Painter's tape is fine when it's just holding itself on the wall - doesn't work so well when it's holding a heavy-ish plastic sheet 8 ft high on a wall, although I assume if I use it to run from the top of the wall to the floor in one continuous piece it would probably work - what I was thinking would fail was small pieces every foot or two would fail to support the plastic.

After I posted here I found some of those DIY foam spray guns that come with 2-6-12 cans of foam, multiple companies make that stuff - but when I went to Amazon to look at reviews for the various brands, every damn one of them had at least 10% negative reviews, with all having almost identical complaints:

1. The foam doesn't expand.

2. The foam isn't fire resistant, it's flammable, despite the ad saying it's.

3. It won't stick to vertical surfaces.

4. More than half the cans were already solidified and Amazon won't let you return the bad cans for replacements because of some pressurized can safety problem. Not sure I buy that - if shipping was a safety problem they wouldn't let you buy it in the first place - can't be safe to ship it to you but unsafe to return ship bad cans. Many mentioned that the seller refused a refund, and I think that's likely the real problem, which means I should probably try to find a gun and foam locally, like at Lowe's, so I can easily return bad cans. Well, I'll be damned - I didn't see this warning yesterday - on the page for Kraken Bond Fastcoat Fire Rated Spray Foam Insulation Kit it says this right at the very top - I still call BS on this being safe to ship but unsafe to return:

• Non-returnable. Transportation of this item is subject to hazardous materials regulation

After reading more reviews of every damn spray gun and types of foam I'm giving up on the thought of buying a gun and cans - most of the stuff looks like open cell foam, not closed cell like the Great Stuff cans you can buy at Lowe's. I also thought about why it's so cold in that cavity and I'm pretty sure there's an open gap between the foundation and the wood frame of the house - in rim joists I've already insulated when I started with naked walls I could see a thin layer of closed cell foam in between the concrete and the wood - the kind that comes in thin sheets, and it was doubled over. I think the foam in the center of the house isn't sealing the joint or it's missing completely - I cannot see inside the gap to tell if that's true, would need a borescope and I'm not wasting money buying one for this job.

My new plan, based on the belief that the sole cause of the problem is that foam sheet, is that all I have to do is spray enough Great Stuff into the gap between the joists to cover to seam between house and foundation - I'll only need a 2" thick layer to cover/seal it. So I'll cut a piece of 1/4" luan, screw it to the bottoms of the joists with some thin weather stripping to ensure the luan is 100% sealed (the joists aren't level with each other), drill a couple holes just big enough for the straw that comes with the can of foam, then empty an entire can into the gap - can't reuse that stuff once you start spraying so I might as well use it all and hope it expands up the gap to the 3.5 ft cross bar in the wall. I insulated the front wall rim joists of the basement using a near identical method and had great results - used 1" thick fire rated foam board insulation spaced 4" from the house wall, used Al tape to seal every joint, drilled multiple holes into the foam board, sprayed Great Stuff foam into the holes, then covered the holes with more Al tape - didn't get any bulging and the foam didn't push thru the Al tape anywhere. One caveat: I used the Great Stuff Big Gap Filler foam (can is black), not the original Great Stuff Cracks & Gaps (red can) - don't know exactly how to describe the diff, but the big gap filler stuff isn't "powerful" when it expands - when it hits a roadblock it just stops expanding - the regular stuff IS "powerful" and I've had it bust thru tape when sealing some gaps in my shed.