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Insulating a 1920 home built with concrete blocks

DarnPotter | Posted in Energy Efficiency and Durability on

Hello. We are remodeling a 1920’s house. Gutting it completely. It currently has no insulation – unless you count 27 layers of wall paper! We are in NE Tennessee – Zone 4A. We plan on framing the exterior walls on the inside (2×4) and filling them with batt insulation. The concrete blocks are “rock face” popular in the area at the time and are about 10 inches wide and shaped much like present-day cinderblock, but the walls are thicker. The rock face block has been painted on the exterior. The mortar, for the most part is in decent shape and we will repoint the bad parts.

My question is: should we (can we?) put a moisture barrier (on the house interior) between the blocks and the batt insulation? I’m thought it is common practice to glue a layer of 1/2 inch foam board to cinderblock walls in basements and then frame in front of it. However, I have been reading articles about the many dangers of insulating inside old brick buildings (with insulation causing wet bricks to stay wet). Here’s one such article:

Does this same thinking apply to concrete block??? Will a moisture barrier and/or insulation cause concrete (and the mortar) to stay wet and result in mold and/or freeze/thaw damage? I have asked this question of a dozen insulating professionals it’s all hemming and hawing at best. Contradictory at worst. No-one in East Tennessee seems to have any idea. Any thoughts or references would be greatly appreciated.

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  1. Expert Member
    Dana Dorsett | | #1

    The CMU blocks themselves aren't damaged by staying damp, but wood in direct contact with damp CMU can be an issue. The details of how floor joists and rafters are mounted to the wall might matter.

    Fiber insulation should not be in direct contact with the CMU either, since it will wick moisture toward the finish wall. If you install pressure treated 1x furring onto the CMU, then a half-inch of rigid foil faced polyisocynaurate (seams taped with a appropriate foil tape) , the air gap provides some space for the CMU to dry into as well as a capillary break, the foil facer & air gap provide another ~ R1 of performance, the polyiso provides an other R3, and the performance of the assembly rises to just above IRC code minimum when the thermal mass of the CMU is factored in. (A 2x4/R13 or R15 wall tight to the CMU does not meet code min.)

    You may or may not need to install weep holes at the bottom and vent holes at the top of the CMU for the 3/4" air channel to provide enough drying to protect a rafter plate or joist ledger, but some amount of intentional venting of the gap with outdoor air isn't a bad idea.

  2. GBA Editor
    Martin Holladay | | #2

    I agree with Dana. Adding insulation on the interior side of your CMU (concrete masonry unit) wall will not cause freeze/thaw damage to the CMUs.

    Dana's advice on insulation technique is also good. What you want is a continuous layer of interior rigid foam, not fiberglass batts between studs.

  3. Expert Member
    Dana Dorsett | | #3

    You would still want batts between the studs in addition to at least some continuous foam layer, unless you're willing to install 2-3" thick continuous foam. For a mass wall (CMU meets the code definition), takes a continuous R13 to hit IRC code minimum, which you could arguably just barely get there with 2" of foil faced polyiso mounted on furring or 3" of foil faced Type-II EPS, but that complicates running electrical wiring, etc. (routing out spaces even for shallow electric boxes , plus channels for the wires could be a real pain.) Installing 1/2" polyiso (~R3) or 3/4" (~R5) along with a 2x4/R13 studwall can be easier to deal with.

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