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Community and Q&A

Exterior Rockwool Comfortboard Insulation and Moisture

MikaelCharrois | Posted in General Questions on

Hi all,

I purchased a home a few months ago that had an interior renovation done last year. It has 2×6 wood framing and 4″ of closed cell Spray Foam in the wall cavities. Drywall for the interior finish and house wrap (Tyvek or similar) w/ vinyl siding on the exterior. I’m going to be replacing the siding this summer with Hardiepanel and could use some advice.

I’m looking to install some insulation on the exterior of the house wrap to add a thermal break. I’ve read articles about keeping a vapour-permeable layer on the outside when the inside insulation is closed cell Spray Foam, so I’m considering using Rockwool Comfortboard. I’d then screw some 1×4 strapping over the insulation (I believe it is a rainscreen) and have the Hardiepanel installed on the strapping. Would there be any issue with this set up from a standpoint of trapping moisture anywhere? Or is there a better insulation product I could use to add a thermal break.

Also the Hardiepanel rep I talked to last week recommended using Tyvek DrainWrap behind any insulation I’m adding over the sheathing. Does anyone see any issues with just stapling DrainWrap overtop of the existing housewrap or should I remove the existing first? Would having 2 layers trap moisture or is it alright?

I live in Alberta, Canada – zone 3 if that affects the installation at all.

Thanks so much for any help/advice on this!

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  1. Expert Member
    BILL WICHERS | | #1

    Rockwool Comfortboard will work great, and is the most vapor permeable rigid insulation product I'm aware of. The only real downside is the speed at which you'll be emptying your wallet buying the Comfortboard. EPS is the next most vapor permeable rigid insulation (much less permeable than Comfortboard though), and is FAR cheaper.

    Drainwrap gives some added insurance for the sheathing. With proper detailing, I don't really think it's necassary. I think if you put up a rain screen, which is easy to do and sounds like you're already probably doing it (just make sure you keep the vertical cavities open and avoid horizontal strapping/furring).

    Housewrap is intended to be vapor permeable, so it won't trap moisture. No need to worry about two layers of housewrap being a moisture trap.


    1. MikaelCharrois | | #2

      Thank you so much Bill for the helpful advice!! Much appreciated.

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