We’re trying to decide how to insulate our roof. We will have scissor trusses with steel roofing on top of felt paper and 1×4 strapping, and we live in an area that has cold winters. It has been suggested to us that we spray foam on the underside of the top chord of the trusses. We would like to choose open-cell foam because it is water-blown. We understand that open-cell foam is vapour permeable while closed-cell is not, but are not fully sure of the significance of this for a roof application in a cold climate. Our questions are: Is it reasonable to use open-cell foam in our circumstances? and What (if anything) would we need to do differently if we go with open-cell instead of closed-cell foam?
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Replies
I like the urythene closed becausae it is moisture impermeable, but if you have great enough R-fator, you effectively eliminate the odds of creating any dewpoint so the icynene open cell is OK then, so the theory goes.
I have not used icy tho, so I can't comment. I have seen the same theory proven wrong with cellulose in a couple places, so I stick with the closed cell and/or VB
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The perm rating of closed-cell foam--how much water vapor can permeate into it--is a function of its thickness. Thinner is more permeable, thicker is less. But it does have a perm rating and, while it won't absorb liquid water as a sponge would, it is possible for water vapor to diffuse into it. The cells in a closed-cell foam will be predominantly closed, but in 2# urethane foam, approximately 5% to 15% of the cells will be open. Additionally, the gas generated during foaming that is originally present in closed-cell foam, and helps to form the cell, will gradually diffuse and be replaced by air and water vapor from the atmosphere (and, incidentally, drop the aged foam's R-value from 7.14/inch to 6.5/inch).
Cloud, do you recommend VB over SPF/PUF? I am thinking a bathroom where I plan to have foam sprayed in stud bays of outside walls, studs wrapped in VB...I just worry about double VB....For attic floor I plan to go without VB....
Paradoxically....for tub/shower surround, I was hoping to use Kerdi impermeable membrane between walls/ceiling and tiles, but decided it would not be a good idea because of impermeability of PUF on the other side (double VB)....I can't make my mind up if I should consider it impermeable or not....