They call the foam in a can like this one part but to cure it really needs a 2nd part and that part is water, normally it gets that water from the air, thus on humid days it will expand and cure faster, get some on your hand and it will cure fast, because of this you have to layer it or it will never set up.
For going under something like a boiler I would think that a 2" thick foam sheet would work, as long as it did not get to hot of course, for higher heat use fire brick or insulating concrete.
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