Insulation of church roof
Hi, in the process of trying to figure out the best way to insulate the cathedral ceiling in a church conversion. Here are the basics from top down:
-steel roof, 12-15 pitch
-metal is fastened to 3/4 inch strapping
-strapping is on top of 2″ foil faced insulation
-1″ roof decking
-roof rafters are true 8″ on 2′ centers
-under decking of roof is not vented
Insulation between the rafters is my area of concern, if price were no object I would fill cavities with closed cell insulation and apply desired 3/4 t&g directly over rafters. Price however is an object, thus I am looking at more cost effective options,
The final finished surface will be 3/4″ T & G
The location is in Nova Scotia Canada.
Any ideas?
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Replies
Jason,
Nova Scotia is in Climate Zone 6. (Here is a link to a climate zone map that includes Canada.)
It's possible to insulated a cathedral ceiling with a combination of rigid foam and fluffy insulation like fiberglass or mineral wool, but only if the rigid foam meets certain minimum R-value requirements. (For more information on this topic, see How to Build an Insulated Cathedral Ceiling.)
In Climate Zone 6, the rigid foam layer needs a minimum R-value of R-25. Depending on what type of rigid foam you now have on your roof, the existing rigid foam insulation probably has an R-value of R-8 or R-10, and that's not enough.
One possible solution is to temporarily remove the metal roofing and strapping, and add an additional R-17 of rigid foam above the existing rigid foam. Then re-attach the strapping and roofing, followed by fluffy insulation between the rafters.
An easier approach would be to install spray foam insulation on the underside of the roof sheathing. If you go this route, you would want to install at least R-17 of spray foam insulation before switching to fluffy insulation like fiberglass or mineral wool. Ideally, you would install an air barrier like drywall on the interior side of the fluffy insulation before installing your tongue-and-groove ceiling boards.
Jason,
One other possibility is to "abandon" the rigid foam installed above the roof sheathing -- discount it and make no use of it.
If you go this route, you could create a vented roof assembly by installing ventilation baffles under your roof sheathing (maintaining a 1-inch to 2-inch air gap under your roof sheathing). To do this, you would need to be able to install soffit vents and a new ridge vent.
Then you could beef up your rafters to provide enough room for about R-49 of fluffy insulation (fiberglass, mineral wool, or cellulose) under your new ventilation baffles.