Hi. I am adding some insulation (blown) into my attic. Before I add the insulation, I have read that I should seal gaps (the stack vent, electrical wires coming up through holes, fxitures, etc)...
Can I use this "great stuff" a foam that I spray out of a can and then hardens to seal the gap...
Can this be used in those spaces above? Will the gap harden so that the vent becomes rigid and does not flex? Should it flex? Also, can it be used to seal the gaps made when I cut the hole for the bathroom fan? etc... thanks

I've used that stuff and it works great. Just be sure you don't try and do a large area all at once. I found that if you do 1/2 of the area, let it cure and then go back and finish the hole you want to seal up, it works just fine. Start with an easy place to fill in and see how it goes for you. The one thing I don't like though about the stuff is that once you start using a can you need to finish the whole can. Don't even think about trying to store a half empty can and using it at a later date. Won't happen as it does not have enough umph in it's wheaties left in the can.

This stuff is great for most of the instllations you mention. It expands and actually has some force behind that expansion. That makes it good for vent stacks, wires but not for the fan box. The expanding foam can deform the fan box but you can buy similar stuff that doesn't expand much at all. We use the non-expanding stuff around door frames and windows that would get too tight if the foam expanded.

This stuff is great for most of the instllations you mention. It expands and actually has some force behind that expansion. That makes it good for vent stacks, wires but not for the fan box. The expanding foam can deform the fan box but you can buy similar stuff that doesn't expand much at all. We use the non-expanding stuff around door frames and windows that would get too tight if the foam expanded.

... what could I use for the gap around the fan box? Could I usesilcon sealant? Caulking?

You must have missed my last sentence "We use the non-expanding stuff around door frames and windows that would get too tight if the foam expanded." Similar can, similar product but labeled for windows and doors, it just doesn't expand much and has no force behind it.

You must have missed my last sentence "We use the non-expanding stuff around door frames and windows that would get too tight if the foam expanded." Similar can, similar product but labeled for windows and doors, it just doesn't expand much and has no force behind it.

You'r right... I missed that sentence... I am going to tackle this job this week... armed with "great stuff" for doors and windows... most of the gaps are smaller (an 1" or so)...

Could I seal the exhaust fan box to the drywall from the inside? And then cover it with the grill? I would use clear silicone to seal the gap (that I see from the attic side)... then, that gap would be filled...
What do you think, ballenger?

I think you can fill that fan box area from the inside. Let it set up and cure and then trim offf and overflow with a hacksaw blade or serrated knife.