How to seal old chimney thimbles?

[MainerGuy]

I was informed by a chimney sweep that before I could have a new woodstove chimney insert installed both of these old thimbles need to be capped off (special insulated cap) or “masoned.”

Since the insulated caps are expensive (I saw one for $500) I thought I would just pull the old thimbles out and seal them with refractory cement.

Any thoughts on my approach?
Should I prep the sites somehow before applying the cement?

Thanks

 

[begreen]

It should be sealed with fire brick and refractory cement.

R

Thread 'Cap off second thimble'

Apr 23, 2026

I have a terra-cotta chimney flue that turns out has two thimbles, one in the basement and one on the second floor. The one on the second floor is A) a nuisance and B) quite likely illegal. It's a standard 8" sheet metal thimble that's in a hole in the cement brick, and the flue liner is clearly visible when the sheet metal thimble cap is removed. It orignally had brick around it as a heat shield, thankfully. The thimble moves around in the chimney, like it's not even attached, but given the gaps and utter lack of seal on the cover, I'm not thinking that's a significant added source of...

• Ragnorok
• Replies: 4
• Forum: The Hearth Room - Wood Stoves and Fireplaces

 

[bholler]

As long as it is getting an insulated liner in this case any brick or block and regular mortar would be fine because it wont be exposed to exhaust

 

[begreen]

But if the liner is not insulated, then...

 

[bholler]

But if the liner is not insulated, then...

Then just patching with regular mortar and masonry would be the same as the rest of the chimney. The firebrick and refractory are only really needed when the clay is still being used