Cores under mudsil , foam board over or fill

This one comes from a homeowner forum, not a news story, but it touches on a question that comes up often when insulating a basement or foundation wall: what to do with the hollow centers of cinder blocks before adding insulation.

A homeowner in a cold climate (zone 6, covering much of the northern U.S.) asked whether to fill the hollow cores in their cinder block wall before installing rigid foam board over it, or just install the foam board on its own. A building science expert replied that filling every core isn't usually necessary. A simpler fix is to use canned spray foam to seal the exposed hole at the top of the wall, near the mudsill (the wood beam that sits on top of the foundation), after the rigid foam board is already in place. Doing it in that order is easier, since the foam board itself covers part of the opening, leaving less to seal.

The reply also mentioned that some people pour in bags of tiny foam beads, each about the size of a BB, to add a little insulation inside the hollow cores. But the expert said this step is optional and rarely worth the hassle, partly because the beads build up static electricity and stick to everything nearby. For most homes, sealing the top of the block cores with spray foam, combined with rigid foam board on the wall surface, is the more practical approach to cutting drafts and heat loss around the foundation.