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Opinions on this Wall Assembly with Spray Foam and Mineral Wool Rigid Board

nwfinch | Posted in Plans Review on

I have a 1960’s cottage, climate zone 6.  There are 2′ overhangs all around the cottage. Original assembly was T&G, 2 x 4 x 24 cavity with R8 fiberglass, 1″ fiberboard continuous, 1/4 painted plywood as siding with strapping covering seams. My plan is to remove the exterior plywood & fiberboard. The new assembly will have the cavity spray foamed (closed cell) from the exterior against the back of the tongue and groove, 1.25″ roxul Comfortboard 80 R-5 continuous, typar housewrap, then reinstall the 1/4 plywood against the comfortboard. I know this doesn’t meet the continuous insulation R-value to cavity R-value recommendations for condensation.  I chose the comfortboard for its perm rating of 30 and think the spray foam will stop most moisture anyway. I also realize that there is no true structural sheathing and the comfortboard is rated as non structural.  The old assembly had this existing issue and building hasn’t shifted at all in 60 years.  The closed cell will add some rigidity to the exterior. What am I overlooking?  Will moisture still be a concern in the wall assembly.  Any recommendations on taping seams and securing the comfortboard?  Manufacturer also recommends securing the comfortboard by sliding boards in between strapping which I don’t have the space form in my assembly.  Will the .5″ air gap between the closed cell and the back of the 2×4 where the comfortboard is attached create an issue? The goal is to leave windows and trim as is.  Thanks in advance.

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  1. Kyle R | | #1

    Have you priced out the spray foam and comfortboard? It might be cheaper to do a more conventional plan with rock wool batts and thicker rigid foam to remove the condensation concern and re-side with new siding. It doesn’t get much more expensive than closed cell spray foam and comfort board.

    How old is the plywood siding? In my experience it will be never go back up right and you will be fighting it the whole way to make it look half way decent.

    1. nwfinch | | #2

      Hey Kyle,

      The plywood actually came off pretty easily. I think only one board broke in the process which is impressive for being almost 60 years old. In a perfect world, I would go with that more conventional approach but my goal was to avoid redoing the trim on the windows. The old fiberboard was about 1" so the 1.25" comfort board replaces it nicely while still being permeable vs. a polyiso.

  2. Jon R | | #3

    Unfaced EPS will also work fine and be much less expensive.

  3. Kyle R | | #4

    Kooltherm K12 by kingspan is a phenolic foam insulation board that comes in 1.2” thickness with an r value of 9.5. It is expensive by might similar to rigid mineral wool.

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