What’s the R-value of Cedar Shingle Siding?
What’s the R-value of Cedar Shingle Siding?
First, measure the siding thickness, taking into account the fact that cedar shingles are usually lapped
White cedar has an R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of about R-1.4 per inch, so it isn't too hard to calculate the R-value of white cedar siding. The trickiest part of the calculation is determining the siding thickness.
If we're talking about cedar shingles, there are usually a maximum of three layers of shingles at any one point in the wall. The shingles are tapered, so the total thickness of the siding includes layers with different thicknesses. (The butt of the shingle may measure 3/8 inch; the top of the shingle may measure 1/16 inch; and the middle of the shingle may measure 3/16 inch).
Let's be generous, and assume that the three layers add up to a total thickness of 3/4 inch. We calculate the R-value of the cedar this way:
(0.75 inch * 1.4) = R-1.05
Next, we'll add a value (R-0.17) for the exterior air film:
R-1.05 + R-0.17 = R-1.22
What if the singles are thicker? Let's do the math for 5/8-inch-thick shingles. We'll be generous (again) and assume that the three layers add up to a total thickness of 1.25 inch. So the R-value of the cedar is:
(1.25 inch * 1.4) = R-1.75
Adding the R-value of the exterior air film, we end up with:
(R-1.75 + R-0.17) = R-1.82
No more than R-1.6 for standard shingles
So, if you're using 3/8-inch shingles, the installed siding will have an R-value of (at most) about R-1.3. If you are using thicker 5/8-inch shingles, the installed siding will have an R-value of (at most) about R-1.9.
If any readers think that the R-value of cedar shingles might be higher, it's worth consulting standard R-value tables for a reality check. Most such R-value tables ascribe an R-value of R-0.87 for wood shingle siding.
At least one cedar shingle manufacturer disagrees
If you were a shingle manufacturer, you might want to fudge these figures a little and exaggerate the R-value of the shingles. If the exaggeration were small, builders might find the claim to be irritating but forgivable. But one shingle manufacturer, Dow's Eastern White Cedar Shingles of Corinth, Maine, went overboard with their exaggerations.
Here are the R-value claims made on the company's web site: “A standard 3/8" × 16" shingle put on a building with a 5-inch exposure has an R-7 insulation factor. A standard shake that is 5/8" × 18" put on a building with a 6-inch exposure has an R-12 insulation factor.”
These R-values are clearly impossible. If we were talking about EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest. foam, R-7 would be a 2-inch layer of foam, while R-12 would be a 3-inch layer of foam. Cedar shingles are nowhere close to these values.
An honest mistake?
Such an exaggeration might be inadvertent — after all, Dow's is a small company, and its employees may be unaware of how R-values are calculated.
I first heard about the R-value claims on the Dow's web site from Michael Maines, a GBAGreenBuildingAdvisor.com blogger and certified Passive HouseA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. consultant. Maines told me that he contacted the company with a few facts, hoping that Dow's would correct the error. Maines reasoned that a small company promoting Maine products should have the opportunity to avoid embarrassment.
No such luck. Dow's response was provided by a company representative who dug in his heels and defended the R-value claims. In an email to Maines, the representative, Jeff Dow Jr., wrote, “In your calculations you didn't take into account the triple coverage and the mixing of grain patterns to get the correct info. 5/8-inch shakes have an R-value of 1.43, [so] 3 layers on a wall or roof ... = 3.432 R-value. White cedar gives you a extra 1.20 R-value every time you overlap... So 1.20 x 3 layers = 3.60 extra R value. ... 3.432 R-value for triple coverage of cedar shakes x 3.60 the extra added R-value of the bridge effect and different grain patterns is 3.432 x 3.60 = 12.3552 R-value ( that is at 6-inch exposure, lower the exposure add more R value).”
Michael Maines wrote back, “I can't find anything reliable that lists cedar at more than R-1.41/inch, which would be less than R-1 at 5/8-inch thickness. My own research for wood and cellulose R-value shows correlation strictly with density; dry white cedar at 22 lbs/ft³ falls just below R-1.4/inch. I assume the additional R-1.2 at each overlap is due to the air film there, but that's much higher than the R-0.17 that is normally used for an exterior air film value.”
In his next emailed response, Jeff Dow wrote, “I worked with the engineering department and they won awards for being energy efficient. I am not going to give you the information that we got. I do not need to explain the findings to you. My products have won energy awards in many states.”
Maines is to be credited for trying to give Dow an opportunity to reconsider the company's exaggerations. But the shingle manufacturer doesn't want to retract the claims.
So now it's GBA's turn to point out that cedar shingles have an R-value that might, conceivably, be a little less than R-2, but which in most cases will be closer to R-1.
I'm a big fan of small New England mills that sell white cedar shingles. But I'm also a dedicated foe of exaggerated R-value claims. So in this case, I'm torn.
Reluctantly, GBA has to call out Dow's Eastern White Cedar Shingles for the company's unsubstantiated R-value claims — claims which appear to be in clear violation of the federal R-value Rule.
Sadly, this cedar shingle mill from Maine must now be added to a shameful list which includes marketers of radiant barriers, foil-faced bubble wrap, Insultex housewrap, and so-called insulating paint.
Author's note: I'd like to thank Michael Maines for alerting me to the R-value claims published on the web site of Dow's Eastern White Cedar Shingles.
Martin Holladay’s previous blog: “Who Can Perform My Load Calculations?”
- Image #1: Journal of Light Construction; photo used by permission
- Image #2: Dow's Eastern Whiate Cedar Shingles web site
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