What’s the best assembly for an unvented attic roof?
WinnDesign
| Posted in General Questions on
We’re about ready to break ground on a new home in Northern Virginia (Climate Zone 4) and we’re planning on having an unvented attic. The assembly I’m considering 5/8″ Zip-System roof sheathing with Henry permiable underlayment, cedar breather and then cedar shakes. My thinking was that the combo of Zip and Henry would be a ‘belt-and-suspenders’ approach to protect against water since I’ll have closed-cell spray foam on the underside. I also wanted vapor permiable membranes and cedar breather to increase the drying potential of the sheathing. Thoughts?
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Replies
Michael,
Sounds OK to me.
Martin,
Am I creating an issue having two WRBs (the Zip AND Henry)?
Thanks,
Michael
Michael,
The term "WRB" applies to walls, not roofs.
Building codes require builders to install roofing underlayment under the roofing. As far as I know, some building inspectors may waive the requirement for roofing underlayment when taped Zip sheathing is installed.
But installing roofing underlayment on top of Zip sheathing won't cause any problems (especially on a roof with Cedar Breather and cedar shingle roofing).
In this particular case, the Henry product is a WRB. They don't have a vapor-permeable roof underlayment, however, my understanding is that some builders have been using their Blueskin VP100 self-adhered WRB as a roof underlayment. The only issue is that it's not made to be walked on and will void ZIP's warranty. Thoughts? Any alternatives I should be considering?
Michael,
Why does the Henry product void the warranty of the Zip sheathing?
If you want a vapor-permeable roofing underlayment, choose a vapor-permeable roofing underlayment (rather than a product designed for walls). Options include:
1. #15 asphalt felt
2. Perma R Products PermaFelt
3. GAF Materials Deck-Armor
4. Cosella-Dörken Delta-Maxx Titan
5. Cosella-Dörken Vent S
6. Cosella-Dörken Delta-Foxx
7. VaproShield SlopeShield
8. Nemco Industries RoofAquaGuard BREA
For more information, see Synthetic Roofing Underlayments.
Michael,
Curious about contractors involved in unvented roof assembly. I am considering conversion to unvented as part of energy retrofit. Not sure who is best able to help.
Scott