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Insulate stud cavities with the possibility to install exterior foam in the future

dgardner88 | Posted in Energy Efficiency and Durability on

Hello!

I am planning the remodel a 1920’s craftsman and wanted to get feedback on how to install the insulate in stages.

The building is a 1.5 story craftsman with a basement. The basement has a 2′ cripple wall. The wall framing is 2×4’s. Exterior to interior the layers are hardiboard, 1×12″ lapped plank sheathing, 2x4s 16″ OC with empty cavities, and then lath and 1/4″ plaster walls. The roof is an A frame with 2×8 rafters.

I live in climate zone 4C.

I plan to remodel the interior of the building first since the roof and siding are in good shape and would like to install exterior rigid foam 5-15 years down the line when the siding and roof need to be replaced.

The plan for the roof is to install rafter vents and fill the rafter bays with mineral wool then use a drywall air seal approach for the air barrier. The attic is a conditioned vented assembly currently and I would like to keep the living space.

My plan for the walls and cripple walls is to remove the lath and plaster, update the wiring and plumbing, install mineral wool insulation in the cavities, and then put up a double layer of drywall. The house is on a moderately busy street so the insulate and drywall is for both thermal and sound insulation. Again a drywall air seal approach for the air barrier. If building code requires a vapour barrier, then membrain sheeting behind the drywall.

Finally for the rim joists, installing rigid foam and filling the remainder of the volume with mineral wool.

With an old house I understand that insulating the walls with batts can lead to moisture problems on the sheathing or bottom plates due to condensation of internal humid air hitting the cold sheathing and the recommendation is to either use spray foam, or cut and cobble rigid foam. https://www.finehomebuilding.com/2016/05/17/insulating-walls-no-sheathing also recommends leaving an air gap. However, if I install foam (spray or rigid) in the cavities then when I go to put rigid foam on the exterior there will be a foam sandwich which could trap moisture between the walls.

The question is, what is the recommended approach to installing batts for an old home like this when rigid foam may be installed on the exterior at a later date. Should I leave an air gap for the walls to help with drying or use rafter vents in the walls to maintain the air gap? Would a vapour barrier be sufficient to prevent condensation on the sheathing?

Appreciate any advice or suggestions,
-David

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    David,
    First of all, forget the advice in the article about houses that have no sheathing. Your house has sheathing.

    The 1x12s are sheathing. The only strange part is that you call the 1x12s "lapped." My reaction is -- really? Lapped like clapboard is lapped? Or are you just talking about shiplap joints?

    Briefly, the plan for your walls is fine. You won't be introducing any moisture problems if you install mineral wool between the studs, with MemBrain on the interior side of the studs.

    It gets a little more complicated when it comes to talking about your roof. Your plan is fine, as far as it goes. But if you decide later to install exterior rigid foam, you'll need to expose the soffit area and the ridge area, so that you can carefully seal the soffit vents and ridge vent. That's fussy work, but it's necessary. (The reason you would need to seal these openings is you can't have outdoor air moving between two insulation layers.)

  2. dgardner88 | | #2

    Thank you for the quick reply Martin!

    You are correct, the 1x12s have shiplap joints they are not lapped.

    I take it from your reply that installing interior foam and then exterior rigid foam at a later date could lead to rot or mold? Otherwise I could avoid the issue with the roof by using spray foam or cut and cobble foam now. Similarly with the walls I could install internal foam now. If the foam sandwich is an issue, would I need to remove the foam from the rim joists before installing exterior foam?

    For the walls with just batts, could I forgo the MemBrain or would that be risky?

    Thank you again.
    -David

  3. GBA Editor
    Martin Holladay | | #3

    David,
    Q. "I take it from your reply that installing interior foam and then exterior rigid foam at a later date could lead to rot or mold?"

    A. I didn't say that. If you know that you will be installing exterior rigid foam in the future, and if for some reason (not sure why) you want to install spray foam between the studs, then you should specify open-cell spray foam, not closed-cell spray foam. That way the wall sheathing will be able to dry to the interior if it ever gets damp.

    For more information, see How to Design a Wall.

    Q. "I could avoid the issue with the roof by using spray foam or cut and cobble foam now."

    A. You could get away with using open-cell spray foam on the underside of the roof sheathing, as long as you install either interior MemBrain or vapor-retarder paint on the interior drywall. You can't use the cut-and-cobble method for unvented roof assemblies. For more information on this issue, see Cut-and-Cobble Insulation.

    Q. "With the walls I could install internal foam now."

    A. Yes, you could, as long as it is open-cell spray foam.

    Q. "If the foam sandwich is an issue, would I need to remove the foam from the rim joists before installing exterior foam?"

    A. No. There is no evidence that a so-called foam sandwich causes problems at rim joists.

    Q. "For the walls with just batts, could I forgo the MemBrain or would that be risky?"

    A. Walls in Marine Zone 4 (Zone 4C) are required by code to have an interior vapor retarder. If you don't install MemBrain, you would need to install vapor retarder paint on the drywall. For more information, see Vapor Retarders and Vapor Barriers.

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