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Community and Q&A

Open-cell foam in exterior wall cavities?

MichDoug | Posted in Green Building Techniques on


We want to insulate several rooms that have plaster interior and no insulation in the wall cavities (as verified in other portions of the house that were gutted). The exterior of the house is 8 inches of field stone over tar paper and sheathing boards. The house has sizeable roof overhangs and no apparent moisture issues. The insulation contractor is recommending injecting open cell foam in the wall cavities (drill and patch holes). Is there any issue with open cell foam retaining moisture from the exterior?

Thanks for the help

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  1. Dana1 | | #1

    If there is an air gap between the sheathing and the fieldstones open cell foam would be fine in US climate zone 5 or lower, but in colder climates you may need to reduce the vapor permeance of the interior side depending on what it's currently painted with. The vented air gap between stone & sheathing is critically important.

    The bigger problem with half-pound slow rise foams is the blow-out potential if they over-fill a cavity. It's usually safer to use blown fiber in that sort of stack up.

    If they are talking about non-expanding injection foam (rather than half-pound polyurethane) the blow out risk is low, but the shrinkage of the material over time makes it a poor performer long term.

  2. GBA Editor
    Martin Holladay | | #2

    I agree with Dana that the injection foam can work, as long as the insulation contractor is experienced enough to prevent the plaster from blowing out.

    The moisture risk to this wall relates to the flashing details and stone installation details, as Dana hints. Ideally, the mason who did the stone work not only left an air gap between the stone and the asphalt felt, but included good flashing details at windows and penetrations. It's almost impossible to verify these details once the house is built, unfortunately, so proceeding with your plan entails a small but real risk. (Insulation of any kind reduces the wall's drying ability.)

    If some of the walls were gutted, you know more about the conditions of these walls than we do. If everything so far looks sound and dry, that's a good sign.

  3. Expert Member
    ARMANDO COBO | | #3

    I would suggest for the applicator to use a Thermal Imaging camera to "see" where the empty cavities are. Not all framing cavities are perfectly at 16" or 24" on center. It would help better if this work is on a cold or hot day.

  4. MichDoug | | #4

    Thanks all! I believe they are using Gacoprofill foam. They do use a thermal imaging camera to make sure the properly fill the cavities. They seem to know what they are doing but wanted to double check their recommendations with the experts here.

  5. Expert Member
    Dana Dorsett | | #5

    Gacoprofill is a slow-rise half-pound pour (0.55 lbs per cubic foot per the spec). That's usually pretty expensive compared to blown fiber. With thermal imaging monitoring the risk of blow out is low, and they would be able to identify unseen voids from unexpected framing elements, etc.

    Photos of the gutted sections might be useful, particularly around window & door openings, to see how bulk water is being managed. With deep roof overhangs you can sometimes get away with poor or absent window flashing. Empty studwalls offer a lot of drying capacity that the foam-filled walls won't have, so it's important to get it right.

    Climate matters too- where is this house?

  6. MichDoug | | #6

    Southeast Michigan

  7. Expert Member
    Dana Dorsett | | #7

    Tuscola, Lapeer, & St. Clair counties are all US climate zone 5, in which case if the cavity between the sheathing and stone is vented to the outdoors you don't have to sweat the vapor retardency of the interior side of the wall.

    In Huron or Sanilac counties you'd be in zone 6, where it starts to matter a bit more.

    If the cavity isn't vented to the outdoors now, you can make weep holes through a mortar joint every couple of feet near the bottom of the wall, and corresponding vents near the top of the wall, which will be enough to purge moist air from the cavity via convection. Sometimes the top of those cavities is vented into a vented attic, which is fine, but you have to be sure to not block that ventilation path when insulating the attic. If it's blocked, adding vents to the exterior is prudent.

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