure and how that can be an issue.

What R-Value Doesn’t Measure

While R-Value does measure an insulation’s ability to stop heat transfer through conduction, there are things it doesn’t measure and it all comes down to how the testing is done.

Insulation materials like cellulose and fiberglass can achieve a high R-Value through these tests but still allow for air to pass through it, thus allowing heat transfer through convection.

You see, the R-Value testing is done in an airtight, zero weather condition capsule. 

When you think about it, this isn’t a great way to measure a material that will be put into the cavities of a home where it will be indirectly exposed to the elements.

Another thing R-Value doesn’t measure is an air seal.

I mentioned above that R-Value doesn’t necessarily apply to foam insulation because of the air seal it creates. 

Annually, Americans spend around $2,000 for energy, but $200 to $400 could be going to waste due to air leaks, drafts, and outdated heating and cooling systems, according to the U.S. Department of Energy.

Foam insulation creates an air seal that prevents air movement into and out of the home. It also helps reduce thermal bridging, because it acts as a barrier between the two temperature differences.

This is why looking at an insulation material’s performance is just as important as looking at its R-Value.

R-Value and Building Code: Prescriptive vs Performance

When it comes to insulation code requirements you have two ways to pass -- meet prescriptive code or show you can pass on the insulation material’s performance.

Let’s talk about prescriptive code first, as it pertains directly to R-Value.

Your state’s Energy Code for Insulation and the International Building Code are what your insulation contractor and building inspector are looking at. These codes are used specifically for prescriptive code and dictate how much insulation you need and what R-Value.

Each state will have its own insulation code, but here in Michigan, we are in climate zones 5 through 7, which means a higher R-Value is required. Sometimes with spray foam insulation, this R-Value can be a tricky number to hit and that’s where performance comes into play.

Performance is a more complicated way to pass code because the insulation contractor needs to prove the insulation creates an air seal, that it has an aged R-Value and a few other variables.

The air barrier that spray foam creates isn’t covered by the prescriptive code in Michigan, but the material passes performance. That’s because the air barrier created by spray foam prevents air leakage into and out of the home.

Traditional insulation, like cellulose and fiberglass, will meet prescriptive code when it comes to R-Value, but they still allow for the movement of air into and out of the home.

Now to prove the performance of the foam insulation.

This is done through technology and more science that is above my paygrade. 

The insulation contractor will compile the insulation data, room assemblies, etc., and plug those numbers into a computer program. Some contractors prefer HERS Index while others utilize REScheck. 

With either program, all of the info provided will let the contractor know if they pass or fail in creating a home that is energy efficient. The contractor can then show these results to the inspector and voilà, they skip over the prescriptive code and pass on the insulation’s performance.

Learn More About Foam Insulation and Air Seals

You now hopefully have more information about R-Value than you did when you started and knowledge is half the battle.

Your educational journey doesn’t have to end here. Head over to our Learning Center where you can find even more information not just about foam insulation, but all things home insulation. Our YouTube channel also has some great information and of course, Foam University.

Related Articles

R-Value vs Air Seal: What is More Important When Buying New Insulation?

How Air Sealing Insulation in Your Home Works Like Your Airtight Cooler

Spray Foam Insulation Code Requirements: How to Pass Code Without Meeting Prescriptive R-Value

RetroFoam of Michigan General Manager and Foam University Professor of Foam, Eric Garcia contributed to this article.

Home Insulation

About Amanda Ringler

Amanda previously has worked as a breaking news and crime reporter, TV news producer, and editor in Flint and Detroit. Throughout her career as a journalist, she has won several awards from the The Society of Professional Journalists - Detroit Chapter and the Michigan Press Association. As part of the RetroFoam of Michigan family, Amanda uses her experience as a journalist to write content that will help educate homeowners on the benefits of foam insulation. When Amanda isn’t writing, she’s spending time with her husband and rescued huskies. She also loves knitting, making art, cooking, and hosting dinner and a movie night for friends and family.