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Insulation between two sheets of plywood?

finePNW | Posted in General Questions on

I have an existing garage that I am hoping to both earthquake retrofit and insulate. The current state of the building is non-structural plywood sheathing over tar paper mounted on 2×4 studs on slab-on-grade. The sheathing is also the siding … previous owner just painted the sheathing and called it good. The inside is unfinished with exposed studs.

I am in Seattle area— climate zone 4C

An engineer has drawn up plans that put shear walls on all 4 walls. It seems to me that the best path for retrofit is to nail (via schedule) structural/rated plywood to the inside of the studs without disturbing the outside plywood… or maybe that’s not a decision to be made without checking with the engineer?

The garage will be unconditioned, at least for now, and I plan to drywall the ceiling and blow in R30+ above that / in the attic  

Can I stick some batting in between those two layers of plywood? Anything weird there in terms of moisture/vapor/mold or considerations I should make in terms of long-term maintenance? 

Does anyone have any better ideas?

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  1. Tom_K | | #1

    You already have an engineer working on this, definitely ask them.

    1. finePNW | | #2

      Well yeah. That’s a reasonable response. I guess I wondered if there was a common approach here.

      1. jollygreenshortguy | | #3

        The engineer is unlikely to be willing to provide an opinion on an insulation issue. They will probably consider it outside their area of expertise.
        I'm not a building science wiz so I'll be interested to hear from more knowledgeable people. But it's my understanding that if the garage is unconditioned, even if you insulate the walls and roof it will come to equilibrium with the outdoors. And if the temperature and humidity level is the same inside as outside then there is little to no vapor drive and no condensation issues in the walls.
        That said, then why bother to insulate at all? Probably because you plan to heat it during the winter, at least eventually.
        So that's the condition we need to look at.
        Will you be using it for something like a wood shop?
        Will you be running a portable heater for maybe a couple of hours while you do a bit of work?
        In that case it should be fine. But again, why insulate?
        If you plan to heat it during winter but just enough to keep things from freezing, again no problem and the insulation makes sense. Keep it to 50-55 degrees and there's unlikely to be condensation issues.
        If you plan to keep it warmer then that, that's when you may start to see issues. And this is where I feel least confident in my answers.
        If the walls can "breathe" in both directions then any condensation should be able to evaporate out. A "smart" vapor retarder on the inside face would be helpful to generally reduce the total amount of vapor making its way into the walls. So consider doing that. Also avoid activities that generate a lot of water vapor.

        1. finePNW | | #6

          Right... why insulate at all? It's something I've thought about but not (until now) had the time to really research. The goal is -- as you infer-- to eventually condition the space, but we only want to spend the effort to insulate if it might have benefits in flattening temperature curves in the summer and -- maybe? -- reaching a substantially warmer-than-ambient equilibrium in the winter... now that I think about it, I'm not sure why we expect the latter, but flattening summer curves should be doable, no? It gets blazingly hot in the summer, which isn't great for our EV or the stuff we store in there.

          Another goal is moisture management. If we can more easily dehumidify the space, it'd be great. But that's more of a sealing thing and should be considered separately from insulation (though, I must admit, my brain often lumps them together).

  2. Expert Member


    There isn't problem with having plywood on both sides of an insulated wall. Plywood acts as a smart vapour-retarder and opens up when wet (from 0.5 perms to 20 when damp). From a moisture perspective, the sheathing acting as cladding is probably the biggest risk of water infiltration.

    How wide are the shear walls? If they are only at the corners then it might make sense to remove the existing plywood and locate them on the outside so you don't end up with something in the way if you decide to drywall the inside. If they cover the whole walls, then ask the engineer if they can be relocated to the interior. If you go that route, make sure everything you might want later - electrical etc. - is in place now.

    1. finePNW | | #5

      Very good points!

      Given the overall long-term annoyance or maintenance issues of double-sided plywood, we are going to try to nail (to schedule) the exterior sheathing, which, it turns out, is quite thick and may pass our shear inspection (we'll see). Good to know, though, that moisture/rot/mold is an unlikely issue if we do need to move forward with double-sided plywood.

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