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My house is in Northern California and was built within the last 15 years. I'm pretty sure I still have the original siding, which is hardboard (a.k.a. engineered wood). I've read that house exterior typically has 3 layers -- sheathing, a water barrier like Tyvek house wrap, and the siding. But when I looked under the siding, there are four layers. Please see the attached photo, which was taken from the ground looking up. What's this pink layer (pointed by the blue arrow)?

You can see the concrete foundation and the sill (pressure treated wood) sitting on top of it. Going from inside to out, the very first layer looks like OSB sheathing, and then the unknown layer, and then a very thin layer (which I think is Tyvek house wrap), and then the hardboard siding.

I think this pink thing might be a rigid foam board for exterior insulation, but it's only about half an inch thick (just like the siding and the OSB). Aren't foam boards for exterior insulation usually at least 1" thick? Thank you.

photo from ground looking up behind the siding

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  • If you scratch it, is it chalky (like drywall)? Jun 2 at 19:48
  • Foam applied over sheathing can be as thin as 1/4" (R1), as in fan-fold panels.
    – isherwood
    Jun 2 at 20:19

2 Answers 2

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It's gypsum sheathing, and it's white (as is the Tyvek). Your camera's white balance is off. In the full-res photo you can clearly see the porous texture and paper facing. Here it is looking a little more natural.

enter image description here

I'm not familiar with fire protection practices used lately in fire-prone areas, but I'd guess it's intended to retard fires that occur on your siding.

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  • Thank you! That makes sense to me. Unfortunately, it looks like those don't have a high R value.
    – James
    Jun 2 at 21:48
  • No. It's not intended to be insulation for energy efficiency. I assume your home was built to modern standards, though, with at least R-19 in the walls.
    – isherwood
    Jun 3 at 12:36
  • Depending on how it was installed, there may be some significant air sealing benefits (i.e. if the edges are taped together and sealed at windows/doors).
    – LShaver
    Jun 3 at 17:41
  • @LShaver, that's the primary purpose of the housewrap. Taped joints would be redundant.
    – isherwood
    Jun 3 at 17:57
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EDIT: Most likely isherwood is correct that it's a gypsum layer due to fire codes in this case, but as rigid foam is a good possibility in other cases, I'll leave this answer in place.


That is most likely rigid foam exterior insulation, as shown in this schematic from the Oak Ridge National Lab Foundation Design Handbook:

Concrete Basement Wall with Exterior Insulation, Footing Detail.

Here's an explanation of the advantages of exterior rigid foam insulation from Green Builder (emphasis added):

BUILDING AMERICA (BA) has been advocating the use of rigid foam sheathing insulation for years as a means to improve the home’s thermal envelope, by increasing R-value while minimizing thermal bridging in wood-framed walls. Research by several teams has provided the critical scientific basis for acceptance of foam sheathing by the codes community, and resulted in best practices for installation to ensure thermal performance—as well as air barrier and drainage plane integrity.

Adding insulating sheathing to the exterior of the assembly is a simple method of increasing the overall thermal resistance of the wall assembly beyond what is possible with cavity insulations, thereby increasing the overall efficiency of the house. Adding one inch of insulating sheathing (R-5) will increase a 2’ x 6’ stud wall from an effective R-14.4 to an effective R-19.4. This represents an increase of 35 percent effective thermal resistance, with only 15 percent increase in the overall wall thickness.

Basically, without exterior insulation the studs act as a "thermal bridge" and reduce the overall insulation value. A solid layer of exterior foam breaks up this bridges, and also provides better air sealing, which improves overall insulation and thermal comfort.

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    Foam insulation would be a standard item, however, this is clearly drywall, probably moisture rated.
    – FreeMan
    Jun 3 at 12:10
  • @FreeMan yeah you're probably right, based on the image detail in isherwood's answer.
    – LShaver
    Jun 3 at 13:13

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