Hi.  We have an older 50’s era brick home in CT which has been added on to.  It is built into a slope allowing a walk-out cellar.  We built a breezeway between the house and garage a few years back, creating a 12 x 12 x 5-ish ft crawl space underneath the floor level that we use to store gardening equipment.  The floor is finished with concrete.  The space has a shed-type door to keep the wind out (and the stuff secure) but otherwise it is not well sealed.

The breezeway floor above is 2×10 joists filled with R30 fiberglass, paper as expected to the finished floor.  An HVAC duct also runs through it which helps warm it a bit in the winter.  Nonetheless, the breezeway floor does tend to get pretty cold despite the insulation, and I would like to try to air seal it somehow.

I understand the ideal scenario is spray in foam into the “ceiling” of the space (breezway floor joists) to both insulate and air-seal, but besides the cost (probably in excess of $1000), I do have a lot of Certainteed Mem-brain “smart” vapor barrier left over from our build.  Assuming I install it correctly (writing facing out so it’s not “backwards”) and tape it well, could that be an effective air seal which would also allow moisture to escape back into the crawl space?  I believe the idea is sound in theory, just don’t want to create any problems with moisture down the road.

Thanks for any advice!