What is R-Value (And Why All R’s are not equal)?

by Erik North on May 27, 2011

What is R-value


When you shop for insulation, they are listed with different R-values.

What is R-value and how the heck does it affect how well the insulation works? Do you save more money with a higher R-value? How is R-value calculated?

(Answers: Measure of an insulation’s thermal resistance, yes, yes and very carefully by smart people in labcoats.)

What is R-value – A Boring Wikipedia-style Definition

R-value indicates a substance’s resistance to heat transfer. The higher the R-value, the more slowly that heat energy transfers across the material boundary. The R-value varies depending on the material and thickness.

When figuring the R-value for a wall assembly (or any layered building element), you can add the R-values of each layer together. For example, a standard partition wall in modern construction consists of drywall on 2″x6″ studs, 6″ fiberglass batts within the wall cavities, plywood or OSB sheathing, a Tyvek water control plane and your siding. The R-value for any cross-section of the wall can be found by adding the material R-value of each layer.

How is R-value Calculated?

Carefully, by scientists with lab coats. Seriously, R-value is tested under strict laboratory conditions to determine the exact rates of heat transfer.

A hot box is set up with a heating element on one side and thermometers closely gauging temperature on both sides. The ambient air temperature is 70 F with as little air movement as possible.

The test material is placed between the heat source and the unheated side of the hot box. Observers closely monitor how quickly (or hopefully, slowly) the temperature rises in the unheated box section. Lather, rinse and repeat until you’ve got a technically consistent set of data points from which to derive the material’s R-value.

You can see the shortcomings. For one thing, I lost half my audience talking about the numbers. Another is that you’re not concerned about insulation in June during a balmy 70 degree patch. It is during the pitch bottom January dead of winter. New Englanders (and most anyone concerned about insulation) are hardly dealing with ideal windless conditions. It is freezing and windy and miserable (hooray, New England) and different insulations perform differently in these less than ideal conditions.

Why is the Advertised R-value Wrong?

R-values are calculated with care and scientific rigor. There shouldn’t be any issue with taking them at face value, right? And it sure would be handy if there was one reliable yardstick for judging insulation. That is true but unfortunately the answer is no. R-values are calculated in a setting that bears as little resemblance to the real world as does Harry Potter or Middle Earth.

Insulation can be compromised in any number of ways: Thermal bridging, irregularity around windows, poor or faulty installation, sagging within wall cavities, aging of the insulation, the nature of the types of insulation, air looping in the wall cavity and wind washing just to rattle off a few ways. Some insulations (reflective barriers) don’t even work by thermal resistance. You could double your wall’s R-value and find the real world results are much poorer.

I’ll detail a couple of these to illustrate the point. Thermal bridging is when a more conductive (or poorly insulating) material allows an easier pathway for heat transfer across a thermal barrier. The most common form is probably within the eyesight of every reader of this article: wall cavity studs. Suppose your walls have 6″ fiberglass batts. I’m sorry. Every 16 inches on center in that wall is a 2″x6″ stud board. The fiberglass has an R-value of around 3.5 per inch and the stud is around .75 per inch.

Here’s a thermal image from my house where you can see the wall studs and corner hurricane brace through the plaster.

Thermal Bridging of wall studs through building cavity

D'oh! Thermal Briiiiiiidgiiiing!!!!

While the advertised R-value would be R-19 for the 6″ batt, the effective R-value is about 2 lower. Bummer.

Wind washing is another common condition that reduces effective R-value. Wind washing is air pressure driven movement through insulation, typically wind driven in a horizontal direction. Some insulations (I’m looking at you fiberglass) are severely compromised by air flowing through it. Fibrous insulations like fiberglass, rock wool and to a lesser extent cellulose (lesser extent because cellulose is made of paper, an excellent insulator where glass and rock are poor insulators) work by trapping pockets of warm air thereby slowing heat flow. The air movement strips out the warm air keeping the insulation from, you know, insulating.

One of my audit customers from this past year had exactly this problem. He lived approximately 3/4 of a mile from the ocean and his house faced directly east. Year-round there is a very significant sea breeze against the front face of the house. The homeowner installed foam blocks in the soffits, sealed against the channel vents in each bay. This forced the sea breeze from blowing through the insulation. You can see the before and after results here:

Before and After Audit

Using R-value as the one stop comparison between different insulating materials is tempting (and easy). But it creates the illusion of better performing house insulation.

So what’s the solution?

It’s the Material, Stupid

Yes, it’s WHAT the insulation is made of, not HOW it’s rated. As tempting as it may be to judge all insulation on one number, each type of insulation performs differently. Fiberglass allows air to flow through it; if applied in the attic you need to seal light fixtures, wire penetrations, etc. to ensure the fiberglass works as best it can. Closed cell spray foam insulation is an air and vapor barrier but needs a fire barrier like drywall.

Knowing the physical properties besides simple R-value will help you get the most out of your insulation. And knowing is half…nevermind.

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