I have a 1953 Cape Cod, climate zone 4a. The first floor is uninsulated CMU with stucco exterior and 3/4″ furring strips plus drywall interior. The attic has ~3″ fiberglass batts on attic floors and knee walls. I have plans to upgrade the attic insulation, but the 1st floor is difficult. Exterior foam would be very expensive, and I hate to tear apart my whole 1st floor to add studs and interior insulation. 

I want to try adding perlite to my CMU cores by drilling ~1.5″ holes through the top plate and pouring in from the attic. I expect it will be tedious, and I’ll experiment before buying all the supplies. I know it will likely only add R-2 due to thermal bridging, but that’s a lot starting from ~R-4. 

1. Are there any risks to this approach?
2. Can I plug the holes with canned foam, or should I glue in wooden plugs?
3. Would it be risky to also pour perlite in the 3/4″ cavity behind the drywall?