# What’s the R-value of Cedar Shingle Siding?

### Image Credits:

1. Image #1: Journal of Light Construction; photo used by permission
2. Image #2: Dow's Eastern Whiate Cedar Shingles web site

1.
Mar 31, 2017 8:33 AM ET

Maybe...
by Dana Dorsett

...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.
Mar 31, 2017 8:37 AM ET

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...

3.
Mar 31, 2017 9:02 AM ET

Edited Mar 31, 2017 9:07 AM ET.

different air film values?
by Skip Harris

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 under 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.
Mar 31, 2017 9:46 AM ET

A few mistakes...
by Michael Maines

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.
Mar 31, 2017 10:59 AM ET

Response to Martin
by Dana Dorsett

"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.
Mar 31, 2017 12:10 PM ET

since when do we calculate the R-value of siding?
by Rachel Wagner

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.
Mar 31, 2017 12:11 PM ET

Oh wait, today is March 31
by Rachel Wagner

Is this an early April Fool's prank? Martin .... ;-)

8.
Mar 31, 2017 1:49 PM ET

Undermined by the paywall...
by Nick Welch

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

9.
Apr 1, 2017 11:50 AM ET

A breakthrough in insulation concepts
by Charlie Sullivan

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.
Apr 1, 2017 7:29 PM ET

Even though their BS is BS
by Dan Kolbert

Jeff makes beautiful shingles. We've used them several times. Ah well.