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Musings of an Energy Nerd

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

Cedar shingles are used for both siding and roofing. Shingles are tapered; the most common type of white cedar shingle is 3/8 inch thick at the butt (the thick end).
Image Credit: Image #1: Journal of Light Construction; photo used by permission

White cedar has an R-value 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…

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13 Comments

  1. Expert Member
    Dana Dorsett | | #1

    Maybe...
    ...if mounted horizontally with both top & bottom 1" air gaps between the shingle with top & bottom air barriers you could hit that level in a ASTM C518 test.

    That's legit because that's EXACTLY how people normally install them! (or maybe not...)

  2. GBA Editor
    Martin Holladay | | #2

    Response to Dana Dorsett
    Dana,
    It sounds like you are describing the mounting method used in tests performed by manufacturers of foil-faced bubble wrap duct insulation...

    -- Martin Holladay

  3. Skip Harris | | #3

    different air film values?
    Let's assume a "perfect" air gap has no wind washing & no thickness limitation

    1) My understanding is that the exterior air film has such a low value because it is subjected to wind washing. This is much less true of gaps within the assembly. Perhaps a "perfect" air gap would have a value of R1.2. (two undisturbed films)

    2) film thickness matters. Not sure of the exact math, but (judging by some numbers I've found comparing R-value of .5" gap and .25" gap (https://www.energydepot.com/RPUcom/library/BUILD001.asp) the <1mm film under each layer of shingles would probably give r0.5 per layer. (i realize i am on very thin ice here.)

    So, I can possibly come up with R1.5 total for the three gaps (probably less). Added to the 1" of cedar at R1.4, I get R3-ish for the assembly. Better than your 1.8, but far below their number.

    What are they thinking? Well, the R1.2 for the air gap seems pretty understandable: they are looking up the R-val for a protected thick air gap. If they actually are using split shakes (nice big air gaps) rather than sawn, they may be getting a far better R-value, perhaps as high as R0.8/gap or 2.4 for the assembly.

    Wood value? Looks to me as though the entire assembly will be about 1" or 1.25" of cedar or about R1.5. To be charitable, perhaps they are taking that to be the R-value of a shingle and then doubling it for the layers.

    Anyway, max I can come up with is R4, but R3 looks far more likely.

    Perhaps their R-value calcs are an honest error, but I must agree that they are definitely an error. Their refusal to correct it? Well, most of us dislike change, especially when it means losing something we have believed in and trusted.

  4. Expert Member
    Michael Maines | | #4

    A few mistakes...
    I've probably earned some bad karma for bringing this to Martin's attention, but I tried my best to reason with Jeff Dow, Jr. before doing so. A few of his mistakes: he is calling each shingle a full R-1.43; he is calling each of three air films R-1.2; and most egregiously, he is multiplying, not adding, the air films with the shingle R-values. Even using his own numbers, adding the R-values properly only achieves R-7 or so, not R-12, for the 5/8" shingles. He calls them shakes but they are sawn, not split. He also advises people to install shingles tight to the housewrap, without a rainscreen, so there are no large air gaps, just the micro-gaps between shingle layers.

    Jeff claims to have done testing with the local university's wood technology lab but would not share any information from the testing, and the person he worked with is no longer there. As Martin said, if he was misinformed or exaggerating a bit I wouldn't get worked up about it, but the occasional unsuspecting customer may actually believe his outrageous claims of R-12 siding.

  5. Expert Member
    Dana Dorsett | | #5

    Response to Martin
    "It sounds like you are describing the mounting method used in tests performed by manufacturers of foil-faced bubble wrap duct insulation..."

    Of course it's true that people installing bubble pack under slabs ALWAYS ensure there is a suffice air space between the bubble pack & slab, and the bubble pack & ground that it hits the marketed performance levels. ;-)

    It's pathetic that a cedar shingle company selling an otherwise decent product would wade into BS this deep.
    Shall we set up a GoFundMe financing page to buy them some manure forks?

  6. Rachel Wagner | | #6

    since when do we calculate the R-value of siding?
    If I'm not mistaken, the only time the R-value of an exterior cladding material comes into play is when that component is part of a thermal mass wall. The moment an exterior air film becomes part of the calculation, then anything outboard of that air film is not considered. That's the scientific basis as I understand it (greatly simplified of course). Common sense would indicate that professing the R-value of cedar shingles is irrelevant at best. Or, better put, "pathetic" as Dana notes.

  7. Rachel Wagner | | #7

    Oh wait, today is March 31
    Is this an early April Fool's prank? Martin .... ;-)

  8. Nick Welch | | #8

    Undermined by the paywall...
    If a gauntlet is thrown down behind a paywall, and hardly anyone can view it, does it make a sound?

  9. Charlie Sullivan | | #9

    A breakthrough in insulation concepts
    Quibbles about R/inch of cedar aside, the multiplication of R values is a breakthrough worthy of April 1st. In a 2x4 wall, you can fit 1" of polysio (on the inside where it will stay warm) for R-6, 1" of EPS for R-4, and 1.5" of mineral wool for another R-6. That makes an overall R-value of 6x4x6 = R-144! My brother is a mathematician and he verified that I did the multiplication correctly.

  10. Dan Kolbert | | #10

    Even though their BS is BS
    Jeff makes beautiful shingles. We've used them several times. Ah well.

  11. Hugh Weisman | | #11

    What some people do to sell a product...as Barnum said, there's a sucker born every minutes. That said, sawn cedar shingles are the defacto siding material on Martha's Vineyard. They're often installed fairly tight with coursing at about 4-1/2: so that might build up the thickness a bit to close to an inch...nevertheless, these guys don't do themselves any favors which such a demonstrably BS claim.

    Actually, I came across this old thread while search for anything to do with shingles and rainscreens.. I've been wondering if the cost and effort of a rain screen is worth it with wood shingles. Shingles (white cedar) are typically installed on the Vineyard directly over asphalt felt and that's over plywood sheathing (or boarding on old houses). Shingle lifetime is usually 40+ years...moisture and rot in the sheathing is unusual except where there might be some flashing issues, etc. The only rainscreen that seems worth considering would be something like Obdyke's slicker. Otherwise, nail base panel would have to be installed first outside the rain screen....I guess that's an option if exterior insulation is used, but if not, it just seems like killing a fly with an elephant....thoughts?

    1. GBA Editor
      Martin Holladay | | #12

      Hugh,
      I installed white cedar shingles on my own house in 1981, so the shingles are now 38 years old. There is no way that they will need replacement in two years. They look perfect.

      They were installed over a rainscreen -- 1x3 horizontal furring strips, installed 5 inches on center (since the shingles have a 5 inch exposure).

      Another way to create a rainscreen gap for shingles is to use a plastic mesh product like Slicker, as you point out.

      Whether or not a rainscreen gap makes sense depends on your budget. If you are getting 40 years out of your shingles without a rainscreen, you might be getting 80 years with a rainscreen.

  12. Hugh Weisman | | #13

    We're now at 40 years on my home ....with no need of replacement...that's why I said "40+"....we have been specifying slicker on all our projects now, but it got me wondering if it was necessary....a lot of contractors say it's doesn't do much except add cost.

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