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Community and Q&A

Keep my old exterior walls dry

McLean28 | Posted in Energy Efficiency and Durability on

Hi Martin I live in New Brunswick, Canada in an approximately 100 year old house. As I am renovating room by room Id like to know how to properly insure my exterior walls stay dry. Ive poured through your blogs and forums and would like to know if my idea is solid.

The exterior wall is cedar shingles on top of rough sawn 2×10(actual) planks nailed to the 2×4(actual) studs. Upon ripping out the interior plaster and drywall (lots of patchwork over the years I see), there are voids in area and fiberglass batts in other. I’ve found no signs of mold anywhere but lots of dirty insulation from air infiltration between the planks.

I’d like to fiberglass batt the cavities and 6mil vapor barrier and acoustic seal over the stud and outlets before drywall to elimnate draughts. We hit -30C here in the winter so the vapor barrier is necessary. Exterior foam is not an option as I cannot afford to redo the entire exterior and am a big fan of the cedar shingles.

My biggest question is I’m considering using a very vapor permeable building wrap acoustically sealed inside the wall cavity before installing the fiberglass batts to prevent the air infiltration into my wall cavity. Is this a good idea or will it cause more hazards? I want my wall to be able to dry out to the exterior as much as possible but read that any airflow in the wall cavity is bad for my R values and let me tell you it’s cold here in the winter.

Also Im painting the exterior and am going for the longest lasting paint job I can pull off which has me worries about moisture from inside pushing the paint off my shingles. Am I on the right track here? Tyvek individual placed in each cavity then batts then vapor barrier and drywall? Any help is greatly appreciated big fan of this site and it’s wonderful info for us DIYers.

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "I'm considering using a very vapor permeable building wrap acoustically sealed inside the wall cavity before installing the fiberglass batts to prevent the air infiltration into my wall cavity."

    A. I don't see why that won't work. But if you're going to that much trouble, why not install rigid foam in that location instead of housewrap? (The old "cut and cobble" option.)

  2. McLean28 | | #2

    My understanding of the rigid foam is that it is a vapor barrier itself and I would be essentially creating a vapor barrier on both the hot and cold. Would this not in fact limit my walls drying potential or am I missing something?

    With how cold our winters get I'm very hesistant not to use my 6 mil poly which is also code in the area.

  3. GBA Editor
    Martin Holladay | | #3

    If you go with the cut-and-cobble (rigid foam) approach, you would make sure that the rigid foam is thick enough to keep the stud bay warm -- and you would omit the interior poly.

    If this approach makes you nervous, go with housewrap and fiberglass batts.

  4. McLean28 | | #4

    Ok thank you for your input greatly appreciated.

  5. Expert Member
    Dana Dorsett | | #5

    If you use unfaced EPS in a cut'n'cobble it will usually be a bit over 0.6 perms at the nominal 3.5" depth of a 2x4 stud bay. EPS would have the same or better thermal performance as the very best stone-wool batts in winter, if ever so slightly lower on the sun-exposed walls in summer. At 3.5" the EPS qualifies as a class-II vapor retarder (comparable to asphalt kraft paper), which should allow you to omit the poly while still providing at least some capacity for drying toward the interior. With XPS or foil-faced rigid foam there would be essentially no capacity to dry toward the interior- and all drying would be toward the exterior (which usually works OK under shingle siding.) The difference in thermal performance between EPS and higher-R foams is negligible when used as cavity fill, due to the severe thermal bridging of the framing, but the drying advantage is real enough to make it better material for the application, however slight that advantage is.

    Shingles are inherently somewhat back-ventilated- enough so that moisture drives from the interior won't affect paint retention. But wind-driven rain getting behind the shingles will wick readily into the unpainted back side, and you may still have a paint peeling problem. In new construction or siding replacements using factory-primed shingles or clapboards gets around that issue, but you'll always have some risk painting over 100-year old shingles. Without back-primed shingles the best you can do is to use a stain (preferred) or high-permeance latex, not a vapor-retardent alkyd or low-permeance latex paint, which will likely peel.

  6. McLean28 | | #6

    Thanks a lot of great info there. Since my studs are actual 2x4s using a 3.5 inch EPS should I choose to go that route will leave me with a 1/2 inch gap. Will this be an issue? Should I fill the gap with more foam or batts or do they make a 4 inch.

    Also in your opinion how much how would I be losing through thermal bridging? What are some realistic options to engage this without foam on the exterior? A layer of foam on the studs uner the drywall? Is this overkill? I'm unfortunately not on an unlimited budget.

  7. GBA Editor
    Andy Engel | | #7

    Chris, the obvious answer is to use 4 in. of EPS, not 3.5. Odds are you'd be buying it in 2 in. thick sheets anyway. I'll defer to those more expert than I about the interior foam, although I don't see a downside other than cost.

  8. GBA Editor
    Martin Holladay | | #8

    Softwood lumber has an R-value of about R-1.25 per inch. EPS has an R-value of about R-4 per inch.

    If the entire wall were covered with 3.5 inches of EPS, it would have an R-value of R-14.

    The 4-inch studs have an R-value of R-5.

    If the wall has a framing factor of 25%, then the whole-wall R-value will be about R-11.75 (instead of R-14).

  9. GBA Editor
    Martin Holladay | | #9

    Leaving a 1/2-inch air space will not cause any moisture problems. Whatever you do, pay attention to airtightness.

  10. Expert Member
    Dana Dorsett | | #10

    Putting perhaps WAY too fine a point on it, the R/inch of softwoods varys quite a bit by species and actual density. At 12" moisture content most Douglas Fir is R1.0/inch, whereas some species of cedar can run R1.5/inch.

    Reference- see the table 4-7 on p.93 (pdf pagination) of this document:

    R1.2/inch is a pretty good ballpark WAG for pines, spruce, true-firs & hemlocks though. I've habitually rounded down to R1/inch, having been born & bred in doug-fir country, (why build with anything else? :-) ) but that's too pessimistic for my current New England location, since local/regional species are lower density, higher-R.

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