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My house sheathing is not a plywood - but some dark brown material.

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Does anyone know what that is?

P.S. here's more higher quality images:
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    It maybe nothing and I am not trying to alarm you, but in your first picture it looks like you may have mold on the wall and on the ceiling joist. I would look at that.
    – WarLoki
    Commented Sep 21, 2015 at 23:39

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It was made by Celotex as well as some other companies. It comes under the nickname Buffalo board and Beaver board depending on where you're at. I don't know that it's manufactured anymore, however I viewed the link from the previous answer and it appears it could be the same thing.

If your house is older then it is probably the original buffalo board material. If it is newer it could very well be the exact same thing that is sold at Home Depot mentioned in the link that was provided in the first answer.

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  • any idea on how does it compare to plywood? Why was such fiberboard used?
    – Dannyboy
    Commented Sep 22, 2015 at 12:19
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    It might have a slightly higher R value than OSB but from what I understand they use this because it did not trap moisture between the sheathing and the house
    – user41750
    Commented Sep 22, 2015 at 12:34
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The generic name is "fiberboard" and it is sold under many trade names like Celotex, Buffaloboard, etc. This stuff is not bad at all, it's just different from plywood and OSB. While plywood and OSB are structural and offer substantial racking resistance against wind load, fiberboard is weaker and does not increase the strength as much. On the plus side, it is basically impervious to moisture due to the chemical additives, so it will never rot or grow mold. It is also highly vapor-permeable, enhancing drying to the exterior should the wall get wet. Also, it is a reasonable insulator, around R-3 per inch. So half-inch fiberboard adds R-1.5 to your whole house. Not bad. Finally, I can tell you from personal experience that termites don't like it. They'll eat wood all around it but won't touch it.

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  • It's Celotex. We use it for sound absorbing material in walls.
    – Lee Sam
    Commented Jul 13, 2017 at 4:43
  • Celotex is a brand name for fiberboard, in the same manner as Hardiebacker is a brand name for Cementboard.
    – iLikeDirt
    Commented Jul 13, 2017 at 21:44
  • Our attic is semi-finished with this stuff. Probably in the 1940s. The wallpaper was peeling off of it where water had been leaking in along the chimney. Behind the wallpaper this stuff was covered in black spots (mold). So I wouldn't say it will never grow mold.
    – Thoracius Appotite
    Commented Oct 15, 2020 at 2:18
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This looks like what we call "asphalt board", a kind of impregnated wood fiber board. Example

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    Definately some sort of fiberboard. I used to run into this pretty commonly when I was putting up siding. It was pretty popular in the 60s and 70s as a cheap sheathing alternative until OSB became more popular.
    – Comintern
    Commented Sep 21, 2015 at 22:54
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Looks like Masonite® to me. The product was "slick" on one side ("finished") and "fuzzy" on the other. Commonly used as "pegboard".

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    It is so NOT Masonite. Masonite is very hard, this board is quite soft and fibrous. I also know it as beaver board. My own home has it for sheathing. I HATE it because it does not hold screws worth a damn.
    – Speedy Petey
    Commented Jul 13, 2017 at 0:02

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