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When houses are constructed of a lightweight frame involving plywood or OSB sheathing, why are these sheets always attached on the outer side (instead of the inner side) of the exterior wall studs?


Context:

Commonly houses are constructed of a wooden structural frame (of vertical "studs" and horizontal "plates") that needs to be braced to prevent the building from skewing. This structural function is commonly performed by attaching a plywood or OSB sheathing (as an alternative to adding diagonal metal straps) to the frame. Insulation is then placed in the cavities between the studs. The finished wall also includes plasterboard installed on the inside, and cladding (such as weatherboard siding) mounted on the outside.

However, to avoid condensation damage in colder climates it is desirable that the layers of the building envelope have vapour permeability increasing toward the outside (to help restrict the humid indoor air from percolating through to colder surfaces of the structure faster than the ventilation by fresh dry outdoor air can remove that moisture.) This would seem to favour having the less permeable plywood/OSB layer be on the interior side of the (typically fibrous) insulation layer. Furthermore, it would also seem preferable if the thermal mass of these dense sheets could be on the inside (rather than the outside) of the main insulation layer. I assume there's some good reason why this isn't common practice.

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  • Further reading with the king of building science: en.m.wikipedia.org/wiki/Joseph_Lstiburek Commented Sep 26, 2023 at 14:35
  • "Does plywood/OSB sheathing ever get attached to the interior face of the building's external frame?" Yes it does! Would you like to come visit my garage to see some? I chose OSB for the interior because it was easy to work with and because I then had a solid wood wall to which I could mount nearly anything nearly anywhere. I did not pick it because it would be better insulated. I also did not pick it for its beauty and/or ease of applying a nice interior side finish.
    – FreeMan
    Commented Sep 26, 2023 at 15:27
  • Keep in mind that while there are lots of places that get cold, there are very few places that are cold/dry enough on a year-round basis to not need AC, and having a vapor barrier on the inside of an air-conditioned structure is a recipe for utter disaster Commented Sep 27, 2023 at 2:15
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    @AloysiusDefenestrate is there somewhere specific that Lstiburek addresses this topic?
    – benjimin
    Commented Sep 28, 2023 at 4:39
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    You have 1/2" sheet rock on your walls and 5/8" on the ceiling or it doesn't meet fire code. Plywood replaced wood siding and you don't have to do "actual carpentry anymore" (bracing). And, you can't have there be nothing behind vinyl siding except some drywall that's 3.5 inches away, that's also the one thing between you and thieves... or wolves.
    – Mazura
    Commented Sep 28, 2023 at 6:42

1 Answer 1

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Well, I'll go for the really obvious answer - you have to have something there, for at least the following reasons:

  1. many finishes (like vinyl siding) aren't rated for spanning studs directly

  2. you need a weather resistant barrier outside your framing

I'm also not an engineer, but I suspect that you're overstating the benefit of having the sheathing inside. You've already got drywall (probably) sheathing the inside anyway. Location of vapor barrier inside the wall is a hotly debated topic though, especially in locations that experience both humid hot and cold weather.

More information on vapor barriers and retarders: https://www.energy.gov/energysaver/vapor-barriers-or-vapor-retarders

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    I think also that drywall is a flatter, easier to flatten/hide seams, to have a very nice flat paint surface.
    – crip659
    Commented Sep 26, 2023 at 14:03
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    Drywall inside is much easier to cut, too, for either assembly or piercing for electric boxes, pipes, etc. Commented Sep 26, 2023 at 14:04
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    Can you imagine running HVAC, plumbing, and electrical with 7/16 OSB on the inside?
    – RetiredATC
    Commented Sep 26, 2023 at 16:48
  • It might also benefit to look at the wall as a system, with each component adding its R-value for an overall insulation rating.
    – RetiredATC
    Commented Sep 26, 2023 at 17:00
  • @RetiredATC presumably services would be run from the opposite side. Wouldn’t reordering the component layers leave the overall R-value unchanged?
    – benjimin
    Commented Sep 28, 2023 at 10:22

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