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Bathroom wall insulation question

mleach350 | Posted in General Questions on

My house is in northern Maryland, Zone 4, which has both cold winters and hot humid summers. My house is unusual for the area in that it’s built out of concrete block, faced with brick and I have plaster walls with no wall insulation currently. It’s built like a cold fortress.

I am gutting my master bath and would like to insulate the walls but fear creating a mold situation. I am reading a lot of conflicting reports on both the insulation type and the placement of the vapor barrier. A lot of this stuff gets over my head and has me spinning in circles waiting to start the project.

I am leaning towards no vapor barrier, dens armour waterproof drywall, latex paint, and unfaced fiberglass insulation. Is this a good or bad idea? I have no idea anymore. Thanks in advance.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Mark,
    Here is a link to an article that describes your options: Insulating Old Brick Buildings.

    Unfortunately, it's hard to insulate this type of building by doing the work one room at a time. You really have to insulate the entire building all at once -- otherwise, you will be left with massive thermal bridging where your insulated wall meets your uninsulated wall.

  2. Expert Member
    Dana Dorsett | | #2

    Are the cores of the concrete masonry unit (CMU) blocks empty, or filled?

    Is there an air gap between the exterior brick and the CMU?

    Fiberglass insulation in contact with CMU can wick moisture (in either direction), but could still be OK if there is a vented cavity between the brick & CMU as a capillary break. If the brick is tight to the CMU and the CMU cores are empty it might still be OK with fiber insulation on the interior, but it's riskier. (Deep roof overhangs might make it worth thinking about at any rate.)

    If the CMU cores are empty it's safe to fill them using non-expanding injection foam insulation (whether there's a cavity between the brick & CMU or not.) But it's expensive in terms of $/perfomance.

    To hit IRC 2012 code-min performance in a MD/ climate zone 4A location would take R13 continuous insulation (see the mass-wall column, move down to the row for zone 4: http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_11_sec002.htm ).

    Filling the CMU with injection foam (CoreFill 500, TryPolymer, etc) would only get you halfway there. The stuff is about R5/inch, but the webbing & mortar joints over 25-30% of the cross sectional area thermally bridges the foam, reducing it's performance to about R5-6 for the whole CMU layer. But you'd then be able to get to code-min performance with just 2" of EPS (or 1.5" of polyiso) on the interior side of the CMU, held in place with 3/4" 1x4 furring through-screwed to the masonry 16" o.c. You can then hang the DensArmor on the strapping. That's a higher performance and less moisture susceptible wall than a 2x4 studwall with fiberglass insulation, and it eats up less than 3" of interior space, 3/4" thinner than a 2x4 studwall.

    Alternatively, you could skip the expensive injection foam and go with 3-3.5" of EPS on the interior, which would eat up a bit more than a 2x4 studwall. If that's too thick, you could make that 2.5" of foil-faced polyiso, or 1" of EPS against the CMU plus layer of 1.5" polyiso. During the very coldest nights 1" of EPS on the cold side would outperform the outer 1" of polyiso, but on average the polyiso would still outperform EPS in your climate, inch-for-inch.

    Plan-C would be 1" of XPS trapped to the CMU with a non-structural 2x4 unfaced batt studwall, no interior vapor retarder. It would hit the code-min thermal performance requirements but would be more susceptible to interior moisture drives than any of the above proposals.

    Plan-D would be a non-structural 2x6 studwall w/ R21 unfaced fiberglass (R23 rock wool would be better, due to lower wicking), no vapor retarder. This would be OK if there's a vented cavity between the CMU & brick and you don't air-condition the house down to 65F in summer. You'd have to make sure the cavity is vented at both the top & bottom to convect & purge moisture (from either direction), but it's lower risk than plan-C due to the higher exterior drying capacity. This eats up a LOT of interior space.

    If insulating the whole house you can hit code-min with as little as R8 foam (or rigid rock wool) on the exterior, which would be 2" of EPS/rock wool, held in place with 1x4 strapping through-screwed to the brick, on which you could hang standard siding materials. You'd have to look carefully at the window & door flashing details- you may need to spray the brick with a liquid applied weather resistant barrier if you can't direct bulk moisture outside of the foam layer. This is a whole other discussion beyond insulating a bathroom wall, but may be worth considering.

  3. mleach350 | | #3

    Thanks for these very thorough answers. - Dana - in this section:

    "Alternatively, you could skip the expensive injection foam and go with 3-3.5" of EPS on the interior, which would eat up a bit more than a 2x4 studwall. If that's too thick, you could make that 2.5" of foil-faced polyiso, or 1" of EPS against the CMU plus layer of 1.5" polyiso. During the very coldest nights 1" of EPS on the cold side would outperform the outer 1" of polyiso, but on average the polyiso would still outperform EPS in your climate, inch-for-inch"

    Does this method use a vapor barrier or is the polyiso the vapor barrier? I have an extremely small bathroom so giving up a space to build non-structural walls isn't an option. This stuff is a good bit over my head but I'm trying to find a DIY method that won't cause problems down the road. Maybe no insulation is safest.

  4. GBA Editor
    Martin Holladay | | #4

    Mark,
    If you install rigid foam on the interior of your walls, you don't need to install a separate vapor barrier or vapor retarder. The rigid foam is an adequate vapor retarder.

  5. mleach350 | | #5

    Martin - I am only insulating a 6 foot section of wall and a 4 foot section. I realize that this isn't going to dramatically change the comfort level of my house but my hope is it will improve the comfort of a cold bathroom.

    Is a reasonable plan to fill in between the studs with rigid foam and air sealing using canned spray foam and caulk? I'm trying to balance being paralyzed on this project and overthinking such a small room with my desire to try to increase the comfort in any marginal way possible.

  6. GBA Editor
    Martin Holladay | | #6

    Mark,
    Q. "Is a reasonable plan to fill in between the studs with rigid foam and air sealing using canned spray foam and caulk?"

    A. Yes.

  7. Expert Member
    Dana Dorsett | | #7

    Begging to differ, NO it is NOT a reasonable plan- it's more work, more rigid foam and more can-foam for significantly lower performance!

    You don't need a vapor barrier on either side of the assembly unless you have a moisture-susceptible layer in-between that will dwell at temperatures below the indoor (or outdoor) air dew point for weeks on end. Masonry is moisture tolerant, wood is not. If you don't have to put the cold edge of the studs in contact with the masonry, you shouldn't.

    A cut-'n'-cobble between full-depth studs dramatically cuts the foam's performance, and is a lot more work than furring over continuous foam. If you want a more substantial studwall and not mere furring, install a 2x4 bottom plate set back 1/8" from the masonry and fill the gap with can-foam to air seal it but also isolate the edge of the 2x4 from masonry moisture. Then install 2" of CONTINUOUS foam up against the masonry, and install studs sideways on the remaining ~1.5" of stud plate. That gives you 1.5" of stud to hang things on, and a 1.5" deep cavity into which you can install split unfaced R13s (into two 1.75-2" thick batts), trimmed to (compression) fit in the spaces between those sideways studs.

    If you used 2" EPS that stackup would have a center-cavity R of about R14 (not counting the masonry, wallboard or air films) but would outperform the cut-'n'cobbled foam approach by more than 25% using the same amount of foam, due to the much lower thermal bridging. The foam would average about R8.4, the somewhat denser compressed fiberglass about R6. But the framing fraction of the 2x4s would end up at at a fairly high ~30% with the edges turned sideways, cutting the combined R6 + stud layer performance to about R3.5, for at grand total of R11.5-R12. If you assumed a combined R2 for the CMU + rick layers, and R0.5 for the wallboard that comes in at about R14-R15 whole-wall, which is about the same as 2x6 / R20 studwall performance.

    Some would add in another combined R0.85 of inside + outside air films, and if the wall is indeed a cavity wall with an air gap between the CMU and brick veneer, the cavity adds at least another R1.

    If you cut'n'cobbled 3.5" of R4.2/inch Type -II EPS into the 2x4 cavity the framing fraction (assuming single top & bottom plates, and 16" o.c. spacing) will come in at about 20%. The thermal bridging of the framing reduces the ~R14.7 center cavity foam down to an average performance of about R9.8. Add the R2.5 for masonry + wallboard and you're at R12.3 (before air films.)

    What's more reasonable, more work, higher cost, for an R12-ish wall that underperforms code-min...

    ...or...

    ... less work, lower cost for an R15-ish wall that meets or beats code min???

    As an aesthetic issue, if the turned studs aren't quite flat enough for your liking, it's fine to through-screw a bowed- out stud to the masonry to pull it in. If you have access to a local supplier, finger-jointed 2x3s would be straigher/flatter than most milled 2x4s, and would reduce the framing fraction for slightly higher thermal performance.

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