How to insulate? Exterior walls clad in asphalt siding.
I have a shed attached to my old 1910 house which I would like to insulate for use as a heated workshop/rec room. My problem is that the exterior cladding is vertical-ribbed sheet steel that has been attached directly over 1930’s-era asphalt/granular sheet siding which I believe they called “Insul-brick” because of its faux-brick pattern. The wall structure is 2×4 wood framing, 24″ o.c., open (unfinished) on the interior side and clad horizontally on the exterior with 1″x 6″ wood planks, with approx. 1/8″ air gaps between the planks. So, effectively, there are 3 external layers: the wood cladding, the asphalt siding on that, and then the sheet metal siding on top of that.
The steel siding is in very good condition and I would like to keep it if possible, so insulating from the inside only would be my preference. What concerns me most is the asphalt siding: i.e.- does it act as a non-permeable layer that will prevent the wood cladding / interior cavity from drying to the outside (as it would with modern housewrap)?
My plan–before I discovered this hidden asphalt siding layer–was to fill the wall cavity with R-14 Roxul batts and add an R-8 layer of foil-faced EPS board to the room-side of the studs and tape it to act as a vapour barrier. I would finish off with 1/2″ gyproc over the EPS, screwed into the studs. This would give me a nominal R22 assemby as I am in Climate Zone 6 (near Ottawa, ON).
Now, I am concerned about using ANY kind of vapour barrier on the warm side as I worry that if the asphalt siding layer does not “breathe” to the outside, I may be in for big mould issues/deterioration of the wood cladding.
I have exhausted searching for answers on the ‘Net so any input from this forum would be greatly appreciated!
Note: I had thought of substituting 1 1/2″ Roxul ComfortBoard 80 (rigid) for the foil-faced EPS as the interior layer, without a vapour barrier, but have never heard of this being done before.
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