Venting Above Cut-and-Cobble Roof Insulation
ventilated cut and cobble?
I’ve got an addition with an EPDM flat roof in climate zone 4a. The main house attic is vented.
I understand that venting low slope roofs generally doesn’t work well, and that unvented cut and cobble roofs are risky b/c it doesn’t stay air-tight over time. So my plan has been to use closed cell spray foam (didn’t use external foam board b/c it was unavailable in my area at the time this project started due to lingering covid shortages).
However, spray foam contractors have been extremely slow getting back to me and cost is high, especially since it’s a pretty small scale job.
My question is whether cut and cobble would work well if I left a ventilation channel above it, running from soffit into existing vented attic? I would think cut and cobble would at least reduce interior air/moisture transfer relative to other insulating materials, so would this offset the poor ventilation performance of the flat roof?
thanks,
Ben
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Well personally I wouldn't try and passive ventilate a flat roof and my first choice would be exterior insulation, second would be spray foam. If you do go your route I would suggest also using a membrane like intello and very good air sealing detailing. I would also put a monitor in the ventilation space. If needed you could always power vent it if it became an issue in the future.
Ben,
The fairest answer is probably "It might". You have to decide if that is a strategy you will be content with.
Thanks for the feedback