Can I double up foam boards to meet my rebate requirements?

You can definitly stack two layers of 2-inch XPS, but you're right to worry about the adhesive. Use a dedicated **foam-compatible construction adhesive** like PL 300; standard Liquid Nails will melt the board from the inside out. When you're stacking, stagger the seams between the first and second layers so the joints don't line up. This creates a much better air seal and prevents the thermal bridging you'd get with a single thick gap. Before you commit to doubling up the foam, check if your local code requires an ignition barrier. Most jurisdictions won't let you leave that much exposed foam in a crawlspace because it's a massive fire hazard and releases toxic smoke if it burns. If you have to cover it with 5/8" drywall or a fire-rated foil anyway, you might be better off doing one layer of 2-inch XPS and then framing a thin 2x4 wall in front of it to hold **R-15 mineral wool batts**. That gets you well over the R-18 requirement without the headache of gluing and bracing 4 inches of foam to concrete. Another option is to look for **Thermax** boards. They're a polyiso board with a thick foil facer that is actualy rated for exposed use in many areas. It has a higher R-value per inch (usally around R-6.5), so you could potentially get away with 3 inches total. You'll likely have to special order it trough a lumber yard rather than a big-box store, but it saves you the labor of covering the walls later. Don't forget the rim joists while you're at it. That's where you lose the most heat. Cut blocks of foam to fit snugly in each joist bay and use **expanding spray foam** (the 'gaps and cracks' stuff) to seal the perimeter of each block. If you don't seal those edges, the R-value of the boards doesn't matter because the cold air will just whistle right past them.

Spot on with the staggered seams and the PL 300. One thing to add—don't rely purely on the adhesive for that second layer. Concrete can get dusty or damp over time, and the weight of two thick layers can eventually pull the foam right off the wall.

I'd suggest using some plastic masonry insulation anchors (the ones you pre-drill for). They have a big flat head that keeps the boards pinned tight against the foundation. It gives you some extra security that those R-20 slabs aren't going to sag or peel off the wall in five years if the glue bond weakens.

Back in the day, we didn't have these fancy rebates or XPS. We'd just slap some tar on the concrete and hope for the best, wich ussually meant a soggy mess five years later. If you're dead set on stacking that foam, make sure you don't forget the bottom plate. I've seen guys glue foam all the way to the dirt floor, and the moisture just wicks up behind the boards untill the whole thing peels off like a scab. Run a bead of spray foam along the bottom edge where the board meets the floor to keep the ground dampness from getting into that gap. It’s a bit more grunt work, but it beats doing it twice.