GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

5/12 metal roof on rigid insulation

steve W| Posted inGeneral Questionson

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 Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  1. Steve Knapp CZ 3A Georgia||#1

    I would consult with your roofer and, possibly, the roofing manufacturer.

    Also... Is 6 inches of xps sufficient to meet code in your area?

  2. Expert Member
    Malcolm Taylor||#2

    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.

  3. Charlie Sullivan||#3

    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.

  4. steve W||#4

    thank you for the polyiso tip

  5. GBA Editor
    Martin Holladay||#5

    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).

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |