Helpful? 0

Plywood faced walls in insulated basement?

I"m insulating my basemen with 1" of XPS glued to poured concrete walls and 2x4 wood framed walls within with R-15 Roxul in the stud bays. I've been facing the walls with mold resistant drywall and latex paint. The basement seems relatively dry with no liquid water leaks. However, based on humidity levels I'm assuming some water is making its way in through the concrete walls and floors.

In one basement area which will be used as a mechanical room I'd prefer not to use drywall and was considering plywood instead. I understand that plywood is less vapor permeable than drywall but that it is still considered "semi-permeable".

My questions:
1) Is plywood (probably 1/2" sanded SYP with interior glue) too impermeable for proper drying of the wall assembly?
2) Even if it is permeable enough am I asking for trouble by installing what could be a growth substrate for mold?

Thanks.

Asked by Andrew Alden
Posted Fri, 04/04/2014 - 16:35

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3 Answers

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1.
Helpful? 1

Andrew,
Your emphasis on vapor-permeability is misplaced. You don't actually want to encourage the flow of moisture from your damp concrete inward.

Fortunately, the 1-inch layer of XPS that you have installed will stop most of the problematic evaporation. I would have preferred your XPS to be thicker, but your assembly will probably work. For more information on this topic, see How to Insulate a Basement Wall.

Whether plywood is a good choice as a wall finish has nothing to do with whether the plywood is vapor-permeable. It has to do with how damp your basement is. If your basement is damp enough to cause mold to grow on your plywood, then you shouldn't be insulating your walls. Instead, you should be addressing your basement moisture problems. To learn more on this issue, see Fixing a Wet Basement.

Answered by Martin Holladay, GBA Advisor
Posted Sat, 04/05/2014 - 04:58

2.
Helpful? 0

Thanks Martin.
I read the material at your links. For the most part I already had. In your musing, "Joe Lstiburek Discusses Basement Insulation and Vapor Retarders". There it said that Joe's stance on whether basement walls should 'breathe' or not has changed. He, you, and others now recommend not designing for walls drying to the inside. Regardless of that change, the XPS I installed against my concrete walls is semipermeable meaning some moisture will get through. The last thing I want to do is trap that moisture within the wall assembly, hence my concerns about the permeability of the wall cladding.

I may have given you the impression that my basement is moist. It is not but I assume that moisture was entering the basement through the concrete walls and floor because concrete is water permeable and its not sealed. I have wood in my basement now but its not harboring mildew. Of course my bare basement walls were allowing any water to freely evaporate into the basement.

My concern now is that using plywood on the walls may lead to problems with mildew that I didn't already have. This concern is reinforced by my understanding that Joe L. and others have not changed their view that interior basement wall cladding should be vapor permeable and that drywall and other wall materials should not be covered with oil based paint, vinyl wall paper, etc.

Answered by Andrew Alden
Posted Sat, 04/05/2014 - 07:44
Edited Sat, 04/05/2014 - 08:16.

3.
Helpful? 0

Andrew,
I wouldn't worry, for two reasons: (1) Very little moisture, if any, is going to diffuse inward through the 1-inch layer of XPS; and (2) If your plywood ever gets damp, it will dry quickly (assuming, as you say, that your basement is dry).

Answered by Martin Holladay, GBA Advisor
Posted Mon, 04/07/2014 - 09:37

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