Roof Assembly in Climate Zone 4C
Hi all…I’m an architect designing my own retirement home on the Oregon Coast (CZ 4C…1,000 feet from the ocean). My career has been solely focused on non-wood, highrise corporate and institutional projects, so I’ve been in an intense learning curve for wood based residential construction and the building science behind it. My roof is low slope (1.5 : 12), simple in plan with no valleys or hips, with standing seam metal (aluminum) roofing, interior clad on the underside of the roof joists (“cathedral”) and unvented. The section I’ve drawn so far is (from inside out):
1 x 6 TG or plywood interior cladding
2 x 4 spacers/chase to allow for ceiling wiring/J-boxes
Intello or Siga Majrex (no penetrations)
2 x 12 or BCI joists w/R-38 batts tight to underside of sheathing
5/8” plywood sheathing
WRB…self adhered; Sharkskin Ultra, Delta-Vent, etc.
Keane Viper CDR Vent mesh
2 staggered layers of 2” OC Foamular XPS (Oregon Code R806 requires R-20 rigid over sheathing for unvented roofs); this gives me a high ratio of 34% exterior insulation
1 x 4 strapping parallel to the joists (under roof venting to the roof peak)
Second layer of 1 x 4 strapping perpendicular to parallel layer for roof attachment
Aluminum standing seam roof
My questions:
1. Do I really need the Intello/Majrex layer to avoid condensation in the cavity?
2. Do I need the Keane Viper layer to allow some breathability under the XPS?
3. What kind of perm rating (if any) should the WRB have (ie, should I try to dry the cavity both to the inside and outside)?
4. Any recommendations of which WRB would be best for my site/application
5. Any other general comments to the proposed assembly?
Thanks in advance for your input.
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Replies
mannarchitect,
1. The assembly should be vapour-open on the inside to allow drying. If you were using drywall on the ceiling you could omit the Intello, but the assembly also benefits from an interior air-barrier, which the T&G doesn't provide.
2. I don't see what the mesh contributes there.
3 &4. I'm also not sure what a WRB, permeable or not adds under the foam. I would tape the sheathing as a secondary air-barrier, and omit the WRB.
5.
- You don't list a roof underlayment. That should go directly under the roof panels.
- Consider using a second layer of sheathing rather than two layers of strapping. The vent space it creates will attract condensation, with no real benefit.
+ 1 on everything Malcom suggests.
Generally roofing polyiso (fiber faced stuff) tends to be the best $/R value for these roofs but you have to be careful to get it covered before any rain.