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How to insulate? Exterior walls clad in asphalt siding.

artichoke522 | Posted in GBA Pro Help on

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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  1. GBA Editor
    Martin Holladay | | #1

    You're right: The old asphalt siding is a wrong-side vapor barrier, so this assembly is potentially problematic.

    The best way to proceed would be to strip all of the exterior layers down to the 1x6 board sheathing. Then you could install a decent water-resistive barrier (WRB) on the exterior, and reinstall the steel siding.

    If that seems like too much work, you could cross your fingers and take a risk by leaving the asphalt siding in place. The key (if you go this route) is to reduce moisture flow through the wall assembly in winter. This could be achieved by (a) Paying strict attention to airtightness when remodeling, (b) Installing an interior "smart" retarder like MemBrain, and (c) Keeping an eye on indoor relative humidity during the winter to make sure that it is low (below 30%).

    If this were my house, I would strip the exterior down to the sheathing.

  2. artichoke522 | | #2

    Greatly appreciate the input. I think I will have to bite the bullet and remove/reinstall the steel siding after removing the asphalt siding. In the short term, I would like to get away with 1 or 2 winters max by taking your suggestion to use Membrain and control the humidity. I assume that using the foil-backed EPS as an extra interior layer of insulation is definitely out!
    To augment the Roxul R14 batts in the 2x4 wall cavity what do you suggest? Can I use Roxul Comfortboard 80 (R8) as an add-on interior layer to bring the R-value to R-22 (I am in Zone 6)? Or is it better to add 2" to the 2x4 studs so they are 5 1/2" deep and use all batt insulation? This seems like a lot more work that just adding 2" of Comfortboard, assuming that is permitted.
    Any other ideas are welcome!

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