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Help with garage insulation in Texas

Hi y'all,

Long-time reader & occasional poster. Much appreciate all the green-building wisdom on this site and forum!

I'm in the middle of converting my garage (500sf, attachedd to home, no living space above it) into shop space. This is early 1980s construction in central Texas, technically within Zone 2B, but right on edge of 3B. I've already conditioned the garage, via ductless mini-split (26 SEER), separate from home's central HVAC, but now need to handle insulation. Currently, it has 1/2" of Thermax polyiso (good for R3 or so) on three side walls, exterior to the studs, and no insulation whatsoever above. The two-car-wide garage door is one of those types with styrofoam sandwiched between metal and vinyl, rated around R6.5. Walls are 2x4 16" c-on-c studs, and there's a gable roof overhead (approx 7:12 slope) held up by 2x6 16" c-on-c rafters. Roof is covered in asphalt shingles (impact resistant to withstand Tx hail storms). As a side-note, garage ceiling (attic floor) is 2x6 16" c-on-c joists (with crosswise central beam to shorten spans).

Key goal is to use garage attic for light storage, so the plan is to put thermal barrier on underside of the roof, to keep the floor of garage attic free from blown insulation, and also to include attic in the (semi) conditioned space.

I was originally considering going with spray-foam insulation on the roof as well as in the garage walls, but after posting here about a year ago, and getting some very good responses, I've abandoned that approach. Current plan is DIY install of Roxul, for a whole host of reasons that have been covered a lot on GBA. I've already purchased R-19 Roxul batts for 2x4" walls and R-23 Roxul batts for 2x6" rafters (and gable side-walls in the attic). I'm leaving the external polyiso as is, and will be replacing the drywall on the garage walls and ceiling (with new) after finishing the insulation install.

As mentioned above, I'm smack in the middle of this work (holiday vacation project, running over) and have a few questions:

  1. I wasn't planning on doing anything in terms of adding or removing vapor barriers on the walls. In searching on GBA, I've read that their use is somewhat questionable anyway for exterior walls such as these, and I don't have access to the exterior of the walls anyway (as I've only removed the interior drywall in the garage, not the exterior facade of stone / wood). Is this approach good-to-go, given my environment?
  2. I'm planning to seal up the garage attic against external air leaks, since it will be within the (semi) conditioned space due to thermal barrier on the roof deck. In my case, that means closing the two gable vents and handling the ridge-line vent. The garage was not built with soffit vents, so no need to deal with them. I've gone back and forth with addressing the ridge-line vent: either (a) hire a roofer to remove the vent, and re-install new hip/ridge shingles; (b) seal up ridgeline vent myself by spraying consumer foam (Great Stuff Pro Window / Door) inside it along entire line. Am I doing the right thing with all this air sealing? Which approach to the ridge-line vent would you prefer?
  3. Should I purposefully leave some openings in dry-wall on garage ceiling so that conditioned air from garage can more easily move into garage attic? The ceiling joists run parallel to the garage door, and I was thinking I could open up the first and last joist spaces in the garage (nearest the garage door, and at the back of the garage). Would this help keep the garage attic in better shape, with respect to both humidity and temperature year-round?
  4. Anything else I should keep in mind? (Given, I've already largely committed to much of the above approach, as the Roxul batts are purchased and at my place ready to use.)

Thanks for all the advice and help --- GBA is a fantastic resource and I'm grateful for all the input.

Happy New Year to all!


Asked by CJ Fox
Posted Jan 3, 2018 2:31 PM ET
Edited Jan 4, 2018 7:09 AM ET


3 Answers

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If the R-23 Roxul batts will be installed between the rafters, then you must include a ventilation channel between the top of the batts and the underside of the roof sheathing. So you'll need soffit vents, ventilation baffles, and a ridge vent.

These required details are explained in this article: How to Build an Insulated Cathedral Ceiling.

Answered by Martin Holladay
Posted Jan 3, 2018 5:45 PM ET


Hi Martin,

Thanks so much for the quick reply. I feel pretty stupid, truth be told. I read the article you linked to a bunch of times, but for whatever reason, didn't think it applied in my case. Okay, so I now get it does, and need to think things through.

Sorry for my delay in responding, as I was trying to read up more about this. I came across the following article, linked from elsewhere on the GBA website (so I'd think it's known / liked by the GBA community):


If I understand this properly, it's saying that I do *NOT* need to control the condensing surface temperature of an unvented roof insulation system, *IF* I live in hot-humid (or hot-dry) climate zones. It then proceeds to give an example of an unvented roof assembly in Houston, which "has no insulating sheathing installed above the roof deck." It seems to say that this is fine, as "there is no potential for condensation on the underside of the roof sheathing until interior moisture levels exceed 50% RH at 70°F."

Anyway, I looked up my Central-Texas county (and a bunch of neighboring counties for a few hundred miles in all directions) at:


And we're all listed as part of the "hot-humid" climate zone as set by this "Building America" effort of the DoE.

If that's the case, then the way I read the BSC article above, I can just putt the R-23 2x6 Roxul batts right in the cathedral ceiling in my garage attic, and I'll be fine.

Yet, this seems to go against the advice of the article here on GBA --- not in general, of course, but with respect to my county, in particular. What to do?

Really grateful for your input.

Thanks again.

Answered by CJ Fox
Posted Jan 8, 2018 2:39 AM ET


There is no reason for you to feel stupid; on the contrary, you've researched this topic more than most code officials or insulation contractors, and you raise an interesting point.

My recommendations are based on code requirements -- specifically, the requirements found in section R806.5 of the 2012 IRC (and comparable sections of subsequent editions of the code).

This section of the building code notes that "Unvented attic assemblies (spaces between the top-story ceiling joists and the roof rafters) and unvented enclosed rafter assemblies (spaces between ceilings that are applied directly to the underside of roof framing members/rafters and the structural roof sheathing at the top of the roof framing members/rafters) shall be permitted" as long as a number of conditions are met.

If you want to combine air-permeable and air-impermeable insulation, there are two possible ways to proceed. One option (according to the code) requires: "In addition to the air-permeable insulation installed directly below the structural sheathing, rigid board or sheet insulation shall be installed directly above the structural roof sheathing as specified in Table R806.5 for condensation control."

Table R806.5 specifies the minimum R-value for the foam installed on top of the sheathing (not the R-value for the whole roof assembly). The table calls for a minimum of R-5 foam for Climate Zones 1-3 (the area where you are located).

All of that said, Joseph Lstiburek of BSC asserts in his article (the one you reference) that omitting the rigid foam above the roof sheathing is safe in your part of Texas. My reaction: (1) I don't have any reason to doubt Lstiburek, but I tend to be conservative on these issues, and (2) Since skipping the rigid foam is a code violation, you'll have to discuss the issue with your local code official.

Answered by Martin Holladay
Posted Jan 8, 2018 8:14 AM ET

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