Sealing Furnace Flue

I'm trying to seal a rather large gap around my furnace flue. The house is on a slab so the furnace is in a closet, the flue goes up into the ceiling, through the attic and out. It's in a really tough place to get through in the attic crawl space. I've managed to cut and lay down some flashing around it, but it doesn't fully cover the gaps. Is there a high temp foaming/expanding sealant I can use to get it done quickly and easily?

How big are the actual gaps? Fill with steel wool, then high-temp silicone sealant (RTV), if big. If small, just use the RTV.

Joe

The gaps about a half inch to 1/4". I like the steel wool idea! The problem I'm having is the working space in the attic is too darn small! I was hoping to find a way to seal it from the inside of the house up into the attic. Any ideas?

There's a product made of rock wool called "zero clearance board." It's about 2" thick sort of like compressed fiberglass insulation, except made of rock wool, so it is fireproof. It's rated for zero clearance to flue pipes and such (hence the name), and would be suitable for filler that would be in direct contact with the flue pipe.

Joe