Is it normal for spray foam to expand unevenly on metal framing?

To actually break that thermal bridge on a metal building, you need at least **two inches** of closed-cell foam. Anything less and you're still going to see ghosting on the steel beams where the heat transfers trough. If the installer says you don't need to worry about the framing, he's probably planning to just spray the bays. But if you want a true air seal and condensation control, you have to 'picture frame' the members or wrap them entirely to stop that cold transfer.

That fear about the foam pulling away is valid if the metal isn't prepped right. If there's residual oil from the manufacturing process or if the steel is below the dew point, the bond will fail. The chemical mix has to be spot on—usually a 1:1 ratio at the gun—and the heaters on the proportioner need to be dialed in to about 120-130 degrees depending on the ambient temp. If the mix is off-ratio, the foam gets brittle or stays tacky, and that's when it delaminates from the roof deck. A good installer will do a flash coat first. That's a thin pass to establish the bond and get the metal surface ready before they go back for the full lift. If they just blast a heavy three-inch pass right away, the exothermic reaction gets too hot and can actually cause the foam to char or pull away as it shrinks during the cooling phase. You want to see them hitting those corners and transitions first to ensure there's no void where moisture can collect.

Make sure they're using a high-pressure system with a clean mixing chamber in the gun. You'll know it's working right if the foam hits the metal and cream-times in about a second or two. And definitely stay out of the workshop while they're spraying. The isocyanates are nasty until they've had a few hours to off-gas and reach a full cure.

I've seen guys skip checking the metal's temp and it's a total disaster. If that steel's too cold, it doesn't matter how good the mix is—it just won't stick right. I'd grab an infrared thermometer and check it yourself before they pull the trigger. Also, watch out for the overspray. That stuff's like permanent glue and it'll drift onto everthing if there's even a tiny breeze. If you've got tools or gear in that workshop, move 'em out or tarp 'em down tight. You'll be scraping it off for weeks otherwise.

Wow, this is all so intense! I’m totally new here and just started researching for my own small project. The way that foam expands sounds like something straight out of a sci-fi movie! Since you guys are talking about the seal, does this stuff acutally help much with soundproofing? My metal roof is insanely loud when it rains and I’m hoping this might kill two birds with one stone. Also, has anyone ever tried using the leftover foam to beef up an old plastic cooler? I’ve got one with a cracked lid and was wondering if a little spray foam would actually fix the insulation in it!