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

Insulation help for attic garage in Zone 2A (while allowing for storage)

C_Fox | Posted in Energy Efficiency and Durability on

Hi all,

Long-time reader of GBA, very infrequent poster — amazing resource here. Thanks to all who contribute.

Looking for input and advice on insulating my garage [in Texas] as part of a conversion into conditioned space for use as a workshop (no longer parking there, so very infrequent raising/lowering of garage door). Just installed high-efficiency (SEER 26) 1-ton ductless mini-split as HVAC (predominantly cooling, of course, but with heat pump for couple of winter months).

Now, I need to upgrade the insulation; currently, there’s very little — insulated garage door (R6) and R3-worth of polyiso rigid boards on garage walls (behind drywall); exterior veneer of garage is stone / wood (for side/back walls). No insulation in garage attic at all (neither attic floor nor roofdeck).

Technical context:

  • Single-family home, built in 1984, in Texas (Climate Zone 2A) with attached garage (~500sf floor).
  • Garage attic is constructed from rafters in gable design with 7/12 roof pitch and completely separate from main attic in house (which is at different height).
  • Roofing is Class 4 impact-resistance architectural shingles atop synthetic roof underlayment and standard plywood decking.
  • Roofing is quite new (installed in 2016 by previous owner of home, while under contract, as condition of sale).
  • Vented garage attic with roofline vent and large (4’x2′) vents, but no soffit vents (for unknown reason, long preceding ownership of home).
  • Attic access is through large, convenient set of pull-down stairs (30″x75″ opening and 55-degree climbing angle).

Goals for attic insulation:

  • Achieve sufficient insulation to make conditioning of garage cost-effective and environmentally responsible.
  • Maintain use of, and easy access to, garage attic for storage (very necessary due to limited storage space overall in house, given lack of basement, etc).

I’ve read a number of articles on GBA that address this, especially the “Musings of an Energy Nerd” to include those on “Creating a Conditioned Attic” and “Open-Cell Spray Foam and Damp Roof Sheathing”.

From what I’ve read so far, I think the best solution might have been to start with Polyiso rigid foam (2-layers with overlapping seams) installed on top of the roof deck and underneath the shingles. (I read on GBA, with interest, of Alex Cheimets’ experiment with this in a cold-climate.) Indeed, I tried to pay for that upgrade when the seller installed the new roof; unfortunately, they wouldn’t work with me to make that happen, and it would be ridiculously wasteful now to replace a brand-new Class-4 roof to sandwich polyiso between shingles and decking.

So, given the scenario I’ve laid out, and my two goals, what’s the collective wisdom of GBA on how to proceed?

Thanks for any suggestions — really appreciate them!

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. C_Fox | | #1

    CORRECTION: Home is in IECC Climate Zone 2 and IECC Moisture Regime A (ie, Central Texas). Sorry for confusion above!!! Above, I mistakenly provided the USDA Hardiness Zone, rather than the IECC Climate Zone (confusingly similar, but almost opposite in numbering system)!

  2. GBA Editor
    Martin Holladay | | #2

    The first question you have to answer is: Do you want to insulate the attic floor or the sloped roof assembly?

    -- Martin Holladay

  3. C_Fox | | #3

    Hi Martin,

    Thanks for your quick reply (as well as correcting my original post). You raise a very good point as to location of the insulation. I've been thinking about that a lot and didn't include it above, only because I felt by doing so that I'd predetermine an answer. I probably should have, though.

    Given that one of my two goals is to achieve convenient use of the garage attic space, I've been leaning towards including the attic in the thermal envelope of the home (making it some form of semi-conditioned space, given that it won't have any direct HVAC vents, but obviously has a lot of air permeability with conditioned garage beneath it). In that case, I'm looking to put the insulation at the sloped roof assembly. I could perhaps be convinced to do otherwise if there were compelling reasons, but this would be my first idea. Thoughts?

    Thanks for all your help (in this particular case, as well as your overall contributions here at GBA),

  4. GBA Editor
    Martin Holladay | | #4

    Your options are all presented in this article: How to Build an Insulated Cathedral Ceiling. You have two main options: a vented approach or an unvented approach.

    1. The vented approach only works if your roof has no skylights, dormers, hips, or valleys, and it only works if you can install soffit vents and a ridge vent.

    2. Since you want to work from the interior (rather than from above the roof sheathing), the only way to create an unvented cathedral ceiling is with closed-cell spray foam.

    -- Martin Holladay

  5. CorporateCowboy | | #5

    C.J. If you have traditional rafters, how are you going to beef them up for storage purposes?

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |