Great stuff foam for chimney seal?

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Burn-1

Feeling the Heat
Jul 13, 2006
446
Lakes Region, NH
I finished a new liner install this past weekend. It went well but one of the issues I had is that the chimney flue is 12x18 rather than 12x12 so the 13x13 flashing top plate in the kit wouldn't work. The existing chimney had a concrete cap elevated by a couple bricks in each of the corners so we knocked the bricks down and the cap fit fairly flush on the top of the chimney. We cut a square hole in the cap and fit the liner through that and put the top plate on the cap with some RTV and that part of the install really went well,(hat tip to MALogger for the idea).

But, there are some small gaps between the concrete cap and the top row of bricks it sits on. Some of the gaps are fairly small like 1/8" but some are closer to 1/2". I'm looking at options to seal up the rest of the chimney in order to keep the wind and water out and keep the liner temp from cooling too much although it is insulated with 1/2" ceramic wool from above my damper to 1 foot below the cap. I was wondering if it would get too hot in the general area to use Great Stuff or some other expanding foam even though the liner is insulated and there is a fair amount of clearance on the 6 inch liner in a 12x18 flue. The foam would be the easiest to apply. I was also thinking maybe RTV or concrete caulk but those would be harder to apply. I was also thinking of just stuffing insulation in the voids as well. Any ideas are appreciated.
 
I assume you mean the outside perimeter of the brick and cap. If so and it was me. I would use 100 percent silicone. It will last the longest. The expanding foam degrades when exposed to UV. By the way. If you are talking about doing it from the inside then there is a Great Stuff Pro formula. It is rated as a fire stop for holes through fire stop blocking. However it requires the purchase of the gun to apply it. $29 at the home depot for the gun and about $10 for the can of foam.

James
 
I'd go with the RTV, and just seal it up and be done with it. The foam stuff really doesn't last that long and degrades with exposure to the elements (even if its ok with the heat... not sure). Insulation will likely get wet and cause problems later down the road.
 
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