Oh, my apologies! I got the impression that the process of making them has the end-result of lots of closed off bubbles. But you're right, reading the wiki page again shows otherwise:

> Sound-absorbing Foamglas® insulation is more than 50% open cell bubbles, and heat-insulating Foamglas® is more than 75% closed-cell air bubbles, which can be adjusted according to the requirements of use through changes in production technical parameters.

https://en.wikipedia.org/wiki/Foam_glass