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Interior insulation of block wall with foam board?

collarless | Posted in General Questions on

My cape style house near Philadelphia was built with hollow concrete blocks in the 50s. The exterior is stucco and the interior is plaster attached with wire mesh and 3/4 in strapping (no insulation). Two of my bedrooms on the northwest end of the house are very hard to heat despite the $9K conversion to a high efficiency heat pump/oil back up system. So I am wondering if I could add 2″ foam on the interior of the outside walls and then glue 1/2″ drywall over top.

My concerns are:
1. will there be a condensation problem between the plaster and the block? There is no roof overhand on the exposed end of the house but I recently primed the stucco well and used waterproof stucco paint over that. Prior to this there was water coming through the wall in places.
2. how would I finish around the new windows that have about a 4 inch sill?

suggestions welcome.

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  1. GBA Editor
    Martin Holladay | | #1

    If you had "water coming through the wall in places," that's a bad sign. Even though you "fixed" the problem with waterproof paint, you can expect the problem to return as the paint ages. A long-term fix might require flashing repairs, roof changes, or even a new water-resistive barrier (WRB) and new siding.

    It's hard to recommend an insulation strategy if your wall has water-entry problems. If you end up needing to address the water entry on the exterior side of the wall, that argues in favor of exterior insulation -- for example, EIFS. You might want to call up an EIFS contractor for advice.

  2. user-4524083 | | #2

    Phil - The other advantage of the exterior insulation is that it completely covers the outside walls. With the inside approach, the insulation is interrupted at any interior partition wall. You wouldn't have to remove/replace electrical outlets, but you would have to come up with a plan for the exterior of your windows/doors, and possibly roof overhangs if these are already minimized. All of these details are not new, and would be well known by the EIFS contractor.

  3. collarless | | #3

    Martin and Kevin. Your answers ring true on every count but the sad thing is that I am committed to a low cost approach. We are entering retirement and plan to live here about 10 more years. I installed low cost replacement windows from HD 4 years ago and spent about 4 days sealing, priming and painting the wall in question last fall. The stucco on all the other walls in the house has been holding. I think the water coming in was caused by a major stucco crack above a window which is now well sealed. I'll keep an eye out for further trouble. Other houses on my street have a similar "no eave" roof on the gable ends.
    So if I went with the interior approach using 2" insul panels is there any cause for concern about condensation behind it? Would it be ok to glue drywall on the surface rather than use strapping? Would it be wise to seal the joints at the ceiling and floor?
    If this is a bad idea I think I will simply live with the current situation and boost the heat with an electric unit. Thanks very much for your input.

  4. GBA Editor
    Martin Holladay | | #4

    As long as you understand the limits of installing interior insulation, you can go ahead. Gluing rigid foam to your interior walls, and gluing drywall to the rigid foam, is certainly possible.

    One problem with gluing drywall is that you have to maintain even pressure on the drywall while the adhesive sets. This can be accomplished with scraps of plywood or OSB to spread the weight, and with long 2-by pieces of lumber that span the width of your room, so that you can jam the lumber in place securely. (Needless to say, protect your finished walls with plywood if you try this method.)

    The disadvantage with interior insulation is all the thermal bridges. You'll have thermal bridges around the entire perimeter of the rigid foam you intend to install.

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