The most common mistake in working with PU foam sealant is not moistening the joint enough.
Foam cures under the action of moisture. What happens when there is not enough of it is what we set out to show in the experiment below.
We filled two cups with the same PU foam sealant. In one of them the foam was moistened, in the other it was not. All the other conditions were identical, yet in one cup the foam cured and in the other it did not. This is why the joint must always be misted with water before it is filled with foam, and then misted once more after filling. And the joint must not be covered with vapor barrier tape until the foam has fully cured.
