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soy-based foam attic insulation

I am pondering how to best insulate the cap of my house--with fibreglass or foam under the roof?...I have a 1938 Garrison with snow and ice dams on the roof. My neighbor installed soy-based foam on the underside of the roof and has no ice dams! Using that foam, could moisture get between the roof boards and the roof and damage the roof or shingles, or is this a good idea? I have inefficient 1938 blown in cellulose under the attic floorboards (and down the walls) right now and use the attic to store stuff currently.
Thanks for any help!

Timothy Miller
Re: soy-based foam attic insulation

If roof leaks it can get water in so is open or closed cell foam better ? I prefer closed it is water proof.. The foamed rafters are keeping the neighbors roof deck cold so the snow is not melting and running down the slope & freezing & building up ice dambs. Have you ensured your roof is properly vented? With eve and roof vents to let the moisture in the air circulate out of the attic? I'd first make sure 1by1' roof vent for every 600 sqft of attic and eve vents and seal around any openings in the ceiling with cans of spray foam such as pipes , wiring etc to help reduce air entering from your home.

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