foam inside cinderblock wall

[donkarlos]

so maybe this is over the top, but my direct vent runs through my basement cinderblock wall - one block down from the sill. I was wondering if heat might buildup inside that space and get too hot or otherwise cause some kind of heat damage, so I thought about using some of that fire-block insullating foam to spray in the block cavities. btw, my basement ceiling is finished with sheetrock so I can't otherwise see the top of the block wall.