Q: Should I spray foam directly on the bedrock in the crawlspace under the house I’m building or put crushed stone on the rock before spraying?
A: If you want to avoid grieving moisture problems, you need to back up with your planning. Building on bedrock is trickier than it looks, much trickier. I know because I see bedrock-related problems in my area all the time. Many houses where I live are directly on bedrock and the issues even baffle professional builders sometimes.
There are two difficulties – the cooling action of the rock that’ll cause condensation and sky-high crawlspace humidity in the summer, and the tendency for liquid water to follow the bedrock and enter your crawlspace during wet times of the year.
You mentioned your plan to spray the rock with foam after the house is up. This will solve the problem of the cool rock causing condensation during warm, humid weather, but it won’t solve the problem of leaking liquid water seeping in between the bedrock and your foundation. I’ve seen foundations parged and tarred and “waterproofed” by all kinds of methods, and water still somehow sneaks underneath the foundation where it meets the rock. Also, if you have spray foam on the rock, you won’t be able to walk on your crawlspace floor because you’ll crush the foam. There’s also still the grim prospect of the foam on the rock getting saturated and moldy during wet times of the year. I’ve seen fundamental problems like this happen with people building on bedrock and there’s no good solution after the fact.
All of this is leading me to a suggestion. Have you considered having no crawlspace at all? I strongly recommend against one in your case. Crawlspace-free is what I did when I built on bedrock back in 2007 with 12” to 24” of soil in the area. I built concrete walls, then filled the space within these walls with compacted fill. I left room enough on top to add R12 of extruded poly foam and a radiant foil barrier, then poured a 6”-thick concrete floor on top. So instead of a crawlspace, I went with slab-on-grade construction. That’s the project you see above. All the mechanicals and plumbing are at ground level and I have none of the problems I’m warning you about. And remember, my warnings are not just theoretical. I’ve seen how ugly, damp and rotten basements can get when their built on bedrock.