Foam Gap Filler and Fire Seal
Great Stuff Pro Gaps & Cracks (and Fire block)
http://www.amazon.com/Dow-Great-Stuff-Cracks-341557/dp/B0013WME6C $8.99 + $4.50 shipping
I use three different types of foam depending on how much expansion
No expansion - Foam Adhesive
Small expansion - Foam Windows and Doors
Large expansion (and fire seal) - Foam Gap Filler and Fire Seal
GREAT STUFF PRO™ Gaps & Cracks - As the first foam sealant to be recognized as a fireblock, GREAT STUFF PRO™ Gaps & Cracks inhibits the spread of flames through concealed spaces. It’s a minimal-expanding, single component polyurethane foam sealant for general purpose building envelope air sealing.