GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X 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

Venting Attic Over Loft

littlebearinthewoods | Posted in Building Code Questions on

I am building a two story – well 1 story w loft with a 12:12 pitch gable roof. On the loft the ceiling pitches up to the 10 ft mark (ridge is about 14′) and on the side I have 5ft walls. So I have three mostly triangular spaces that run the length of the house (28′). I was thinking of just air vents/grills to let air in/out of these spaces – no ducts/fans etc.   But am not clear on what code demands (IRC 2018) for this situation. The ceiling is 2×10 with rockwool inside and poly outside so no foam fire risk as I understand can be issue.

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.


  1. user-2310254 | | #1

    Where are you located? Poly is almost always problematic. And you want to keep conditioned interior air away from exterior walls.

  2. Deleted | | #2


  3. littlebearinthewoods | | #3

    I am in N Az - flagstaff- zone 5 , 6500 ft. Dry most year except for monsoon seasons july-aug. R38 required for ceiling. Would you suggest a different approach?

  4. user-2310254 | | #4

    It might be helpful for you to read about building a "pretty good house," which is really a home that maximizes efficiency and comfort without going to extremes. This article is a good starting point:

    You haven't provided much information, but your comments on poly and R-38 suggest you have some specifications that are no longer consider best practices. Getting rid of the poly, focusing on air sealing, and setting your roof insulation target for R-49 to R-60 probably would be good first steps for getting your project on track.

  5. littlebearinthewoods | | #5

    Hmm I don't have access to that article but not clear on the poly issue. Some kind of rigid insulation has to go outside. The hours wrapped in zip sides and roof. So from inside out rockwool, zip, foam board, more wood, underlayment, air gap strips, metal roof

  6. littlebearinthewoods | | #6

    Evidently its not currently possible to sign up for prime access but the very latest article from Martin is exactly what I am doing. I did throw the poiyiso in there as a general rigid foam board catchall and will do final research on exactly what type I will end up using. In meantime I am still left with the tiby attic space to living space venting. I am considering a large closet so I think simple large vents to keep the air flowing should suffice but open to thoughts and potential issues I am not considering

  7. user-2310254 | | #7

    Yeah, there seem to be some coding issues with the site at the moment.

    So polyisocyanurate rigid foam and not polyethylene plastic. The plastic was my concern.

    If you are combining insulation types, you need at least 41% of the total r-value to be above the sheathing. With an r-49 roof, that's about R-20 (4 inches) of polyiso on the exterior. (See

    With those triangular spaces, you might want to use a layer of closed cell foam on the exterior walls to air seal and bring everything into the conditioned space. (See

  8. littlebearinthewoods | | #8

    Ah sorry - yes polyisocyanurate! Phew. Had me worried. Yes I have the 41% all setup etc. When you suggest CC foam - do you mean for that hard to seal area where the eaves come out? I see people adding eaves after but frankly that looks like a PIA so gonna notch the zip and squeeze foam in the gaps before applying tape/windshield caulk as I see being done.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |