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Best practice for extending out a wall

giantsean | Posted in General Questions on


I have a 2nd floor gable end wall which is constructed of (from outside) brick, air gap, block, air gap, 3/4″ plaster/blueboard. There is no framing/insulation in the wall. We are converting an existing closet to an opening and to part of the above wall will be furred out with 2×4’s. I would like to insulate it while we are in there but as always don’t want to create issues.

I plan to use Roxul with Certainteed Membrain as a “smart” vapor barrier on the inside. My question is whether it would be necessary to demo the plaster against the block. These walls seem to dry well to the outside, but my thought is that leaving plaster there would create a double vapor barrier on the inside. Appreciate any advice!


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  1. Expert Member
    Dana Dorsett | | #1

    First, what climate zone/location?

    Is the 2x4 being applied as furring (flat side to the concrete masonry unit), or it it a non-structural studwall (edge on to the CMU)?

  2. giantsean | | #2

    Southern CT right on the zone 4 and 5 border. The finished wall will be non-structural but it may not actually contact the CMU, rather I am estimating it will be framed in front off the existing wall unit by a few inches (we are headering an opening and it depends on how many jack studs we'll install. I could always build it straight off the wall but then it's a no-brainer to demo the plaster as it's real cinder block and I'll need to find a good mortar joint to have a chance for any fastener to hold.


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