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Vaulted ceiling assembly

Hi I am looking at building a vaulted ceiling with 11 7/8 I joist 24" oc drywall on the interior, 1/2 osb 4" xps board over osb, and synthetic tar paper on the foam. On top of the tar paper 1x3 or 1x2 furring strips over and parallel to the joist.(to space up the 2x purlins for ventilation and drainage plane) and 2x4 24" oc perpendicular to joist to support a standing seam metal roof. At the eve there will be drip type flashing and 1" thick coravent to ventilate the roof and allow condensation out, there will be a ridge vent on the roof as well. The I joist will be insulated with fiberglass bibs. I prefer cellulose but insulation installers in the area prefer to install fiberglass maybe just because its easier to install.
There will be no vapor barrier poly.
I've seen similar assemblies but they typically have an additional layer of sheeting, which I am avoiding because I think my assembly will cost less than sheeting in labor, sheeting vs 2xs, and less long screw fasteners.

I like this assembly because the foam forms a full thermal break and true r-20 and the I joist allow for a lot of blown in insulation.

It will be constructed in the northern part of zone 5a.

Any thoughts from the experts?

Asked by tobias larsen
Posted Sun, 05/04/2014 - 11:48

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Tobias,
Your planned assembly will work well. While most green builders would suggest another type of rigid foam -- either polyiso or EPS is more environmentally friendly than XPS -- there is no building science reason why XPS won't work well.

Taping the seams of the rigid foam will make the assembly more airtight, which is always a good idea. Foil-faced polyiso is the easiest type of rigid foam to tape.

For more information on these issues, see How to Build an Insulated Cathedral Ceiling.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 05/13/2014 - 06:13
Edited Tue, 05/13/2014 - 06:15.

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