myredracer wrote:
agesilaus wrote:
Styrofoam absorbs water?
It will depending on type. The open cell rigid expanded polystyrene XPS type absorbs moisture and moisture migrates through it. It's the white stuff disposable coffee cups are made from. It's the type that is used in exterior walls of RVs and is a really bad choice because of moisture migrating through exterior walls to a cold exterior skin and condensing.
Closed cell rigid extruded polystyrene EPS type does not absorb moisture and cannot migrate through it and acts as a vapor barrier. It's the pink or blue or sometimes green stuff you see in building supply places. It is denser and has a much higher R-value but is more expensive and is why it's not used in RVs.
Both types come in spray or rigid sheet versions. A better rigid or spray foam is icynene which has a higher R-value but costs even more. It's not fun to work with and makes a nasty dust when cut.
I believe you have that backwards....
XPS (blue/green/pink at big box stores) is considered non-absorbing and EPS (white) is water absorbing.
Owens Corning XPS vs EPS water absorbing foam
