Any benefit to foam board under steel roofing on a vented roof?
We’re getting ready to replace the asphalt shingles on our cottage in upstate NY with a steel roof. We are wondering whether it makes any sense to include a few inches of foam board under the new roof, even though the roof is vented.
The cottage which was built around 1990 has a cathedral roof and poorly installed fiberglass between the 2×8 (?) rafters. There are venting channels from the soffit passing above the fiberglass and to the roof ridge. There is significant heat loss through the roof, as snow melts in uneven patterns all over it in winter.
Redoing the insulation inside and replacing it w/ spray foam would probably be the best plan, but a job that we don’t foresee doing anytime in the near future. Since we have to replace the roof anyway, we’re wondering whether it would make sense to install a few inches of foam board to reduce some heat loss, even though it’s not an ideal situation. Or would this really be a waste? Any thoughts?
Thanks!
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Replies
It would help with ice damming and reduce solar heat gain, but would not help retain heat in winter (well maybe a little).
The real solution is not spray foam, but creating an unvented assembly with exterior insulation.
https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
SEC,
As Freyr-design said, you don't get appreciable benefits while the vent channel still exists. As an unvented assembly the roof will perform well, and it's nice not to have to work from the inside with all the disruption that comes with that, but to make it unvented does come with complications.
When you close off the vents you are left with the existing air-space. On roofs with exterior foam, building codes require that the permeable insulation be in contact with the sheathing above, because otherwise that gap is one of those in between spaces where moist air may accumulate in unpredictable ways.
To fill that gap you need access from one side or the other, complicating what was a fairly simple roof replacement where you just added a bit of foam. So perhaps it still makes sense to go ahead, with understanding that the benefits are down the road, but also that to get those benefits there is more work involved.
Maybe now is the time to pull up the plywood fill the air gap with more fiber glass and reunstall the plywood add the new foam under the new steel roof.
Walta