Are you hoping to improve the energy efficiency of your home? If so, you may have heard a contractor talk about “R-value,” or seen R-value ratings on windows or insulation packaging. Did you have any idea what it meant? If not, don’t worry. Even many well-educated people find themselves scratching their heads in confusion at the term. Fortunately, it’s not hard to explain.
What is R-value?
Simple. R-stands for “resistance.” R-value is a measure of thermal resistance. Without getting technical, that means it’s a measure of how hard it is for heat to pass through a material.
Now here’s where it gets a little more complex. You can’t measure the resistance to flow. You can only measure the flow of heat. Heat flow is measured as U-value. A U-value of 1 equals 1 BTU per hour per square foot per degree of temperature differential.
“Now wait a minute,” you say. “You just said R-value is a measure of resistance. Then you said you can’t measure resistance. Now I’m really confused. What gives?
It’s true, you can’t measure resistance. But when we’re talking insulation, we think in terms of resistance. Like, when we put on a pair of mittens it keeps the cold from getting through to our fingers. We want a number that reflects the way we think. So we make one up. Like this:
We take the U-value (a measure of heat flow through a material) and find its reciprocal. And that’s the R-value.
To find the reciprocal, take your number and convert it to a fraction, with one as the numerator (on top) and the U-value as the denominator (the bottom part of the fraction.) If the U-value is a decimal, multiply to make it a whole number, then simplify if possible. (For instance, 1/2.5 = 10/25 = 2/5.) Then convert to a decimal by dividing the numerator by the denominator (2 ÷ 5 = .40). So a U-value of 2.5 = an R-value of .4.
Fortunately, we don’t usually have to do the math. Most building materials are already rated in R-value, so that’s the only number most of us need to be aware of most of the time. All we really need to know is this:
The higher the R-value, the better the insulator, and (at least in theory) the warmer it’ll keep our homes in winter, and more effective it’ll be in keeping cool air in when it’s hot out.
Here are some typical R-values for some common building materials per inch of thickness:
How to Use Your Understanding of R-Value to Calculate Insulation Value
The beauty of R-values is that, unlike U-values, they can be added together. So if you’re planning, say, an addition on your home and want at least R-24 in the walls, you can count up the R-values of all the materials you have to choose from and figure out your best value.
For example, say you have a 4” wall cavity. You could fill it with dense-pack cellulose to get R-14 (R-3.5 for dense-pack times 4 inches.) Then you’d have to make up the rest with, say, 2 inches polystyrene board on the outside (R-5 times 2 = R-10).
Or you could accomplish the same thing by filling your cavity with expanding foam (R-5.5 x 4 = R-22) and only using 1/2” board.
R-Value isn’t the only factor to consider when shopping for insulation and other home performance improvements. You’ll also want to look at air infiltration qualities, ease of installation, product life expectancy and budget. But now that you have a good basic understanding of what R-value is and how to apply it to your situation, you’ve taken a big step towards making intelligent value choices for your home.
Some Industry Links
Bambrook, S.M., AB Sproul, and D Jacobb. “Design Optimisation for a Low Energy Home in Sydney.” Energy and Buildings 43.7 (2011): 1702-11. Science Direct. N.p., n.d. Web. 9 Oct. 2017. <http://www.sciencedirect.com/science/article/pii/S0378778811000910>.
Hoicka, Christina E., Paul Paker, and Jean Andrey. “Residential Energy Efficiency Retrofits: How Program Design Affects Participation and Outcomes.” Energy Policy 65.(2014): 594-607. Http://www.sciencedirect.com. N.p., n.d. Web. 9 Oct. 1920. <http://www.sciencedirect.com/science/article/pii/S0301421513010768?via%3Dihub