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Spray foam for fiberboard sheathing

midwest | Posted in General Questions on

I apologize if this topic has been brought up in the past but I couldn’t find it while searching.

I am working on a house built in the 60’s in Minneapolis, climate zone 6.  The house has one layer of painted brick on the outside, about a 3/4”-1” air gap, fiberboard sheathing (no WRB) , and finally fiberglass insulation with a kraft facing on it. This house also has air conditioning.  Most of the exterior walls are staying as is but I had to gut three walls down to the studs.  I didn’t see any moisture concerns on these three walls.

My question is whether spray foam is a good idea for this situation.  When I got to this job, spray foam was already in the bid but no one knew about the fiberboard sheathing.  I feel like it will cause issues (with moisture not being able to dry to the inside and no WRB) but maybe with the good sized air gap, it’s not going to be a problem.

Also, along the same topic.  This job seems like a good candidate for AeroBarrier, especially with moving quite a few walls and disrupting the poly that was on the ceiling. On a house like this and air sealing all the cracks, does anyone see any issues? The best option for applying this  product would be after drywall.  I’m trying to think of any negatives for a product like this.  Please let me know what other information/details are needed or wanted about the spray foam or AeroBarrier.  Thanks in advance.


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  1. Peter Yost | | #1

    hi Casey -

    1. brick veneer: the 1-inch space between the brick cladding and the sheathing is a drainage gap. Even painted brick leaks like a sieve if it gets wet (primarily through the hairline cracks in the mortar at the brick-mortar lines, cracks paint can't span). Sometimes mortar droppings can block off this drainage gap as they accumulated at the bottom of the wall cavity during installation of the brick veneer. Inspect wherever you can to make sure that your drainage gap is clear all the way down the wall assembly.

    2. fiberboard sheathing - is this asphalt-impregnated? The asphalt coating does impart some moisture resistance.

    3. penetrations - This is the place to look for moisture problems, particularly if the sill of the exterior brick walls are made of brick (rowlock) and there is not integral flashing at the window sill. If your assembly has stayed dry over the years in this weakest of locations (particularly on the WINDWARD side of your building), that is a good sign as you consider better insulating the wall and making everything to the exterior of the insulation colder.

    4. AeroBarrier - you should check with AeroBarrier of course, but my understanding is that you can't use their system with fiberglass insulation; just too air-permeable for the aerosol to span and close off.

    5. Spray foam in MN CZ 6: this is a tough call: you are right on the border of concerns with brick spalling when water in the brick freezes. I would be looking for other projects with brick veneer, which have made the brick as cold as when spray foam is applied to its interior.


    1. midwest | | #2

      Hi Peter, thanks for the response.

      1. Yes, my mistake. It is about a 1” drainage gap but like you said, it does have a bunch of mortar droppings. I have opened up a couple small areas but it’s hard to tell if there’s a clear path all the way down. This is a 2 story condo, and the homeowner lives on the 2nd floor. It would be difficult to verify if there’s a clear path.

      2. It’s usually called buffalo board around here and I don’t believe it is asphalt impregnated.

      3. There is a limestone cap at each window that’s butted up against the underside of the sill and I haven’t seen any integral flashing. There has only been one exterior wall on the windward side and I haven’t seen any signs of damage.

      4. My plan for using AeroBarrier would be after new drywall is installed. All the fiberglass will be covered by the existing rocklath and the AeroBarrier will seal at the bottom plate and all small cracks.

      5. We use quite a bit of spray foam around here but with the uncertainty of this situation, I’m leaning towards insulating the 3 exterior walls with rockwool and adding a smart vapor retarder like Membrain. And then the AeroBarrier after drywall will create a good air barrier. I think this would be a safer option but definitely open to any ideas you have other than spray foam. Thanks.


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