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