Why Closed Cell Foam Works So Well (And When It’s Worth the Cost) Closed cell foam is one of the best-performing insulations out there and yes, it’s also one of the most expensive. The reason it performs so well is simple: once it cures, it becomes rigid and dense, almost like concrete. Air can’t pass through it, and that changes everything. That rigidity is why closed cell foam: Stops air movement Prevents condensation Keeps outside conditions outside and conditioned air inside Think about a re