The thermal expansion coefficient is the same for all eps densities: 0.000035 in/in (degree F) linear, and 0.001/cubic inch/degree F volumetric. As a typical SUP has a volume of 16K cu inches, this means internal volume will increase by 16 cu in. for every degree increase in temperature. Bouyancy is also the same between 1-3 lb density; 50 lbs /cu ft. The only differences when density changes upwards from 1 lb/cu ft to even 10 lb/cu ft are the insulating properties (not applicable), strength properties (not applicable after glassing), and moisture penetration (not applicable). These are the ASTM 578-92 material spec's that 98% of all eps is made to. http://northwestfoam.com/fact-specs.htm

So the only difference is the finished weight (design attributes like flex and such excluded). However, this assumes that the glassing or shell construction is far stronger than the eps core strength properties. With plain glassing this isn't the case, and the core contributes to the total strength. Compression, shear, and flex strength for 2 lb is almost twice that of 1 lb. Experience does show that 2 lb is acceptable with plain glassing, but 1 lb isn't. However, the use of carbon fiber, sandwich construction, etc. to increase the shell strength makes 1 lb a viable option. The trick is to maximize the shell strength to weight ratio to take advantage of the 1 lb core. In 14+ ft SUP's, 1 lb is a huge difference over 2 lb. Keep in mind that spec'd density is really the average density over an enormous block; so most "2 lb" foam is actually about 1.75 lb and "1 lb" about 0.80 lb.

In terms of eps characteristics, bead size doesn't matter nor any other geometric factors in terms of density. You basically have X amount of air + Y amount of bead gas + Z amount of polystyrene in whatever shape. To get different densities, Y and Z are varied and X fills in the total. The real story is rather complex and unecessary since all we need to know is what is available in eps densities and the strength properties relative to how we're going to glass it.

Taking an eps/epoxy board on a plane is irrelevant since all cargo holds are pressurized like the cabin. Hell they put pets in cargo. So unless your airline has roof racks, this isn't an issue. It is a simple fact that if you seal 14.7 psi in a container and take it up to 7000 ft where the air pressure is 11.34 psi, the internal container pressure is now 3.36 psi higher than ambient. That's no different than heating the container at sea level so that pressure increases to 18.06 psi; if the container can take it then everything is OK. Solar radiation is also stronger at altitude, so you can have a double whammy effect.

I must agree with Greg that eps is not a "crappy" material, and that it adds a new, big dimension to design. Ask any of the major industry players where they would be today without SUP sales, and this wouldn't have happened without eps/epoxy technology and Greg himself. As with anything new, we have to first understand the processes needed to work with it, and in parallel understand what happens to the finished product after the average dumb ass buys it. To obtain this understanding, you can: (1) Guess, (2) Copy somebody else, (3) Figure it out analytically, (4) Or like some import manufacturers, copy somebody who already did figure it out analytically. However, you will not truly have the understanding unless you pay the dues and find out for yourself. This knowledge is what leads to the cutting edge.