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Old house retrofit using Demilec foam in wall and polyiso board OK?

GBA Editor | Posted in Energy Efficiency and Durability on

I live in the Western Upper Peninsula of Michigan in a 100 year old house. I have taken the inside down to the studs. I have removed all exterior wood siding. Western exposure was taken completely down to the studs. Plywood sheathing installed, All other exposures kept 1 by sheathing boards, I have covered exterior with 1 inch polyiso board with fiber cover (warm wall).
All boards taped and caulked at joints. Insulation board then covered with Tyvek. Metal flashing above all openings. RainSlicker on top of Tyvek to provide a separation for ventilation of the rainscreen of wood siding. I want to put a 3/4 layer of foil polyiso on the inside beneath the sheetrock. I plan on using energy recovery ventilation. Is the interior poly iso an issue? I have tried to be meticulous in the sealing around windows and doors.
It is my understanding that I really do not need a vapor barrier as most moisture movement is due to air flow and that is unlikely in a foam filled cavity.

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  1. GBA Editor
    Martin Holladay | | #1

    It certainly doesn't sound as if you are going to be getting any air moving through your wall. But the interior foil-faced polyiso will make it impossible for the wall to dry to the inside; that's a risk. It's always possible that a flashing defect will allow wind-driven rain to enter your wall. If that happens, it will be unforgiving.

    If you really want to add another layer of polyiso to the wall, why not add it to the exterior, where it belongs? Then your wall will have some ability to dry to the interior.

  2. Riversong | | #2


    I agree with Martin that it would be a mistake to have two vapor-impermeable wall surfaces. But I disagree that the outside is "where it belongs".

    1" of polyiso may not be enough to keep the sheathing above the dewpoint, since that's dependent on the R-value ratio between outside and inside insulation, but it will prevent the sheathing from drying in the dominant vapor drive direction in a cold climate, which is inside-to-out. With a rainscreen, there will be little solar radiant vapor drive toward the inside, so it makes much more sense to apply a vapor impermeable layer (if it makes sense at all, which it almost never does) on the inside to prevent the exfiltration and diffusion which creates condensation potential.

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