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!