Understanding R-Value
R-value measures heat transfer by conduction, convection, and radiation
Posted on Mar 24 by Martin Holladay, GBA Advisor
The job requires special equipment. Measuring a material’s R-value according to the ASTM C518 test procedure requires a heat flow meter like this German appliance manufactured by Netzsch-Gerätebau. After a sample is placed between a hot plate and a cold plate inside the calibrated testing apparatus, the heat flow occurring at a defined temperature difference is measured with a heat flux sensor.
R-value measurements are subject to a fair amount of ridicule, especially by marketers of radiant barriers. As it turns out, however, the ridicule is mostly unwarranted.
R-value is a measure of a material’s resistance to heat transfer. Before 1945, resistance to heat flow was measured by referring to a material’s U-factor. The lower a material’s U-factor, the better the material is at resisting the flow of heat. Because many people assume that high numbers on a scale are “better” than low numbers, insulation manufacturers found it hard to market insulation by bragging about low U-factors. So the R-value — which is simply the inverse of U-factor — was proposed by Everett Shuman, the director of Penn State’s Building Research Institute. Since R = 1/U, the higher the R-value, the better the insulation.
Does It “Only Measure Conduction”?
Some manufacturers of radiant barriers falsely claim that R-value measures only conductive heat flow while ignoring the other two heat-flow mechanisms, convection and radiation. In fact, R-values include all three heat-transfer mechanisms.
The usual procedure for testing a material’s R-value is ASTM C518, Standard Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus. The test method requires a technician to measure the thermal resistance of a specimen placed between a cold plate and a hot plate.
When a fiberglass batt is tested, heat flows from the hot side of the batt to the cold side. Wherever individual fibers of insulation touch each other, heat is transferred from fiber to fiber by conduction. Where fibers are separated by air, the heat is transferred from a hot fiber to a cooler fiber by radiation and by conduction through the air. Finally, the effects of any convective loops within the insulation are also captured by the test procedure.
Since a material’s R-value is the measurement of its resistance to all three heat-flow mechanisms — conduction, radiation, and convection — it is a useful way to compare insulation products.
R-Value Matters, But So Does Air Leakage
Of course, an insulated wall is affected by many factors not addressed by R-value testing. Although the R-value test captures the effects, if any, of convective loops within the insulation, it obviously cannot be expected to measure the amount of air leakage through a wall assembly once the insulation is installed. The rate of air leakage is affected by the density of the insulation, the presence or absence of an air barrier in the wall assembly, wind speed, and the stack effect.
Because of these factors, a leaky wall assembly insulated with fiberglass batts will usually perform worse than a wall assembly insulated with spray foam having the same R-value as the batts. The performance differences are due to spray foam’s ability to reduce air leakage, not to any difference in R-value between the two materials. It doesn’t make any sense to blame the R-value test for differences in air leakage between the two wall assemblies.
To obtain the best performance from fiberglass insulation, the Energy Star Homes program requires most fiberglass-insulated framing cavities to be enclosed by air barriers on all six sides. While the recommendation is sensible, it’s hard to achieve in the field. If such a six-sided air barrier can be created, however, fiberglass insulation will meet the performance expectation promised on the product’s R-value label.
Some marketers of radiant barriers or spray foam insulation imply that R-value measurements are meaningless. On the contrary, R-value is a useful measurement. But just because you know a product’s R-value doesn’t mean you know everything necessary to predict heat flow through a wall or ceiling. R-value is just one factor among many to be considered when deciding which insulation to use. Builders must also understand many other topics, including air leakage and moisture movement. No one has yet invented a “magic number” that replaces the requirement for builders to study and understand building science principles.
Does Radiant Heat Pass Through Insulation Like Radio Waves?
Another scare tactic employed by some marketers of radiant barriers is the idea that conventional insulation materials — sometimes called “mass insulation” — allow radiant heat to pass right through them. Scam artists have been known to warn builders that “mass insulation is transparent to radiant heat.” The implication is that a layer of aluminum foil is necessary to prevent radiant heat from traveling like radio waves right through a deep layer of cellulose.
In fact, most mass insulation products do a good job of stopping radiant heat flow. Radiant heat easily travels through air (for example, from a wood stove to nearby skin) or a vacuum (for example, from the sun to the earth). But radiant energy can’t travel through a solid material.
If the sun is shining on a concrete patio, for example, the heat travels to the soil below by conduction, not radiation. Here’s what happens: the concrete is first warmed by the sun (by radiation), and then the warm concrete gives off some of its heat to the ground below (by conduction). There is no radiant heat transfer from the sun to the soil.
When radiant heat hits one side of a deep layer of insulation, only a tiny percentage of that heat is “shine-through” radiation that manages to miss all of the fibers in the insulation blanket and emerge unscathed on the other side.
The fact that heat flows through a layer of insulation doesn’t mean that the insulation isn’t working. By definition, insulation slows down heat flow; it doesn’t stop it. Heat will always flow from hot to cold. The more insulation, however, the slower the heat flow.
Image Credits:
- NETZSCH-Gerätebau GmbH
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Apr 6, 2009
3:51 PM EDT
3:51 PM EDT
R-value and radiative heat
by Brent Eubanks
You said
Some manufacturers of radiant barriers falsely claim that R-value measures only conductive heat flow while ignoring the other two heat-flow mechanisms, convection and radiation. In fact, R-values include all three heat-transfer mechanisms.
I wonder if (intentional marketing obfuscation aside) this issue arises from a subtle distinction between how the energy passes through the wall assembly (or test coupon) vs how the energy is absorbed into the assembly/coupon in the first place.
As you explain, the R-value test captures the effect that the insulation has on all three heat transfer modes. However, your description of the test -- with the coupon between two plates -- suggests that the heat is transmitted to the hot side of the coupon exclusively through conduction. Is this a correct understanding?
If so, one could argue that the test fails to capture the real-world performance of a wall, which is subject to heating predominately from convection (which is essentially conduction from the air film layer) and direct radiation from the sun and other radiant heat sources.
Apr 6, 2009
4:29 PM EDT
4:29 PM EDT
How about real-world performance?
by Martin Holladay, GBA Advisor
Brent,
You're quite right: the real-world performance of a wall assembly includes many factors not captured by an R-value test designed to calculate the R-value of a piece of insulation. However, it's impossible to begin understanding how a wall assembly will perform until we know, as a starting point, the R-value of each of the components in that wall assembly. That's why R-value testing is so important.
In addition to knowing the R-value of the insulation, we need to know many other factors to understand how the wall assembly will perform. These factors include: the R-value of other materials used in the wall assembly (for example, exterior sheathing); the emissivity of any surface in the wall assembly that faces an air space; the rate of air leakage; and the Delta-T. (This is just a partial list.) However, knowing the R-value of the insulation is an essential starting point.
Feb 15, 2010
11:36 PM EST
11:36 PM EST
R Value misuse
by wda
One aspect of ASTM C518 that wasn't addressed is that the test is typically conducted at a 75 degree temperature and 0 mph wind. When manufacturers describe a material as being a specific R value they have to include those test parameters as part of the information. All materials will vary in R value as the temperature varies. An R19 batt will not be R19 at 32 degrees nor at 97 degrees. When you also introduce varying wind speeds, fiberglass is known to be a poor performer. The R value number as typically used is a poor indicator of performance because it is not a constant value. Yet it is allowed to be stated and specified as if it was non-varying. From that perspective, R value is essentially useless and its use should not be allowed in building codes. Real world simulation of building materials should be the acceptable test method rather than laboratory measurements.
Feb 16, 2010
4:49 AM EST
4:49 AM EST
Response to WDA
by Martin Holladay, GBA Advisor
WDA,
R-value is useful, because it is a measure of the thermal performance of a material under certain known (defined) conditions. Knowing a product's R-value, a product specifier can compare that material to others.
Such comparisons would be impossible if we allowed each manufacturer to devise their own test, with some manufacturers testing at 0°F and others at 70°F.
That said, it's still important to keep your thinking cap on. Just because you know a material's R-value, doesn't mean you know everything. It's still important to know something about the material's resistance to air flow, for example.
But there is no reason to deride the usefulness of R-value just because it doesn't tell you everything.
Mar 17, 2010
3:26 PM EDT
3:26 PM EDT
could i test R-value in my science class?
by Anonymous
We talk about r-value in my environmental science class but I've always wanted to conduct a lab where we measure R-values for different materials. Could I set up an in-class test to look at these issues or do I need a fancy machine.
Mar 17, 2010
3:46 PM EDT
3:46 PM EDT
You need a special machine
by Martin Holladay, GBA Advisor
Anonymous,
You need a special machine (see the photo and caption).
A variety of simple hot-box displays can be used to demonstrate the effectiveness of insulation -- but beware, it's also possible for such displays to be misleading. (The classic example is the radiant-barrier display you see at trade fairs -- the one with the incandescent light bulb. It doesn't represent typical conditions affecting a wall or ceiling.)
If you're tempted to set up a display for your students, consider controlling for differences in air leakage and consider whether any radiant effects are an accurate representation of real-world conditions.
Aug 25, 2010
6:34 PM EDT
6:34 PM EDT
RADIANT BARRIER
by ED GILSON
jUST A COMMENT.. Oakridge National Labortories (dept of energy) Florida Solar Energy Center and Texas A&M University tests state that R-19 and radiant barrier are the equivelant of R-30 or better. We all should disregard some sellers of radiant barrier who make unnessary claims about R values or effectivenes because thier national sales are just a tiny sliver of the market and from what I have heard they go out of business as fast as they start.. Most installations of foil insulation and radiant barriers are new construction.
Aug 26, 2010
4:16 AM EDT
4:16 AM EDT
Response to Ed Gilson
by Martin Holladay, GBA Advisor
Ed Gilson,
No responsible researcher or scientist would make the claim that R-19 insulation and a radiant barrier are equivalent to R-30. A radiant barrier has no R-value.
It is possible to build a radiant barrier assembly (a radiant barrier facing an adjacent air space) in such a way that the radiant barrier boosts the R-value of the air space from about R-1 to R-3. However, it's the air space that has a measurable R-value, not the radiant barrier. And you are still a long way from R-30.
Aug 30, 2010
12:38 PM EDT
12:38 PM EDT
Re: to Martin Holladay
by Sylvain Dutil
There are now triple-pane windows that get R-8 to R-10 value rating, so those responsible researchers that won't test a combination of materials and give it value are out to lunch there.
A single-pane glass has no R-value so what does that say? Think on that, so-called researchers.
The first inch or two on the heated or cooling side is the most important as well, and should be the biggest part on how to make a high R-value wall or ceiling system.
Let me give you a hint: the smaller and closer together a material like a common made bubble wrap for example you can make those R-values go off the chart on the side you want to keep from escaping temperatures.
Aug 30, 2010
1:02 PM EDT
1:02 PM EDT
Response to Syvain Dutil
by Martin Holladay, GBA Advisor
Sylvain,
If you are talking about whole-window U-factor (NFRC rating), the best operable triple-glazed windows are rated at U-0.17. That's about R-6, not R-8 or R-10. European windows that claim R-8 or R-10 are using a rating system that is not comparable to the system used in North America. (Many are providing glazing-only numbers, not whole-window numbers.)
I'm not quite sure how you think that bubble wrap can make your R-value go "off the chart." I suppose if you want to install 20 layers of bubble wrap, eventually you'll get a respectable R-value. But there are much more affordable ways to get R-40 than to install 20 layers of bubble wrap.
Martin Holladay has worked as a plumbing wholesale counterperson, roofer, remodeler, and builder. He built his first passive solar house in northern Vermont in 1974, and has lived off the grid since 1975. In 1980, Holladay bought his first 
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