Local builders don’t like my unvented roof assembly
HeikeU
| Posted in Green Building Techniques on
Greetings-
I read every article on unvented roofs at GBA and designed the roofing approach for our house as listed below. My husband talks to a lot of local builders and their reaction to our unvented roof assembly plan ranges from suspicious to outright “no you can’t do that”. Can you help put us at ease or if the assembly can be improved, let us know how? We’re in zone 4C (marine).
Ply ceiling w/ site-fabricated rabbeted and sealed edges
2×10 rafters with fiberglass batt insulation
5/8 ply sheathing, all edges/gaps sealed and taped
2” of XPS (min. per bldg code) – no purlins
3/4 ply sheathing attached through XPS to rafters below
MetaLayment
Metal roofing
Sealant and tape remain flexible and per tech sheet withstand temps up to 170 degrees. Thanks in advance for your insights and for all the information on your website, it’s been a great resource. My apologies for bringing up the topic of unvented roofs again 🙂
Heike
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Replies
Heike,
If local builders aren't familiar with unvented roof assemblies, you can steer them to the following resources:
1. Section R806.4 of the 2009 IRC. This is the section of the 2009 building code that allows your type of assembly. (In the 2015 IRC, the comparable code requirements can be found in Section R806.5.)
2. The following two GBA articles:
"How to Install Rigid Foam On Top of Roof Sheathing"
"Combining Exterior Rigid Foam With Fluffy Insulation"
Your suggested assembly raises two questions:
1. I have my doubts that you can seal the seams of a plywood ceiling in a way that is both (a) airtight enough and (b) attractive. Gypsum drywall is a safer and easier option.
2. Most green builders avoid the use of XPS, which is manufactured with a blowing agent that has a high global warming potential. For more information on this issue, see "Choosing Rigid Foam."
I would make few modifications to your plan:
1. Install Polyiso on top of the roof decking.
2. Install dense pack cellulose instead of batts.
3. Install drywall before ply ceiling.
4. Tell your local builders to study the code books, and to join this blog so they can educate themselves. ;-))
Thank you, the ply ceiling should be changed to gypsum then. Re: XPS v. Polyiso, we made that choice partially because several builders told us that ants prefer polyiso over XPS. Could that claim have any validity?
At 2" you can't assume XPS will remain R10 over the lifecycle of the roof. The manufacturer's will only warrantee R9 (knowing that the risk of ever paying on that is vanishingly small) and there is reason to believe it would drop to the R8.5 range over the lifecycle of a steel roof.
If using foil faced goods rather than the more commonly used fiber-faced roofing polyiso, sealing seams with foil tape and sealing the perimeter edges with foil tape would deny access to insects.
Using blown cellulose rather than fiberglass in the cavities also reduces insect penetration from below, since the borate fire retardents kill the gut flora that allows wood boring ants/bees/wasps/termites to digest the wood fiber.
We use Polyiso in wall and roof assemblies on almost all houses without ant problems. Ants love to nest near moisture in any kind of wood and insulation. Cover and seal your roof assembly, specially the rigid insulation and you should have no problems. You could also install stainless-steel mesh around the assembly for more protection.
Great insights, thank you so much.