Ventilation channels for conditioned attic under a slate roof?
I own an 1890’s home in Boston (IECC zone 5) with an original slate roof. The roof is in good condition after a fair amount of recent maintenance replacing slates and installing new copper valleys. I’d like to convert the attic into conditioned space, and I’ve received several recommendations to use closed cell spray foam in the rafter bays and walls because it maximizes R-value and serves as a vapor barrier . In talking to my roofer, the general advice has been to assume that the roof will leak over time, which leads to questions about potential water infiltration. I’ve been receiving conflicting advice from my architect, roofer, and various spray foam contractors regarding whether I should have spray foam applied directly against the roof decking, or whether I should create some sort of ventilation channel underneath the decking to allow water and air movement. (For creating a channel it’s been suggested to use polyiso insulation sheets laid into the rafter bays underneath of 1-inch furring strips of insulation or wood.)
I’m having a hard time sifting through the options for moving forward and would appreciate the advice and insights of the GBA community. On the one hand, I’m concerned that with closed cell, it will be difficult to identify roof leaks, leading to possible rotting of the sheathing or rafters. On the other hand, it’s a fairly significant investment of time and money to install ventilation channels, and also to retrofit soffit vents and roof or gable vents for airflow. What approach makes the most sense? Or should I consider using open cell in lieu of closed cell, since it would be water permeable in the case of any leaks?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
B.D.,
First of all, can you tell us your name? (I'm Martin.)
The biggest factor affecting your decision is the type of roof sheathing on your house.
If your roof has skip sheathing -- that is, 1x3 or 1x4 boards installed with a gap between each board -- then you definitely don't want to install spray foam on the underside of the sheathing. If you do, it will glue everything together, including the slates.
If your roof has solid board sheathing, or solid board sheathing covered with plywood or OSB, you have more options. Installing closed-cell spray foam against the underside of the roof sheathing in this case isn't too unusual, and I don't think there's a lot of evidence that roof leaks are harder to notice than when the roof assembly has a ventilation channel (although you hear that a lot).
All of that said, a slate roof from 1890 with new copper valleys is a precious and wonderful asset, and worth preserving with special care. If this were my home, I would definitely install the ventilation baffles, because this step will keep your sheathing dryer and will facilitate future repairs.
Note that you don't need soffit vents and a ridge vent with a slate roof. As long as you install the ventilation baffles with an air space between the top of the baffle and the underside of the roof sheathing, you'll get all the benefits of the air gap. Slate is vapor-permeable, and this type of roof can dry outward even without air flow.
Thanks Martin, for your response and guidance. My name is Brian! To answer your (other) question, I have solid wood boards for decking (i.e. not skip sheathing). Not surprisingly given the age of the roof, there are locations here and there where slates are visible from knots falling out of the wood, small gaps between boards, and the like.
Based on other GBA info I would plan to create ventilation channels using foam boards. The rafters are true 2x6 or 2x7 (can't remember at the moment and I'm away from my home this week), so not a ton of room for spray foam on top of the baffles. Given that, would you recommend a 1-inch or 2-inch baffle? I'm not expecting to get to full R-49 specified by guidelines, just want to do a reasonable job of insulating given the constraints of the old home. I should mention that it would be straightforward to add foam boards over the rafters to add insulation and decrease thermal bridging, if that would be useful.
Brian,
Either 1-inch-thick rigid foam or 2-inch-thick rigid foam can be used to create ventilation baffles, as long as it is a type of foam that is stiff enough to resist cracking or extreme deflection when it is installed. For more information on this issue, see "Site-Built Ventilation Baffles for Roofs."
If you end up adding a continuous layer of rigid foam on the interior side of your rafters to reduce thermal bridging, you'll end up with an excellent assembly. For all of your options, check out this article: "How to Build an Insulated Cathedral Ceiling."