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Community and Q&A

Thoughts on this Wall Assembly Retrofit

j_prescott | Posted in Energy Efficiency and Durability on


I’m planning renovations for a 110 year old house in southern VT (zone 6).  The house has never seen any insulation or material modifications to the wall and roof assemblies since it was originally constructed.  While that’s horribly inefficient from an energy perspective, it means that the house has avoided any major issues with rot and decay as everything dries despite neglect, no WRB, some leaks, no modern flashing, etc.

Existing wall (interior to exterior):
Cementous plaster on cement board
2×5 full-dimension studs – 16″ oc
1″ board sheathing
Clapboard (cedar)
Lots of lead paint

I plan to strip to studs and sheathing so we can produce a high performing wall with new windows, etc. and make sure there are vapor, air, & water control layers, proper flashing, etc.  Ideally we design so the house lasts at least another 100+ years.  The house will have AC in the summer when we’re done so vapor drive to the interior / buffering and drying potential both ways is a consideration.

Our post-renovation wall assembly that might look like this (from interior to exterior):
Intello or Membrain smart vapor retarder
Dense-pack cellulose cavity fill (5″)
Existing 1″ board sheathing
Henry Blueskin VP100 WRB
3″ rockwood comfort board 80
1/2″ furring / rain screen gap
Fiber-cement clapboards

Given challenges finding construction labor I’m assuming this assembly with potentially two effective AB layers (intello or membrain and the blueskin) will give me some margin of error for imperfect install. I like the idea of a more vapor open exterior with continuous insulation and rain screen to avoid wet walls.

My rough calcs (attached) show this assembly would have an a U of .043, not including windows.  Assuming I can get the vapor retarder and WBR reasonably well detailed and make sure junctions to the foundation and roof manage the AB I’d assume I can get a reasonably tight assembly from this without resorting to spray foam in the walls despite current challenges finding labor in my area.

My end goal is to have a pretty good, comfortable and resilient house, where we can avoid fossil fuels and offset our energy use with renewables.

I’d interested in hearing the community’s thoughts on this plan for the walls.

Is the smart vapor barrier and blueskin overkill or relatively cheap insurance?

Can I put blueskin on top of old board sheathing or would this assembly require adding something on top? (Anyone done this before?)

This is starting to look like a “residential perfect wall”.  Do I even need an interior vapor retarder in zone 6 for a wall like this with the CI and drying potential?  (Maybe with a bit more CI?) This could become more important if the cement board has asbestos as we may consider dense packing and encapsulation within the existing interior wall vs. disturbing it.  Maybe this is a good idea and cheaper anyway, assuming we can get a good exterior AB with blueskin, etc.?


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  1. brian_wiley | | #1

    That's a beautiful home. That porch seems built for summer.

    >Can I put blueskin on top of old board sheathing or would this assembly require adding something on top? (Anyone done this before?)

    That's what I'm planning on doing. I spoke to the Henry rep a few months ago, and he told me that as long as the board sheathing is reasonably clean (he suggested a rigid broom if need be) and dry, then VP100 should stick well. There is an engineer, Patrick Vandoorne, that did the same thing to his house (in cz6, I believe) that might be worth taking a look at.

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