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Community and Q&A

Insulating a concrete block house

finefinish | Posted in GBA Pro Help on

I am looking at a renovation job that involves adding a second floor to an existing one-story house. The existing outside walls are 8″ concrete block with stucco.

The homeowner wants to keep these walls intact since the stucco is in great shape and the windows are new.

In order to comply with the stretch energy code we may need to insulate these walls. Is there a viable way to insulate these walls without disturbing the exterior and/or causing moisture issues? I am in climate zone 5a – New England.

Thanks for your thoughts and suggestions.

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  1. GBA Editor
    Martin Holladay | | #1

    The best way to insulate this house is on the exterior.

    It's possible to insulate the house on the interior, but there are several areas that are tricky. If the foundation is a slab on grade, you'll need to cut the slab on the interior side of the exterior walls and insert vertical rigid foam insulation, in order to interrupt thermal bridging through the slab. That work would be difficult, and you would have to consult an engineer to be sure that the plan would work.

    If the house has a basement with poured concrete walls, the job would be easier. You would need to insulate the rim joist areas and the interior of the basement walls.

    In either case, you need to pay attention to the continuity of the thermal barrier at the tricky area where the top of the walls meet the roof.

  2. Expert Member
    Dana Dorsett | | #2

    If there is no existing insulation and the CMU cores are hollow you can fill them with non-expanding injection foam (TryPolyMer, CoreFill-500, etc.), but it's not cheap, and the net-R value is still well below code for new construction.

    Injection foam is fairly water-vapor permeable, but won't wick water, and won't alter the moisture dynamics of the wall very much, in most instances. If there isn't access to tops of the CMU core openings, they can be filled from ~1" holes, drilled from either the interior or the exterior. That adds a bit to the cost due to the drilling & patching labor.

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