Foam filled panels

I am in the process of building the fuselage sides and bulkheads for my SAL 2/3 Mustang. They are built up with a sitka spruce frame and skinned on both sides with birch plywood. When completed, these pieces will have sealed sections that will be hollow. My question is, can I fill these cavities with foam before gluing the second skin on? My concern is that when completed, these sections will be sealed and won't be able to breathe. Temperature variations from summer to winter and from the aircraft being in a hangar to parked outside in the sun may cause these sealed panels to expand and contract. Possibly damaging the paint, and just putting a little more stress on the glue joints. The frame work is 20 milimeters thick, so I'm thinking that 3/4" foam would be a good fit. Gluing the foam in could add strength as well. One 4' x 8' sheet of 3/4" foam would do the entire fuselage. I have seen the Corning pink foam in 3/4" at the Home Depot by my place. That is when I started thinking about this. What are your thoughts guys? What type of foam is most suitable for this? Are there any special preparations that have to be done prior to gluing foam to wood? All input is most welcome. Thank you.