Still debating insulation and interior finishing

[CV428]

I am stuck in analysis-paralysis for insulation and interior finishing options on our 24x50. Electrical is done.

My original plan was:
Insulation: PolyISO panels with a small air gap (furring strip width) from the metal siding, then skin over it.
Wall/Skin: Debating between BlueTex fabric insulation vs Birch paneling vs HDPE paneling
Climate: I plan on adding a mini-split, but this is for "partial" heating/cooling to take the edge off (maybe 60f in the winter, 80f in the summer). Zone 8B, winters are mild, summers are toasty.

Primary use is automotive/equipment repair and storage.

Any suggestions? I can't seem to find any consistency on answers, even contacting pros. So much conflicting information on "best practice" for insulation. The spray-foam guys vs sheet/panel guys seem to disagree on everything.

 

[Codyboy]

My understanding when I researched insulation, is that if there is a gap between it must be ventilated or the metal will sweat on the inside.

 

[CV428]

My understanding when I researched insulation, is that if there is a gap between it must be ventilated or the metal will sweat on the inside.

That's my thought as well. The wall gap would continue up along the soffit and ceiling to ridge gap as a continuous air channel. I would also leave a very small controlled gap on the lower edge of the polyiso in each wall cavity (likely furring strip width again).

Once it's finished, moisture ingress would be minimized except ambient air intrusion from soffit gaps and ridge vent.

 

[sansbury]

Up here in Mass. our codes on insulation are getting pretty strict. For a utility building like a detached garage, you can build it insulated or uninsulated, but if you insulate it, it must be done to full residential standards, which dictated R-60 ceiling and R-30 wall as the minimum. As a result closed-cell spray foam is pretty much the default for walls unless you want to spend a lot more money on something else.

In my case, I was lucky that I was in a town where the building inspectors used common sense, and did not require us to do things like a blower test or insulate buried interior walls that already had 2" of foam on the outside.

In any case the building I built last year got 4" of foam on in the walls, which were finished on the inside with 5/8" plywood sheathing and painted a bright white. The sheathing is the typical Zip green board with clapboard siding over it to match the house. It is entirely a functional building, so everything is surface-mounted on the interior walls, nothing buried. While I cursed a bit about the cost of compliance, it has been incredibly comfortable from 90+ summer days through the extremely cold December we just had with many days in the single digits. Heating cost is a bit brutal due to our electric costs (33c/KWh) and the very low temps, but would be much less so in your climate where the heat pumps are more efficient.

While I swore I wouldn't run the A/C in summer, and generally didn't need to even though the building gets a ton of direct sun, with it being so well-insulated I can knock 10-15° off on the hottest days for the cost of a couple cold drinks.

I also had an ERV installed in case I fill the place with smoke and want to dump it quickly in the winter, and for general air health when I'm working in there for more time. In a milder climate (I'm zone 5) probably easier to just crack some windows.