Hot Roof – Ventilate or not with foam insulation.
I am building a new house in climate zone 4, house is a skillion type roof system with low pitch roofs 3/12 and 4/12 pitch standing seam metal roof. I was advised to ice guard the entire roof system due to low pitch metal roof, which we have done. We have 16′ cathedral ceilings at the peak, with 16″ TJI for rafters. We are attempting to achieve a R-38 or better at the roof. Question is to ventilate or not-ventilate the joist space with foam insulation. I was going to ventilate joist space with 2″ joist baffle sheets and spray closed foam to it. Spray foam insulator said not to ventilate joist space that I was only drawing the moisture into the house like condensation on a soda can. He is stating he can keep moisture out by sealing all penetrations and then foaming 10″ with open cell foam. I have not seen an article doing a ceiling with open cell, only closed cell foam. I have read closed cell non-ventilated but with permeable barrier above sheathing to allow moisture upward. Any advice how to proceed?
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Replies
wmorey,
Not a direct answer to what you asked, but with a 16" deep cavity you could use r-40 batts and still have a 5" air-space left over for venting. That's a well performing roof with none of the downsides of spray foam.
i agree with the above comment. On ours we did go with spray foam directly on the sheeting and unvented attic. If you are going that direction i would certainly go with closed cell and not open.
You want closed cell foam under the roof sheathing. I would absolutely NOT use open cell here. Open cell foam in that location comes with risks, and while sometimes it's OK, it's a BIG problem when it doesn't work. Closed cell is safer from a moisture standpoint.
Sounds like you don't really have an attic here, so the space right under the roof is conditioned space. This means you don't want to vent the assembly. If you were to do any venting, it would have to be ABOVE the spray foam, to vent the roof assembly and not the conditioned space. This would also need vent channels and some type of intake venting around the soffit area.
It's a good idea to air seal the other parts of the structure regardless.
Bill