Rigid foam vs. mineral wool in a exterior retrofit
I've been reading GBA and FHB articles on how best to retrofit exterior insulation on a wood-frame addition of my otherwise brick and block 1950s ranch. There seems to be some debate about whether or not an exterior layer of insulation should be vapor permeable. I'm debating between the two but leaning toward rigid foam as it seems a bit simpler to construct.
I am in zone 5 although I hope to install R10 or more of additional insulation. Framing is 2x4. There is currently no vapor barrier. The existing construction from inside to out is: Drywall, faced fiberglass batts, tar paper, sheathing, house wrap (I think), siding.
1) Since there is currently no vapor barrier, my initial thought is rigid foam would be the better approach since I would be both insulating and creating a vapor barrier (while also not creating a moisture trap). Is this a fair assumption?
2) If rock wool panels are preferred over rigid foam A) how (and where) do I then create a vapor barrier since none currently exists? (keeping in mind I do not want to touch the interior drywall) B) where in the heck would I buy these rock wool panels? and C) Are they cost-competitive with EPS or XPS?
3) A friend of mine suggested that the greenest (and easiest) approach would be to keep the entire wall assembly intact (eliminating a landfill-bound dumpster of siding, sheathing, & fiberglass batts). He suggested I simply apply rigid foam over the existing siding (which is sound although weather worn) and then side over that. The existing siding is ~inch thick vertical tongue & groove pine which I suspect would also make it easier to install the rigid foam and new siding. Keeping that siding would mean that my screws will still have a good grip even if I miss a stud. The siding is also essentially flat save for the small vertical indentations where the tongue meets groove. So this leads me to my final question - given the current construction of the walls (noted above), is there any merit to the idea of not touching any of the current wall assembly and simply applying a thick layer of rigid foam over the whole lot, adding vertical furring strips as a rain screen/siding nailer, and finally siding?
Lastly, the new siding will be Hardieboard cement siding.
Posted Jun 20, 2014 10:49 AM ET
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