Sealant for air sealing masonry chimney chase
To air seal between framing and an interior masonry chimney, does anyone have product recommendations?
Code requires a noncombustible caulk and it seems this is interpreted as meaning ASTM E136 rated, which is very limiting. They have to survive 1380 F temperature in a furnace for 30 minutes, which seems like extreme overkill for this purpose. Also, I’m dubious of the long term effectiveness of these products as air sealing agents.
The product sheets describe them as ‘silicate mortar’ or ‘latex mortar’.
The 3M product FB136, says in it’s data sheet:
“This product is not flexible when dry and may not be suitable for application to surfaces where vibration, expansion and contraction is present.”
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The solution I'm going to try out is securing the metal flashing like a term bar. I'll put sealant between the flashing and cinder block surface and then secure the flashing with tapcons through the sealant into the cinder blocks. The flashing will be an L shape - about 1-1.5" bent 90 degrees to provide the sealing surface and then the rest of the flashing will extend out to bridge the gap between framing and the cinder block.
I'm doing a 'garage test' with a cinder block and I'll be trying the 3M 136 sealant as well as Silicone RTV sealant with 500F continuous operation rating, to see how well it sticks and how durable the joint is after curing.