What does R-value Mean?

March 4th, 2020

Dear Readers,

With William having his degree in architecture, as well as years of experience working with his father’s business, A Squared Carpentry, he sometimes explains things to me beyond my level of understanding. In an attempt to keep up, and perhaps provide him with new learning material as well, I have been reading The Superinsulated Home Book by J. D. Ned Nisson and Gautam Dutt. The book was published in 1985, but much of the information they provide on building superinsulated and energy efficient homes is still relevant. They explain the home building process in a simple, easy to understand way. They even provided the equations necessary to find a home’s total heat loss coefficient in a way that even I (someone who struggled through every basic math class) could understand. Heat loss coefficients, by the way, deserve a blog of their own, and will be explained at a later time… From this book, and with some of William’s guidance with his William Words, I finally came to understand what in the world ‘R-value’ means. 

R-value is a measure of thermal resistance. And, thermal resistance is based upon conductivity. Lots of big words, and if you are anything like me (and could read War and Peace faster than you could read a book about insulation), when you first read them you don’t see the relation. Sooooooooooo…… let’s break this down. 

William’s Warning: We are using imperial units in our equations! If you live somewhere other than the United States (I believe we are the only country left using them), and if you use the metric system instead, your equations and R-values will have slightly different values.

Conduction

Conduction is the movement of heat energy through a material. And as a rule, heat will always conduct, or move, toward the colder surface of said material. So, let’s take a window, placed in a house in the middle of January, as an example. It’s 20 degrees Farenheit outside. It is 70 degrees Farenheit inside. The side of the window on the interior of the home is warmer than the side of the window on the exterior of the home. However, due to conduction, that glass window is not keeping all of the home’s heat on the inside of the home. The heat energy on the interior side of the window is traveling towards the cold, exterior side of the window, and therefore heat is being lost to the outside. 

Glass has a high conductivity of heat, also referred to as a high ‘thermal conductivity.’ Wood, in comparison, has less thermal conductivity (heat energy does not travel through wood as fast as it does glass). When measuring the thermal conductivity of a material (which you will see in the equation for R-value) the conductivity is translated as a k value. A k value is the number of BTU’s (British thermal units) transmitted in one hour through one square foot of the specific material, one inch thick, when there is one degree Fahrenheit difference in temperature across the sample (whew, lots of ones…).

When you look at the measured k values for wood and glass, you can see why having a house made entirely of windows is just not a great idea for heat savings (pg. 22, The Superinsulated Home Book): wood (softwood) has a thermal conductivity of 0.8 k… while glass has a thermal conductivity of 7.08! The higher the k value (or thermal conductivity) the faster the heat loss through that given material. 

Thermal resistance

Thermal resistance is how quickly, or slowly, heat moves through a solid. Thermal resistance is the opposite of thermal conductivity. The higher the thermal conductivity of a material, the lower its thermal resistance. For example, windows have a high thermal conductivity (a high k value) but a  low thermal resistance. Which means that they allow heat to quickly move through them. Wood, in comparison, (which has a low k value, or, low thermal conductivity) has a high thermal resistance. Wood does not allow heat to move through it as quickly as glass, or windows, do. 

R-value

So, given this information, how would we find the R-value for a given material? The R-value for a 1-inch thickness of a material is the reciprocal of the k value (the thermal conductivity of the material). Because we are finding the reciprocal we are essentially finding the thermal resistance of the given material. And the higher the thermal resistance, the higher the R-value. It comes down to this: the lower the thermal conductivity, then the higher the thermal resistance that material will have, which means it has a higher R-value. The higher the thermal conductivity, then the lower the thermal resistance of the material, which results in a lower R-value. 

Let’s say the material is a 1 inch thick piece of soft wood (such as pine). That one inch thick piece of soft wood has a k value of 0.8. The R-value reciprocal would be 1 / 0.8, which is an R-value of 1.25 per inch (R-1.25). Now, what if you had thicker wood? If the wood is three inches thick, you would take 3 multiplied by R-1.25, which equals to an R-value of 3.75. You just take however thick the wood (or other material) is, and multiply it by the R-value for its 1 inch thickness.

In conclusion…

Because we are building our own passive house, and a passive house is a highly efficient retaining box of either warm or cool air, the R-values of the used materials is of great importance. You will most likely see R-value return in many a blog. However, the R-value of a material alone does not make a passive home. It is how the insulative material is used that also composes an efficient, living dwelling. We look forward to continuing this educational, crazy, slightly extravagant, adventure! And we look forward to sharing it with you! Stay tuned!

Sincerely,

Shelby Aldrich

Dutt, Gautam and J. D. Ned Nisson. The Superinsulated Home Book. New York, John Wiley & Sons, Inc., 1985.

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© 2020 Sustaining Tree