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I'm currently doing a full house renovation, taking off all the internal drywall and moving a few walls to reshape the layout with a new kitchen and bathroom. I'd also like to update the insulation while the framing is exposed.

Plan

My intention is to fit rigid foam board between the studs and seal the foam board to the studs (unsure exactly how yet, maybe insulation tape) to create an airtight seal, followed by a layer of plywood fixed to the studs to increase structural strength/ shear strength of the house (as we live in Japan - an earthquake zone), followed by another layer of foam board, followed by drywall.

Question

My concern is about condensation forming inside these layers (or anywhere, really). The climate is hot (often 30°C+/ 86°F+) and humid in the summer, while very cold (0°C ~ -10°C/ 32°F ~ 14°F) and snowy in the winter. Winter seems to last about 4 to 5 months, so I'm really want to get in as much insulation as possible.

Can anyone weigh in on the safety of this strategy??

Also, I intend to leave a small gap between drywall and foam board, but is this necessary?

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  • Will you have any ventilation?
    – Solar Mike
    Commented Apr 2, 2020 at 8:05
  • Note that adding plywood will increase the shear resistance of the wall but will only do so much as at some point rather quickly the failure point will now occur at another deficiency. To get further gains (read: use the full strength of the now upgraded wall) it will then need be necessary to add hold downs, probably more bolts into the footing, and increasing of nailing of the roof and/or floor diaphragm to the shear wall. And other items. Consider that it is the combination of many things that provides lateral support and shear walls are just one element
    – Ack
    Commented Apr 2, 2020 at 18:44
  • Solar Mike Yes, plan to put in hrv or similar. @Ack Thanks, valid point. The weight is an issue I was thinking about.
    – samgled
    Commented Apr 4, 2020 at 4:00

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Where in the world do you live? Most earthquake zones are temperate climates in the US. It sounds like your sealing may create more of a problem as you will end up with a double barrier or even 3 if the house is wrapped trapping moisture. The Pacific Northwest in the US is one of the cooler earthquake zones but also the wettest and this sandwich would setup a great home for mold. The insulation in the walls with an additional layer of plywood would help seal. I think the extra layer of foam is asking for trouble long term.

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  • Hi Ed. We're in Japan.
    – samgled
    Commented Apr 3, 2020 at 5:45
  • The house is wrapped, with some kind of wrapping foam that's about 2-3mm thick. It's not taped so it is a water barrier but air can flow through between where each wrapping overlaps.
    – samgled
    Commented Apr 3, 2020 at 5:51
  • Your climate is similar to Oregon more quakes over there so the plywood may be a good idea but I would be cautious about layering the insulation and wood. I remember wood being very expensive there would steel straps diagonally work? Some of the homes I built in the San Francisco Bay Area had combinations of wood sheer walls and steel strapping the straps would provide the sheer you want without thickness or creating the sandwich that could be a problem.
    – Ed Beal
    Commented Apr 3, 2020 at 15:26
  • Thanks Ed. I'll look into the cost of steel straps. There are already some diagonal 4cm x 10.5cm beams there but the framing is quite different here to what I'm used to - lots of 1.5cm x 4cm or 3cm x 4cm twig framing between the main structural posts that is there basically just for the drywall to be attached to. I was planning to get that out of there in order to put in the insulation then overcompensate with the ply. The issue is that twiggy stuff takes up a lot of space within the wall cavity.
    – samgled
    Commented Apr 4, 2020 at 3:59

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