Open-cell spray foam wall assembly in Zone 5A
I am currently in the design phase of a small home, 50 miles north of Philadelphia, Pa, Zone 5A. The home will be a single level, 1248sq feet, with the following wall assembly…
exterior siding, Hardie
Rain screen
2″ PolyISO
Zip Sheathing
2X6 Framing
Open Cell spray insulation
The roof is a fully vented attic.
I have read many articles discussing the potential issues with Open Cell spray insulation being used in a roof assembly in Zone 5, however, I am still uncertain if those same issues are an equal concern when used in a wall assembly in the same zone 5. Should I have the same concerns using Open Cell spray insulation in the exterior walls? Will it limit drying to the inside? or would it be considered a safer wall assembly using Rockwool insulation?
Thank you,
Ron
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Replies
It's not nearly the same issue in walls, and even less of an issue when there is exterior insulation.
Roofs experience much bigger daily temperature swings, due to both nighttime radiational cooling to below the ambient air temperature, and the intense solar gains during the day. The cooler temps overnight cause the roof deck to take up moisture from the interior (if can), and the intense heat of the mid-day sun causes it to release that moisture. That doesn't happen to nearly the same degree with walls, or when there is insulation exterior to the roof deck in roofs.
Hi Ron -
In cold climates in the winter warm moist air, which is less dense than warm dry air, rises to the top-floor ceiling or up into the attic. The open cell foam--without a vapor retarder--sees more moisture than the walls. And stack effect also moves warm moist air up the building.
These are other reasons why we end up with tougher moisture management in attics and roofs than walls.
Peter
Ron,
Q. "Will it [open-cell spray foam] limit drying to the inside?"
A. No. Open-cell spray foam is vapor-permeable.