There has been quite a bit of discussion about whether ventilation is required for the soffits/roof. Logic, the BC Building Code, and past experience have told us that of course we need to ventilate to prevent water damage from temperature differentials/condensation. The boys have put ventilation strips in the soffits and the apex of the roof has a ventilation strip over its whole length. But, the spray foam insulators (and their consulting engineer) insist that ventilation is not necessary, and that all their jobs have been passed by the inspectors. When batt ventilation is installed, there is a natural space for ventilation between the batt and the roof sheathing. Spray foam fills all voids. How can it ventilate? Is ventilation necessary? Here are the results of one study.
Action Spray Foam’s consulting engineer, who is a building envelope specialist as well as a structural engineer, visited us today. She says there will never be condensation in the roof, because there will never be an interface between warm and cold. The spray foam is itself a vapour barrier on both sides of the foam. Nevertheless, if we want to ventilate the uninsulated soffits, we can install a firm substrate (plywood) to spray the foam against at the outside wall in the roof joists. So, that’s what the boys will do tomorrow. Then, the sprayers can come back to finish the ceiling, the engineer will sign the letters of assurance for our inspector, and we can proceed to drywall. Just a bit of a delay. We’re used to bits of delay.
...and the foam is applied (Photo 3/3)
A firm base of plywood needs to be inserted in the roof spaces above the walls, to support the spray foam, yet allow ventilation in the soffits. (Photo 1/3)
the blocking is installed in ceiling cavities above the wall (Photo 2/3)
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The engineer had better be right, or it’s “Leaky Condo Gibsons Style”.
Hi Gail,
I sent this article to my Dad because he’s always been very keen on vapour barriers, insulation and ventilation, and his opinion is that ventilation is a good idea!
In normal roof construction a vented cavity is required to help dry out any condensation that may form when warm humid air hits the cold underside of the roof plywood. When a closed cell polyurethane foam is sprayed tight to the underside of the roof plywood, there is no space for the condensation to form. The spray foam that Gail is using is Polar Foam 7300 Soya which is a rated vapour barrier and air barrier when applied at more than 2″ thickness. No condensation can form between the foam and the plywood and no condensation can form inside the foam since it is a closed cell foam. For anyone interested in reading further about this principle there are many good articles at http://www.buildingscience.com such as ” BSD149 all unvented roofs”
Thank you, Brenda for your comment and the link.