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Fiberboard as a rain screen?

user-641887 | Posted in GBA Pro Help on

I am re-siding my 1964 house, probably with fiber-cement.  Location is mid-Atlantic, zone 7 wet/humid.  The sheathing is asphalt impregnated fiberboard. Some of the interior wallboard has been removed, and I plan to remove the rest.   I wanted to create an air barrier and am concerned that housewrap alone will leave holes.  One idea is spraying 1″ closed cell foam on the inside of the sheathing to create the air barrier and fill the rest with fiberglass.  The exterior would be housewrap over the fiberboard, then the siding. It appears that the contractors here do not typically install rainscreens. My questions are a) is the closed cell foam a good idea/needed? b) how much benefit would I get from a rainscreen? c) does the fiberboard allow enough air movement to act as a rainscreen?

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  1. Expert Member


    You may find this discussion useful:

    a) There are a number of ways to create an effective air-barrier. Foam, a robust house-wrap, a variable-perm membrane, or air-tight drywall. Given the choices, I don't think foam is worth it.
    b) As fibreboard is very vapour-open you would see significant benefits to including a rain-screen, which will allow your walls to dry to the outside.
    c) Rain-screens are gaps between the siding and sheathing that allow water that gets in from the outside to drain, and provides a path for water-vapour in the walls to dry to the outside. No gap, no rain-screen.

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