Collar Ties and Above the Roof Rigid Foam
Hi,
We’re building a 16×20 building in Zone 6, Nova Scotia, Canada
We’ve framed a complicated roof with rafters (no trusses). Imagine a 16/12 gable intersecting a Saltbox. Lots of valleys. Not enough ventilation.
So we realized our options were limited to a non vented cathedral ceiling with above the roof rigid foam insulation.
One of the challenges with this framing is the collar ties near the top of the rafters.
Would anyone have an idea how we can detail an air barrier around it ?
And would running the drywall under the collar ties create a problem because of the triangle shaped gap it creates between the insulation ?
We thought of contracting out spray foaming the whole roof but we already have lots of 2” XPS insulation leftover from a foundation to use up and we prefer to do everything ourselves!
Here’s the layout and rough drawing
- Standing seam metal roof
- High Temp Ice and water shield
- 3/4” ply
- 6” XPS rigid foam (staggered and taped) R-30
- Peel and Stick Membrane
- OSB sheathing
- R-30 Rockwool (We would prefer to go with a lower R-Value to be safer but we didn’t want to create an air gap by using thinner batts since we’ve already framed with 2×8)
- Smart Vapor Retarder like MemBrain
- 1/2” ply because of the convoluted roof framing
- Drywall
– Jeanne
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Replies
Jeanne,
Run the drywall under the collar ties, but keep the batt insulation in the rafter bays. Having the airspace underneath is fine, it’s airspaces directly under the sheathing that can be problematic.
Great! Will do. Thank you for your answer Malcom!
If you haven't installed the exterior insulation yet, another option is to remove the collar ties and install a metal strap across the ridge, over the sheathing instead. At least that's allowed under the IRC in seismically mild zones.
Yes, we also thought of putting metal straps, but we liked that we could run wires and hang a ceiling fan for our wood stove on the only flat part of the steep ceiling! Thank you Michael for the answer! We’ll keep that in mind!