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Insulation advice for concrete block house

isomer1 | Posted in General Questions on
Some background information:
I live in climate zone 4A in central Virginia.  I live in a single-wythe concrete block house built in the 1950s.  The majority of the house exterior walls are uninsulated and have no siding/cladding.  The master bedroom in the house needed 4 old double hung windows replaced.  The concrete block interior walls in the bedroom are plastered and are not insulated.
I have recently completed replacing the four old bedroom windows with 0.25 U-factor casement windows.  These windows were flashed with Prosoco FastFlash and air-sealed using backer rod and Prosoco AirDam.
Current project:
I would like to finish out the interior of the master bedroom now that the windows are installed.  Some of the goals for finishing the interior are to add electrical outlets, increase comfort by adding insulation, and making the area aesthetically pleasing.
What would be the recommendation for a wall assembly for insulating on the interior with a reasonable margin of safety against condensation and mold?  It looks like it is pretty easy to get this wrong according to the articles and forums I have been reading on websites like,, and
Thank you,
// Brent

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  1. Expert Member
    Dana Dorsett | | #1

    If you were to insulate on the exterior it would only take 2" of Type II EPS (R8.4) to meet IRC 2018 code-minimum performance. This is taking advantage of the thermal mass of the CMU being inside the thermal boundary. If you keep the drain plane at the CMU layer (where the flashing is draining to) its advisable to use a textured drainable EPS of the type targeted toward foundation insulation, etc, eg DuraDrain (there are others.) If the drain plane is moved to the exterior of the foam, the flashing has to be extended outward too, and some amount of drain space needs to be provided behind any siding. Using 1x4 furring through -screwed to the CMU provides an adequate nailer for most siding types. If finished with an EIFS (faux stucco) applied directly to the EPS, no drainage gap is needed.

    To do it on the interior requires the thermal performance of R13 continuous insulation (or equivalent.) You can get there with the least loss of indoor space with 2" of foil faced polyiso secured with with 1x4 furring through-screwed to the CMU, hanging the wallboard on the furring. Even though the foam is only labeled R12, the foil facer next to a 3/4" air gap bumps it to a legitimate R13.

    If rather than foam + furring a non-structural insulated studwall is prefered, 1/2" of polyiso or 3/4" EPS (either foil or plastic faced) strapped to the CMU with a 2x4/R13 studwall gets you there.

    Facers on foam are usually true vapor barriers, and blocks the high moisture drives of the rain & dew wetted CMU from the moisture susceptible studwall &/or finish wall materials. In your climate R3 on the exterior of an R13-R15 studwall is more than sufficient to mitigate against wintertime moisture accumulation inside the studwall from interior moisture drives, and plain old interior latex paint on wallboard would be a sufficient interior side vapor retarder.

    My personal preference would be to insulate on the exterior, if possible. That puts all of the potentially moisture susceptible materials on the conditioned space side of the assembly where it stays warmer & drier in winter, and is dried by air conditioning in summer.

  2. isomer1 | | #2

    Thank you for your detailed response.

    I have experienced the thermal-mass effect in this house. But right now it is hurting instead of helping - radiating warm during the hot summer months and sucking away heat during the cold winter months.

    I have thought about insulating on the exterior. Your suggestions have given me even more to think about too. I was unaware of the possibility of keeping the drainage plane at the CMU outer wall by using EPS foam with drainage channels. The Insofast EXe 2.5 panel looks to be a good candidate with its rear drainage channels, tongue and groove assembly, integrated studs, and built-in rainscreen. Securing the panels would be fairly easy using the appropriate length Tapcon screws. I think I would also want to apply a rolled-on fluid (vapor-permeable) WRB to the block wall before attaching the foam panels.

    There are some things working for me and some things working against me with regards to insulating the exterior. All of the windows have been installed about 2.5 inches outset of the block wall including the casing thickness. This means that I wouldn’t have to extend the jambs too far to accommodate the increased thickness. The most difficult problem to address would be what to do about the gutters and downspouts. And a secondary problem is how to handle the tops of the gable walls.

    The house has no overhangs and has a standing seam metal roof that extends roughly 1.5 - 2 inches past the block wall face. I have read articles that show that verge trim flashing is a popular option in the UK for flashing above the exterior insulation on the gable ends. So appropriate z-flashing could be attached on the gable ends over the insulation and cladding. To insulate behind the gutters and downspouts would mean I would need to move the gutter and downspouts outboard of the insulation and siding. The primary question then is how to handle the flashing details from the roof drip edge so that rain water is channeled into the gutters. It is this detail where I get stuck and don’t really know the best way to proceed.

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