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

Still struggling with insulation

sleaton | Posted in Energy Efficiency and Durability on

Austin TX area
House 2×6 framing 16 OC. Rafters 24 OC. Built on 18″ crawlspace.

The house is a complicated roof structure, with a 12×12 running down the length of the house framed in 2×8. There is then wrap around porches 4×12, a front gable 8×12 framed in 2×6. Then there is a large area of roof that is 2×6 construction, with a third of it being 2×8.

I made a huge mistake in that talking with the so called local experts, they were all selling me on open cell spray foam 5.5 inches everywhere in the roof, unvented. I figured cool, all good, put a thin closed cell spray if I wanted to over kill. Then I found out that is no where near code.

Looking at how to fix that, I am starting to wonder about options.

1. Closed cell on the whole thing would do a good job, but be super costly.
2. I am thinking of abandoning my unvented roof, and venting it. Then it would be a bit of a pain, but I could put the ducting all in the crawl space below the house. Then just blow in the required thickness on the roof floor. I might still need to get a bit creative on the 12 x12 part, but that could still be vented foam, roxul, and covered with some interior insulation to get up to code.
3. A hybrid. Closed cell unvented on the 12×12 and dormer area, and turtle vents on the other area of the roof that is not enclosed by the 12×12. keep as much ductwork as possible under the encapsulated 12x 12 pitch.
4. go crazy with open cell on the roof. 9 inches everywhere, which then covers the rafters also which helps with bridging. I plan to have aiprilaire to try and keep humidity down. Not sure if I should vent in that situation or not after reading about osb rotting under open cell.
5. an inch of closed and then filling the rest with open cell.
6. last option I am not sure I can do. I would put 6 inches of polyiso over my 5/8 osb decking. Then instead of shingles, I would attach furring across the polyiso and attach exposed seam metal roofing to the furring.

At this point, I am thinking #2 might be best. I was all excited about keeping the attic in the conditioned space, but maybe that isn’t best.

sorry for the detailed question, and so many questions lately. I just have a limited amount of time to set things right.

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. sleaton | | #1

    I guess, I could also do some kind of flash and bat on the roof ? a few inches of foam on the deck and rafters, fill the rest with bats, and then maybe a few inches of rigid under the whole thing ? sounds painful. with closed cell at the deck, would I need to frame in a vent too ?

  2. GBA Editor
    Martin Holladay | | #2

    I hate to say this, but your questions are all over the map. You need to hire an energy consultant who can look at your plans, talk with local insulation contractors, get some bids, and help your formulate a plan.

  3. GBA Editor
    Martin Holladay | | #3

    Here are just a few issues that come to mind:

    1. We can't visualize your roof shape from your description.

    2. It's hard for us to know if it is even possible for your roof to be vented.

    3. If your house is already framed, it's often too late to make a drastic change like moving the HVAC ductwork from the attic to the crawl space. It's hard to know if that is even possible without reviewing your plans.

    4. If you move the insulation layer from the sloped roofline to the attic floor, you are also moving the air barrier. For air barrier continuity, it's usually important to know where the air barrier is located before framing begins, in order to anticipate possible air leakage locations.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |