RichK wrote:
"dpb" wrote in message

Virtually any RTV silicone is good to well over 200F.

I don't think there's any chance at all the temperatures at that point
will be a problem for any high quality silicone caulk.

How deep is this crack and is there anything backing it to help fill the
void?

It's not a crack, but a 1/4" (or so) gap, the depth of the crown, left by
the mason, between the flue and the crown. When the cap mortar sets, I want
to fill that space with some flexible sealant.

Some people recommend using this method to prevent crown from cracking.
Sounds good, but now I'm having problems finding something easily available
to use for that purpose. DEFY product is expensive and sold 1gal
quantities.


Fire rated from DAP...note even here its service rating is 160F
http://www.dap.com/docs/tech/00077364.pdf

DAP Premium Roof and Flashing Sealant
http://www.dap.com/docs/tech/00077348R.pdf
Physical & Chemical Characteristics:
Volatile: Xylene
Solids: 90% minimum
Service Temperature Range: -40°F to +185°F


Note it's rated higher actual temperature than the fire-rated product.

"High temperature" is only used/needed around the actual outlet of a
stove or similar heating source into the exhaust duct or chimney and
will typically be rated at something like 2000F, not 200F.

I've used just ordinary "off-the-shelf" clear adhesive caulk/sealant for
making probes into large diameter pulverized coal feed pipes in power
plants operating w/ air temperatures of roughly 190-200F and had no
problems with them withstanding the positive pressure and temperatures
for weeks.

There's no way the temperatures at the outside of a chimney flue at roof
level are going to approach 200F during weather during which a
fire/furnace will be operating.

I think you're into the way over-specification region here for an
unwarranted concern.

As for the "crack", I understand the gap, I was trying to find out what
was behind the gap to judge the difficulty of filling the gap. If it's
more than 1/4" deep or has no backing I would get some of the gap-filler
material for backing caulking an insert it first.

HTH...

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