Dense foams have what’s
known as a closed-cell structure, which means that the
gas bubbles that form during the application process
remain permanently locked into the cured foam. The result
is something like a three-dimensional bubble wrap with
extremely tiny bubbles. Because there are no interconnections
between individual bubbles, the foam absorbs little water
and also resists the passage of water vapor. According
to most building codes, a vapor retarder must have a
perm rating of less than 1.0 and some foams meet this
standard.
Low-density open-cell foams, on the other hand, have
a structure more like a very fine-grained sponge. The
cured material consists of a series of tiny interconnected
passageways. These open cells are too small to permit
the passage of much air, but they are more permeable
to water vapor than closed-cell foams.
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