Enclosing a flat roof carport
I am enclosing an area under a flat carport roof to be a workshop. The area beneath the carport roof to be enclosed is approximately 20′ x 30′. Roof construction is 2×10 joists, 16″ on center, 3/4″ plywood sheathing, maybe 1/2″ – 1″ of rigid insulation on the roof side of the plywood, with an EPDM roof. The “flat” roof is slightly pitched for rain runoff (no pooling water). I live in Kentucky – at the far northern side of climate zone 4A.
This room will be conditioned with a mini-split. However, to save on utility bills, when the shop is not in use (which will be the majority of the time), the thermostat will be turned way down in the winter (50 deg. F?), and the AC may be turned off in the summer. There is no plumbing in this room.
My question has to do with insulating the ceiling. Due to the fact that the roof is only a few years old, this needs to be done from the inside, which I know is less than ideal. If I had known when I had the roof done several years ago that I was going to build a room under this ceiling I would have had gone with added rigid insulation on the exterior side of the roof sheathing. Is closed cell on the underside of the roof basically my only option? Or closed cell to a certain thickness (how thick?) + air-permeable insulation to fill up the joist space. My plan is to drywall the ceiling but there will essentially be no perforations. Any wiring for lighting will be conduit/boxes mounted on the drywall.
What are my best ceiling insulation options based on my situation? Thanks for any input.
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Replies
Ross,
Q. "Is closed-cell [spray foam] on the underside of the roof basically my only option?"
A. Yes, unless you want to replace the roofing. The closed-cell spray foam can be supplemented with some "fluffy" insulation like fiberglass or mineral wool if you want -- using the flash-and-batt approach -- but only if the spray foam is thick enough. For more information, see these two articles:
"Insulating Low-Slope Residential Roofs"
"Flash-and-Batt Insulation"
What about using only batt insulation between the joists (no spray foam) and then using a vapor-retarder membrane (like Intello Plus)? I was reading about this installation technique and was wondering if it was viable?
Ross,
The method you suggest is a code violation. It can lead to moisture accumulation in your sheathing and possible rot.
Once again, I urge you to read the articles I linked to.
If you want more information on the controversy surrounding the use of Intello Plus on roof assemblies that lack code-mandated ventilation channels, see this article: "Smart Vapor Retarders for Walls and Roofs."