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Community and Q&A

Insulating single slope metal roof with no attic

Todd76 | Posted in General Questions on

I am in zone 3 (DFW area) building a 16′ x 16′ conditioned room, using 2×6’s for walls and 2×10’s for the rafters. I have been reading a ton of articles on this site and elsewhere, trying to understand everything but am a little overwhelmed.

I have the walls covered in OSB. I was thinking about putting a layer of 3/4″ polyiso over that, wrapping, and attaching metal siding.

For the roof, I was thinking of covering in OSB or zip sheathing and taping and then 2 layers of 1/2″ polyiso (was a little confused if I need some other type of underlayment).  Screw 1×4’s to the foam, running the length of the rafters and attaching metal roof to that.  Not sure if the 1×4’s are needed  and if I should do the walls too for some sort of rainscreen.

For the interior, I was originally going to spray open cell foam but not sure after all the reading. It seems like I see a lot of recommendations for blown in cellulose. 

Am I going down the right track or do I need to do something different? Slowly learning and thanks for any help. 

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Todd.

    I think you are on the right track.

    On your walls, you can use some rigid foam insulation as your water-resistive barrier (WRB) if it is approved for that use, and not include housewrap or an additional WRB in the assembly. Some builders install housewrap over the rigid foam to integrate with outie window flashing, some install housewrap behind the rigid foam to protect the sheathing, and some builders consider it best practice to install the rigid foam over a drainable housewrap, particularly with commodity OSB sheathing. There is a slight loss in thermal performance, but the extra protection against trapped water and wet sheathing may make it worth doing.

    You'll also need to decide how you want to install your windows--at the sheathing level with exterior extension jambs or in plane with the rigid foam with a window buck--and how you will air seal the sheathing. You can caulk the perimeter of the sheathing and tape the panel joints. You can use your WRBs as the air barrier (self-adhering products and fluid applied products are better air barriers than mechanically fastened products). Or you can detail the rigid foam as the air barrier. A rainscreen gap is generally a good idea on the walls, though it is possible that your metal siding has some integral venting potential depending on the profile: All About Rainscreens

    If you don't use ZIP sheathing for the roof, you'll want to create an air barrier at the sheathing. Again, you can tape the sheathing seams and then use a mechanically fastened roof underlayment or use a fully-adhered underlayment. Either way, you want airtight sheathing before you install the rigid foam, and you want to include an underlayment beneath the foam detailed as if you we installing a more conventional roof. This may be helpful: How to Install Rigid Foam on Top of Roof Sheathing

    In your stud and rafter bays, you can open cell spray foam, but as you have pointed out, many GBA users do prefer cellulose. Among other benefits it is one of the more environmentally friendly insulators.

  2. Todd76 | | #2

    Awesome....thanks for the reply!

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