As you research different types of foam you will often hear “open cell” or “closed cell” thrown around. Both types of foams have their place, but they most certainty are made for different applications.
All foam is made up of cells. The cells hold air. That is what gives foam its’ cushion. Open cell foam consists of cells that are all interconnected meaning air can travel freely through the foam. This can be seen with a household sponge. Because the cells are all connected, it allows water to be absorbed within the foam. (When submerged the air is replaced by water)
Closed cell foam on the other hand also has cells, but the cells are not interconnected. The air trapped in those cells cannot freely move between each other. This can easily be observed when swimming with a pool noodle you might find at a pool. Because the cells are all closed, water cannot absorb into the foam making this type of foam required for water applications.
Each foam has pros & cons. When it comes to cases we tend to choose the best foam for the job. If the foam is going to be deployed in wet environments or the gear going into the case could be oily, closed cell foams are the best choice as liquids will not be absorbed.
Alternatively if the foam is to be deployed in varying altitudes it is often wise to use open cell foam as the air pressure within the cells will remain consistent with the surrounding air pressure. Using closed cell foams in this situation could result in foam which changes shape due to expanding and contracting in various altitudes.