PeteC is correct. This is basic physics – Gay-Lussac’s law, Boyle’s Law, Charles’ Law.
The lower the foam density, the higher the gas content. A glassed foam board is basically the equivalent of a fixed volume.
Simple experimental verification. Take an empty plastic milk jug and place it on the roof of your car with cap removed on a hot sunny day. Allow it to sit there for 5-10 minutes. Replace the cap tightly. Then place the sealed jug in the CA Pacific Ocean. Watch the jug collapse inward.
Now do the reverse. Take an empty plastic milk jug and place it on the surface of the CA Pacific Ocean with the cap removed (without submerging the opening). Keep it there for 5-10 minutes. Replace the cap tightly. Then place the sealed jug on the roof of your car on a hot sunny day. Watch the jug expand like a balloon.