Hi all,
Please take the scientific sound of this post with a grain of NaCl as it is from my memory of sprayfoaming my header space in my basement and other renos.
Most of the foam in a can type are isocyanurate foams, a polymerization process that produces a closed cell foam. Insulation values are high and they also provide a vapour barrier making them a great insulation. The insulation values between the low expansion and high expansion is insignificant in the case that you gave. That is, you should
use the high expansion low density foam for your suggested case because it will cost less and be easier to use. Also, the pressure built up by the low expansion high density version can pop drywall off of walls so low density version should be used for filling any enclosed space. Best practice says to allow one can to fully expand and cure before using the next in the same area because the chemical process at work produces heat. Sometimes this type of foam is labelled as suitable for windows and doors because the low pressure build up will seal but not bow the window/door frame.
Other extraneous information: Even the fire retardant added ones are not good in a fire (the fire retardant decreases/stops the spread of fire/flame by them but the smoke produced by heating them is still mucho toxic). That is, don't breathe unless the other option is death by another route so avoid using them anywhere in any quantity larger than just filling cracks and holes unless they are covered by a fire stopping material such as drywall. A brick size chunk of polyurethane foam that is in a fire situation can fill a small house with not-so-good-for you smoke. Anyway that is just an aside for those who, like me, try to take into account all of the factors in a decision (and sometimes therefore never get around to accomplishing the task
.
Cheers,
Benphyr.