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Foam insulation question for 100-year-old brick home

wleland | Posted in Green Building Techniques on

The attached illustration depicts the current condition of my wall. The plaster and the lathe is currently removed from the wall.

What I’m thinking of doing is spraying 1/2 to 1 inch of closed-cell foam along the inside of the stud and on top of the sheathing, which will serve as a vapor barrier. The remaining space will be filled with fiberglass insulation, and topped by 5/8-inch of drywall and latex paint.

Another option would be to spray low-density insulation between the brick and wood sheathing by drilling holes in the sheathing.

I’m currently looking for the best way to handle insulating my wall in its current condition, and would appreciate any suggestions you may have.


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  1. Expert Member
    Dana Dorsett | | #1

    Where are you located? (Climate matters!)

    How deep are the roof overhangs?

    Is there anything on the exterior face of the plank sheathing, such at asphalted felt or rosin paper, or is it just bare-wood facing brick across a 3/4" gap?

    Low density foam in the gap is never a good idea, since it provides a capillary path (however poor a path) from the masonry to the wood, and prevents convective drying of that channel.

  2. wleland | | #2

    We are in zone 5A (Pittsburgh, PA) climate wise. The roof overhangs are about a foot and a half. Right now we are working on the first floor (the house has two floors and a basement).

    Yes, it is just bare-wood facing brick across a 3/4" gap. I had a feeling foam in the gap might lead to problems, thanks for the clarification on that.

    So do you think my first idea would be the most effective? Or is there a better or more advanced method that I could implement?

  3. GBA Editor
    Martin Holladay | | #3

    The approach that you are suggesting (1 inch of closed-cell spray foam on the exterior side of you stud cavity, with the rest of the stud cavity filled with fluffy insulation) is called the "flash and batt" approach. For more information on this techniques, see Why Flash and Batt Makes Sense.

    Although you refer to the closed-cell spray foam as a "vapor barrier," its main purposes in your case will be to act as an air barrier, as a useful layer of insulation, and (in some respects) as a type of water-resistive barrier (WRB).

    In your climate zone, the minimum thickness of closed-cell spray foam that will work for a flash-and-batt job is 1 inch -- so make sure that your spray foam installer doesn't skimp. If the surface is bumpy, you want a minimum of 1 inch, not an average of 1 inch.

    As Dana correctly pointed out, the air gap between the brick veneer and your wall sheathing serves several important purposes. It is a capillary break; a drainage cavity for rain that gets past the brick veneer; and a ventilation gap that helps help keep the sheathing dry. So don't be tempted to fill this cavity with insulation.

  4. Expert Member
    Dana Dorsett | | #4

    With 18" overhangs on a one-story building direct rain-wetting of the brick will be limited, which is a good thing. If it's 18" of overhang for a 2.5-3 story it could be an issue. it would be better if there were asphalted felt on the exterior side of the sheathing, which would limit the uptake of moisture into the wood when the cavity air is super-damp, but it is what it is.

    An inch of closed cell foam is MORE than good enough for dew point control at the foam/fiberglass boundary in a 3" nominal cavity but reduces the drying rate toward the interior to about 1 perm. That's fine if the brick doesn't get frequent rain wetting, not so fine if it does. A half-inch flash could be a bit shy for dew point control, and hard to control, for depth, but would allow better drying toward the interior for the sheathing.

    You can mitigate risk (even in a rain-wetting condition) by ensuring that the brick cavity is vented at both the top & bottom. Most brick veneers were originally built with weep-holes in the vertical mortar at the bottom course (or a few courses above-grade, if the brick extends below grade), and either fully open at the top (often venting into a ventilated attic). If there is not venting at both the top and bottom, it can be retrofitted to vent freely to/from the exterior. That's usually pretty easy by drilling a 3/8" hole in the vertical mortar every other brick at the top course, with similar hole spacing at the bottom course.

    For more robust venting that can be 1" holes cored in with a masonry hole saw every 18" or so, with 1" round screened vent-plugs to keep the bugs & rodents from setting up critter condos in there.

    It's unlikely that the 4x3" stud cavities are anything like a standard batt-width, and there may be severe fitting issues with standard batts. Compressing batts is fine (it raises the R/inch), but edge gaps or waves/wrinkles or other voids will undercut performance significantly. Blowing fiberglass or cellulose in netting would be easier to get a perfect fit, and while it's DIY-able with rental blowers, there is a learning curve to it, and reading up online would be advisable before embarking on that path.

  5. iLikeDirt | | #5

    Here's a question I've been wondering about: would it be acceptable to fill that gap in such a brick wall with an insulation that didn't impede that gap's drainage, ventilation, and capillary break functions, such as loose fill perlite?

    What about filling the stud cavities with mineral wool batts? They're easy to cut to size for a snug fit, and offer better R/inch than fiberglass, too. And the new layer of drywall or plaster is likely to be its own air barrier, right?

  6. Richard Beyer | | #6

    Mineral Wool would be my first choice. It gives the home tremendous fire protection @ 2000+/-F, unlike spray foam @450+/-F. Mineral Wool will allow the wall assembly to dry accordingly and will not harbor moisture causing premature sheathing & framing rott, unlike foam. Mineral Wool can be installed safely every time, unlike foam. Mineral Wool can be installed without extensive training, unlike foam. Mineral Wool will not off gas toxins, unlike foam. Insects do not like Mineral Wool, unlike foam. Mineral Wool does not need intumescent paints and flame retardants applied over or in the product, unlike foam.

    If your concern is an air barrier one method is to caulk or tape all sheathing seams and then insulate. Another method of creating an air barrier is by use of 475 Building Supplies product(s) as shown here eliminating the concern of an environmental catastrophe from poor spray foam installation methods.

    Air barriers...

    EPA consumer guidance and warnings about Spray Foam...

  7. Expert Member
    Dana Dorsett | | #7

    You can't fill that exterior void with ANY insulation without forming a capillary bridge. Perlite would be about the WORST form of insulation in there- it's used to redistribute moisture evenly in potting soils and has very high capillary draw!

    Caulking or taping dozens of ship lap plank seams would be a ridiculously tedious and unreliable method of air sealing. To air seal the sheathing without foam, use housewrap, uncut, carefully wrapping it over each stud, stapling it to the sheathing at the stud/sheathing corners. Go back over it and seal any staple tears or other small holes with caulk before insulating.

    It's unlikely that standard rock wool batts would fit into those cavities, but blown rock wool could work, as well as blown cellulose. Batts aren't designed for full-dimension 2x4 depth, let alone the likely odd cavity spacing of your 3x4 stud elements.

    With a vented cavity between the brick and planking you would still meet code in a zone 5 A climate without an interior side vapor retarder, since the cavity qualifies as the "Vented cladding over wood structural panels." exception written into the code:

    The vent space allows the wood planking to dry fairly well toward the exterior even during the winter months. The bigger risk in your stackup comes during rainy shoulder seasons, but you have pretty good overhangs to limit direct wetting of the brick.

    Blown cellulose would be more protective of the wood than rock wool since cellulose shares the moisture burden. But as long as the venting is properly attended to in the cavity, rock wool would be fine.

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