I’m looking to improve the insulation in my pole-built garage, which currently has only the standard R8 vinyl-faced fiberglass between the metal siding and the 2×6 girts/purlins. No sheathing or housewrap.

I’ve read Martin Holladay’s “Insulating a Pole Barn”[1] article along with many Q&As and understand this isn’t an easy problem, particularly air sealing. I’m not trying to turn the garage into living space, though — my goal is just to keep it above 40 degrees in the winter. I’d like to add a bit of insulation to reduce the (electric) heating bill without spending a fortune on materials or risking moisture issues down the road.

Since I don’t have a ceiling, Dana Dorsett’s suggestion in “How to Insulate my Ceiling / roof deck”[2] seems like a good one: “If you can put up a layer half-inch OSB on the underside of the pre-existing purlins you could safely blow in low density cellulose which would fill nicely, and then detail the OSB as an air barrier…The 2″ of existing fiberglass is about R7-R8, which means you can install up to about R35-R38 under it without adding a vapor barrier without much risk of mold on the vinyl in your climate.”

For the walls, I’m thinking about the rigid foam option Martin suggests in [1]: “…install one or more layers of rigid foam on the interior side of the girts, followed by a layer of plywood or drywall on the interior, screwed through the rigid foam to the girts.” I like that this approach doesn’t involve adding studs.

My question is: what should I do with the vinyl-faced fiberglass on the walls? If I screw foam to the girts, that’ll leave an air gap between the foam and the vinyl. If the foam is vapor impermeable, that’s a double vapor barrier that could trap moisture in the air gap. Should I slash open the vinyl before installing the foam? Or use a vapor permeable foam and leave the vinyl intact?

I’m in zone 4C, Seattle-area.

[1] https://www.greenbuildingadvisor.com/article/insulating-pole-barn
[2] https://www.greenbuildingadvisor.com/question/how-to-insulate-my-ceiling-roof-deck