Styrofoam vs fiberglass matt insulation

The bleu and pink foams will also had rigidity to the structure, you can also fill the gap with spray foam, the less expandable one.
Gaetan

.....like Buzzcut suggests, don't fool about with anything other than closed cell foam. There are MANY block foam compositions out there. Check the PERM rating and compressive strength (ASTM D1621) for the foam FIRST before settling on ANY for product (Water Absorption, ASTM C272, % by volume, max: 0.3 or better) ! Also, if the better PERM comes from a thin plastic veneer over the foam board, ask yourself: what if this thin veneer is scratched, perforated or abraded? If this happens, then you have just usurped your PERM/vapor barrier spec!

Remember this: try and build a toy boat out of glass wool (if you can) and a 2nd toy boat out of XPS closed-cell block foam, drop them in the bath, and see which one survives after a few hours. Now, remove from tub, and dry both off with a hair drier (you've only got 1 minute to dry both: which one will not dry, even after days?).

When glass wool takes on even a small quantity of humidity (forget about water!), it will virtually lose ALL its insulating value.

a third choice that is waterproof, lightweight, insulates well and easily cutable to fit the spaces is closed cell foam. You can pick up 4'x 8'x 1" sheets at most stores that sell foam for $75 per sheet.

Foam sheet are much better in campers. If the fiberglass gets wet, it loses it's insulation factor. Tends to sag over time from vibration and gravity. plus, when installed with pressure, foam also can provide rigidity to the structure, another plus.

Foam insulation has many variables which determine it's R value. The cheap white bead foam board is lowest, but high density foam actually surpasses fiberglass. 4" of high density with foil siding can exceed R-30 while the best batt fiberglass is R-15 for 3.5". If you want better insulation, do some research on different foam types and select a better one, though it will cost more.