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Roof insulation and air sealing; advice and input appreciated…

peaceonearth | Posted in Energy Efficiency and Durability on

I’m thinking ahead about a room addition to my house. This would be a 16′ x 15′ room with a gable roof of at least 6/12 pitch with no dormers, chimneys or other obstructions. I’d like to do a cathedral ceiling for the aesthetics, and since I can plan to insulate well, I’m okay with the small energy inefficiency of the higher ceiling (not really that high).

I’m thinking unvented with rigid foam above the exterior decking. I live in zone 6, so would need R-25 there. Type 2 EPS has about an R 4.2 value, so I would need 6″ of thickness, a bit daunting but I think unavoidable with this design. I don’t think using XPS to save 2″ of thickness is warranted, but I could do 3″ of polyiso with 2″ of EPS above to keep polyiso warmer.

I know I would need another layer of sheathing above the foam, or could use horizontal purlins since I will be using screw down metal roofing. Between the rafters I’m thinking of 12″ fiberglass batts (6″ batts are short on R-value). I can use 2″ x 10″ or 2″x 12″ rafters that would compress the batts a bit but not enough to threaten my getting more than an overall R-49.

What about below the batts (underside of rafters)? I don’t think I can use taped rigid foam as an air seal here as that would be an inappropriate moisture barrier and limit drying to the interior, correct? (would even 1″ of taped EPS be risky here)? Should I use a smart membrane here for air seal, and what would be good options? The ceiling will be sheetrock. so maybe no need for the membrane, but I’m okay with that insurance unless a complete waste of money.

If I went with all interior insulation (avoiding exterior rigid foam), could it work to have 12″ fiberglass batts (R-38) between the rafters, and then perhaps 2″ of polyiso on underside of rafters, well taped? The felt faced roofing stuff is said to be R-13, giving me R-51. Must this design become a vented one?

Thanks for listening!

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  1. user-2310254 | | #1


    It is good that you are asking these questions at the design stage. Too many posters seem to start thinking through their options (or lack thereof) after construction begins.

    In general, GBA posts suggest that putting all the insulation on the outside of the sheathing is the best option if you can make it work. (Be sure to check into reclaimed rigid foam that can be purchased for one-third to one-half the cost of new material.) But I think you can solve this particular challenge with any of the strategies you have listed.

    It might help to know if there is an aesthetic or budget concern that might push you in a particular direction.

  2. Jon_R | | #2

    Consider a raised heel scissor truss. Can be insulated with loose cellulose and is a robust vented design.

  3. Expert Member
    Michael Maines | | #3

    I agree with both Steve and Jon--it's great that you're asking questions at this stage, and you don't have to use foam if you use a raised heel design. It doesn't even have to be a scissor truss; I've used parallel chord trusses in this application several times, with excellent results. A parallel chord truss is like a scissor truss but the inner and outer slopes are the same. Either way you don't need a ridge beam.

    If you choose to use foam, XPS ages to about R-4.2 or 4.4, so you wouldn't be saving any energy there. (EPS' R-value remains constant.) If you want to beef up the R per inch, consider either polyiso panels, or Neopor--graphite-injected EPS that has a constant value of R-5/in. I just visited an architect-designed job that I was the energy consultant for, which used both of the systems above--a raised heel scissor truss with R-120 cellulose, and R-60 walls with 2x6 framing and 8" Neopor nailbase panels on the exterior. The owner/builder/designer was happy with how it was all coming together.

    Read this and the related articles listed for more information:

  4. peaceonearth | | #4

    Thanks Gents. I've read Martin's excellent article re cathedral ceilings several times, and is what gave me a large part of what (I think) I know. I need to further consider truss rafters. I do understand their benefit in insulating well over the outer wall upper plate.

    A couple of specific questions:

    1)if I go with the exterior foam and 12" fiberglass batts (unvented), what is a good finish below the rafters? A smart membrane for air sealing, just taped sheetrock, both?

    2)with exterior foam, deep rafters and an overhang, can I get adequate insulation above the outer wall without raised heel trusses?

    3)is my all interior insulation idea a bad/lesser one? I think it needs to be vented, correct? It certainly is an easier overall application.

    4)is there a good link to how the trim is finished to look good with 6" of added thickness resulting from the exterior foam? This seems like a challenge and maybe big expense.


  5. Expert Member
    Michael Maines | | #5


    1. In zone 6 you need at least 51% of the R-value on the exterior, to meet code and for a safe assembly without doing a hygrothermal analysis. 12" batts are R-38, so you need at least R-40 on the exterior--about 10" of EPS, 9" of XPS or 7 1/2" of polyiso. If you use less exterior insulation, the sheathing is at some risk of accumulating moisture. With the proper amount of exterior insulation, all you need on the interior is painted drywall. If you want additional insurance or if you skimp on exterior insulation, you could add a variable permeance membrane like Siga Majrex, Pro Clima Intello or DB+, or Certainteed Membrain.

    2. Code requires at least R-38 over top plates in zone 6, so it depends on your assembly. It is certainly possible, and sounds likely, based on where you're heading with the design.

    3. Vented rafters are a time-tested solution. Read Martin's article on designing a cathedral ceiling.

    4. There is a cost, and various approaches, depending on your overall design. It is possible to make it look fine, and very easy to make it look bad.

  6. user-723121 | | #6

    I like a parallel chord truss (24") with a dedicated 1 1/2" minimum airspace below the roof deck with fiberboard or plywood as the spacer. Vent at both the eave and ridge. Blown insulation (22") can be used to fill the rafter space, use a warm side air barrier that is regionally appropriate for your area. This configuration gives full insulation throughout the ceiling and very good insulation fill around the roof truss webs. I will attach the ceiling drywall before insulating and leave an 18" or so space at the ridge for the blown insulation installers, they will net this final area and complete the blow. I have used this method for an art studio built over a garage in the Minneapolis/St. Paul area. The ceiling is R-65, the walls R-30 and the floor R-50 and built very airtight (1 ACH50 or less). I was there yesterday to visit the owner and with my infrared thermometer recorded a ceiling temperature at the 12' ridge of 71.5 F and a floor temperature of 70.5F. Well built and highly insulated, airtight structures will have very even thermal comfort.

  7. peaceonearth | | #7

    Michael - thanks for the specifics and actual numbers. 10" of rigid foam is quite daunting, so I'm feeling less confident about this scenario. I could skimp a bit (no code people here and low winter humidity in my house, with little moisture source in this room save for people), but thats not a good way to start or think. Interior insulation and venting is gaining favor. I could do 12" batts, a secure vent channel above, and rigid foam below rafters via 2" or more of well taped polyiso. Not quite as robust at r-51 or so, and less able to get as much insulation above outer wall, but doable, I think.

    Doug - thanks for the info on trusses. That scenario is gaining based on your explanation. Since this is not a dense pack fill, is it a realistic DIY job. I have done blown cellulose with the standard rental machine a few times.

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