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Community and Q&A

How to Protect Old Brick Before Applying Spray Foam Insulation?

smoke_teff | Posted in General Questions on

Hello everyone! I am planning to apply spray foam insulation to the inside of my exterior walls. (I live in an old brick rowhome in Philadelphia.)

There is a lot of mixed or contradictory info out there on the best way to protect old brick while insulating. Similarly, lots of mixed info on the effects of spray foam insulation applied to the interior. 

My main questions…
-Should I do anything to prep the old brick walls before applying the spray foam, e.g. parging?
-Do I need to retrofit weep holes (currently there are none) or do something else to allow ventilation? 
-Is spray foam on the inside totally the wrong way to go? Should I apply to the outside and add siding over it? (Would prefer to avoid this extra difficulty and cost)

Extra info…
When I removed the lathe + plaster (probably over 50 years old) there was a ton of dust, from plaster and brick. One or two soft bricks remain, one or two loose/missing bricks, some missing mortar. The rest of the bricks seem solid.
I think there may have been some previous moisture damage–the downspout is on the other side of the wall and needed replacing when we bought the house four years ago, also we stuccoed the bottom four feet of the exterior. No moisture issues since.
I’ve read very mixed things on the effects of spray foam on the interior side. The spray foam salespeople say no extra parging, mortaring, or weep holes are needed…
The original pitch was that a) the foam would add to the structure of the wall by filling all the gaps, and b) the spray foam would eliminate the need for weep holes–the claim was that the foam would fill all the cracks and keep any water/vapor from getting to the other side of the brick, so any moisture on the brick/mortar would, I guess, either remain on the outside or dry to the outside? Not sure how this works in Philly winters…
Others say weeps are still necessary…

But still others say weeps are only effective/necessary when you have an air gap of more than 1″
And still others say that the ideal solution would also involve exterior insulation and siding up to the level of the joists to keep the brick dry and warm, but we don’t quite have the budget for that right now, maybe down the road…

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  1. DC_Contrarian_ | | #1

    Brick and mortar are somewhat porous. The danger is if water gets trapped on the surface and freezes, the expanding water can crack the surface of the brick and the mortar. Insulating the interior increases the risk in two ways: first, the brick is colder so more freezing happens. Second, heat flowing through the brick tends to dry it out.

    What's hard is that it's hard to predict what a particular wall is going to do. Sometimes insulating a wall on the inside causes it to crumble completely, sometimes nothing happens, sometimes you get something in between.

  2. Expert Member
    Akos | | #2

    As DC points out, insulation brick from the inside is never black and white.

    Generally in climates that aren't too cold, zone 5 or less, interior spray foam tends to work. You do have to make sure your window and roof flashing details are good and don't go overboard on R value, 2" of ccSPF is good enough.

    For more reading you can check out:

    There are many more articles about insulating brick on that site.

  3. MartinHolladay | | #3

    This article should answer your questions: "Insulating Old Brick Buildings."

  4. smoke_teff | | #4

    Thanks to all who have responded. I believe I have read nearly to the end of the internet on this issue, and the conclusion seems to be that there is no way to proceed with certainty, as you've all indicated.

    This seemingly well-researched article seems to argue for a minimal ccSPF application as the best way forward for load-bearing brick walls, acknowledging that risks cannot be entirely eliminated:

    That article suggests ccSPF can still offer some degree of vapor diffusion through to the inside (the product I bought lists a perm rating of 3.2 for 2", i.e. semi-permeable). Hopefully that strategy will make this application (in our climate zone 4 house) a risk worth taking. Maybe I'm engaging in a bit of motivated reasoning (having already bought the foam), but at least I'm not totally out on a ledge.

    That still leaves unanswered the question about whether to retrofit weep holes where there are none. I've read accounts advocating for them, but have also read others suggesting that the retrofit is unnecessary and even ill-advised, given the challenge of properly executing the retrofit. I guess my current plan will be to not install them unless someone cares to make a convincing argument for it.

    1. Expert Member
      Akos | | #5

      A lot of older double brick houses get insulated here (Toronto) with spray foam and I've never seen any weep holes added in. The spray foam explosion is about a decade old here, so hard to say if it creates long term issues. The walls don't seem to be failing so far.

      The important part is to deal with bulk water, which sounds like you have already doing. If there any additional areas with effervescence, you need to find where the water was coming from and fix it.

      P.S. Just making sure, the foam you got is one of those two part spray foam kits not rigid insulation.

      1. modulerics | | #6

        What would be the concern w rigid insulation?

        1. Expert Member
          Akos | | #7

          The rigid is not directly bonded to the brick surface like spray foam. Because of this there is a small gap where mold can grow.

          I've tried this (double brick + foil faced polyiso), mix in a lack of overhangs and solar vapor drive and it turned into a moldy mess, had to rip it all out.

          If you don't want to go for SPF, I would go for fig 11 in BSD 114. Key there is the vapor open WRB on the inside of the brick and MW tight against it with no gaps.

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