Cathedral roof with external XPS and embedded 2×4 nailers
Hi folks, I’m back with another question on a topic I’ve tried to research but have so far been unsuccessful in finding this exact situation.
I’m building a small cabin with a cathedral ceiling and 5″ exterior XPS insulation (one layer 3″ and second layer 2″). I’ve been thinking about the likelihood of success in hitting the rafters with long screws and the cost of the screws themselves. I’m not overly concerned with energy performance, but longevity and avoiding rot.
From interior to exterior the roof assembly will be as follows:
1. 3/4″ shiplap maple
2. Solotex Mento 3000
3. Layer of 3″ xps then layer of 2″ xps
4. Grace Ice and Water Shield
5. 2×4 purlins parallel to rafters for ventilation gap
6. 5/8″ plywood
7. another layer of Mento 3000
8. Metal roof
I’m thinking a way to reduce both cost and frustration would be to implement a 2×4 nailer system similar to the one in the attached photo (pink xps photo). What I’ve been trying to determine is whether this opens up the possibility of condensation on the underside of the 2×4’s “embedded” between the 2″ and 3″ insulation layers. It’s quite possible the underside of the 2×4 against the xps would be warm and the exterior (Grace side) very cold.
I’d hate to have water collect and be trapped within the insulation. The photo is an example of installation over top an insulated attic, rather than cathedral ceiling.
Swapping the layers of insulation and installing the 2″ layer first would also reduce screw length by an inch and make it easier to simply stack two 2×4’s to achieve correct embedment dimension.
I feel like the two air barriers should prevent any real risk of moisture but would like to hear people’s thoughts. Perhaps the grace isn’t necessary and simply taping the seams would be enough as well?
I’ve also attached a picture of the build so far to give some more context – test fitting my rafter templates.
Appreciate any input you guys might have!
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