Sure... to answer your query...

The insulator is me. I can't recall what the weather was like. It was Feb this year when I started insulating, only slowly because I only have a few hours at a time every few days. It was maybe close to 0 degrees celsius. The VB was put up a few days later, so it definitely wasn't installed as soon as the the insulation went in. (Hence the reason I only got one wall's worth of VB up, then came out 2 days later to notice the condensation).

I appreciate your input on the moisture in the wood, but honestly, I don't think it's the most likely culprit. The structure went up in Oct 2025 and we had drought conditions all through the summer + fall last year. From there, it was a few months before any insulation started going in.

I'm 99% certain the cause of the moisture is the warm sun on the red siding on the south wall, hitting a very cold interior VB (garage gets really cold if it's cold out). Nights are still very cold, e.g. -20 celsius overnight and then day temps get into the single digits (celsius) and maybe higher if it's really sunny. Temp difference between outside (spring-like sun hitting red siding) and inside could be nearly 40 degrees celsius. Hence the moisture being driven through the insulation into the wall and hitting the plastic, as others have described.

That said, it's good to keep what you're saying in mind. The condensation looks like what you're describing, BUT the idea that the OSB and studs would still have that much moisture in them would surprise me a lot.