5/12 metal roof on rigid insulation
steve W
| Posted in General Questions on
the project is a small restaurant. the Owner wants the dining to be open to underside of osb sheathing. The proposed roof system is: 5/12 wood roof trusses @ 24″, 1/2″ osb, 30# felt, 6″ extruded polystyrene, metal roof. i have called Tech support for both owens corning, and DOW; both said that it was ok. no offense to either Tech, but some of the things they each said, made me have some doubts.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
I would consult with your roofer and, possibly, the roofing manufacturer.
Also... Is 6 inches of xps sufficient to meet code in your area?
Steve, A couple of thoughts:
Has the owner seen actual sheets of OSB? They have various grade and proprietary writing stamped on them and the edges are typically painted a bright colour.
It's worth checking your local code to see wether OSB yields a sufficient fire rating for the roof assembly in a commercial building.
I've seen OSB used as an interior finished wall surface in at least one high-end commercial project. They sanded it and varnished it so it looked like an expensive exotic material (to some observers). But perhaps the owner wants a rough industrial look. In any case, the textures vary and it seems like a good idea to give the owner a chance to look at and perhaps choose the specific product.
Since you are asking on a green building site, you should know that 6" of XPS is not at all green. XPS is made with an extremely potent greenhouse gas, more than 1000 X worse than CO2. You can avoid that impact and get better insulation if you choose polyiso instead of XPS. EPS and "neopor" graphite-loaded EPS also avoid the problem. Various combinations of those three can beat XPS on R-value per dollar and/or R-value per inch, depending on your constraints and objectives, so there's no reason to use XPS unless you came upon a great deal on reclaimed XPS.
thank you for the polyiso tip
Steve,
There are lots of issues here. Several have been pointed out already: Does the owner really want to look at an OSB ceiling? Will and OSB ceiling meet requirements for a thermal barrier (usually defined as 1/2-inch gypsum wallboard), which is needed on the interior side of rigid foam insulation? Do you really want to use a type of rigid foam that has a high global warming potential (since either EPS or polyiso is preferable)?
Here are three more issues to add:
1. The way I read the code, you are required to install roofing underlayment under the roofing. I'm not sure whether the felt layer under the rigid foam is good enough -- I would prefer to see a layer of underlayment above the rigid foam.
2. Where is your air barrier? Probably at the OSB layer -- but if so, you need to tape the seams of the OSB with a high quality tape.
3. As a roofer, there is no way I would screw metal roofing through 6 inches of rigid foam. You need either a continuous layer of roof sheathing above the rigid foam (OSB or plywood) or some type of purlins (1x4s or 2x4s).