I-joist cathedral roof / ceiling insulation and moisture control
I'm new here, and I really appreciate all the great information that has been provided by GBA and its users.
I've been reading a lot about cathedral ceiling insulation, and it seems like it is a fine line between success and disaster when it comes to moisture control. So now I have a few questions myself that I hope someone could help me answer.
We just bought a house that is going to get a new roof, and we're thinking about using I-joists for the rafters, and a combination of a combination of XPS rigid foam and closed cell foam spray as insulation. The roof will also be vented (even thought many seem to go with a unvented roof when using closed cell foam spray). See the sketch here: http://oi61.tinypic.com/33z4en6.jpg.
With the I-joists at 24" o.c., our idea is to glue/silicone (?) the XPS rigid board on each of the I-joist flanges, forming a vent channel above it (see sketch). Then using foam spray for the remainder of the I-joists depth, and having another XPS board between the drywall and the I-Joists to minimize thermal bridging.
The vent channel will be 1" to 1.5", XPS board below the vent will be 1" to 1.5", foam layer 8" to 11", and the XPS board above the drywall will be 1.5" to 2".
Does this seem like a good approach (from an insulation and moisture point of view)? Should we put any kind of vapor barriers between the inner XPS board and the foam (or between the drywall and the XPS board)?
Maybe I'm overly concerned about this, but I just like to minimize any risk of having to redo the roof in the near future due to moisture issues.
Posted Aug 18, 2014 5:21 PM ET
Edited Aug 19, 2014 8:17 AM ET
Other Questions in Energy efficiency and durability
Can unvented roof assemblies be insulated with fiberglass if I use rigid foam on the exterior of the sheathing?