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Community and Q&A

Spray Foam + Vapor Barrier?

marcr | Posted in General Questions on

Hi, we recently purchased 4 floor townhouse style condo in New Hampshire (6A). It is poorly built and extremely leaky.

Our walls assembly consists of (from outside to inside):
* Rough exterior vertical pine board.
* Some sort of plywood sheathing.
* 2×4 wall cavity filled with fiberglass bats.
* Rough interior vertical pine board with lots of gaps and cracks. 

I don’t believe there is a weather barrier between the siding and the plywood sheathing, or anywhere else in the wall assembly. 

The interior wood boards have small gaps between each other, and frigid cold air just streams through like the window is open. It’s hard to overstate how drafty this room is on a cold day. 

Because this is a condo, messing with the exterior isn’t a feasible option. I’m thinking the best approach would be to remove the wood boards from the interior wall, install closed cell spray foam, and finally replace the wood wall. The spray foam would act as improved insulation and an air barrier. 

I’ve read on here that closed cell spray foam can shrink a bit, opening up tiny cracks. Given that the spray foam would be the ONLY air barrier in the wall assembly (no drywall to act as a second barrier), I’m concerned that spray foam alone is enough. In this case, would adding a vapor barrier (like Membrain) between the foam and the interior wood planks be prudent as a second layer of defense? Is there anything else we should be considering?

Thanks!!!

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    markr,

    I think you can do this much less intrusively, without the use of spray foam, by simply removing the interior wood boards and adding an interior air/vapor barrier. You can do this in stages, and may want to fur out the walls to add more batt insulation at the same time.

    1. marcr | | #3

      Malcolm, thanks for the reply! I'd be curious to better understand your line of thinking. I figure if we're going to rip off and re-install the entire wall, we might as well upgrade the insulation while we're at it? Are you questioning whether swapping the fiberglass batts for spray foam in a 2x4 wall would be cost effective?

      One additional detail I forgot to mention - this is roughly 300 sq feet of wall.

  2. walta100 | | #2

    Before you start be sure you understand your condo contract. Often you do not own the interior of the wall so they are not yours to insulate.

    Consider removing the interior surface insulate if allowed install painted drywall as an air and vapor barrier then reapply the wood if you like it.

    Look to get blower door directed air sealing it locate the leaks and plug them.

    Walta

    1. marcr | | #4

      Hey Walta, good point about the condo docs. I'll double check but I know other units have upgraded insulation, so I'm assuming it's compatible.

      > Consider removing the interior surface insulate if allowed install painted drywall as an air and vapor barrier then reapply the wood if you like it.

      If I'm understanding correctly, you're suggesting we use drywall as a second air barrier between the wall cavity and the interior wood wall instead of something like Membrane? Wouldn't this be much more expensive?

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