Sealing near insert or top of chimney flu

[SantaCruz22]

Hello, I have put an insert in with an 8" exhaust where old fireplace use to be. In the process of clearing an area for the 8" flex pipe, I have a pretty big hole to seal near the exhaust pipe at insert per my picture and also a giant fireplace area. My question is, can I seal at the top of the chimney flu where the pipe comes out and what materials would you use for that? Would sealing at top be just as efficient with keeping heat in the house since there would be a void from top of chimney down to insert? I would much rather do this option because the it is a huge space just above the insert.
If sealing near the top is not as good of an option, what do you recommend for this huge gap in the picture and what materials?
Thank you so much

 

[EbS-P]

You want an insulated blockoff as close to the insert as you can and you want to seal the top cap to the chimney. Insulation around the top of the top of the chimney just absorbs moisture and falls down.

A sheet metal blockoff plate sealed well (high temp silicone) with mineral wool above / resting on it, is what to use. The sides and back of the insert can also be insulated with mineral wool the the front surround added. Good air sealing isn’t necessary there as you already sealed the block off plate.

 

[SantaCruz22]

You want an insulated blockoff as close to the insert as you can and you want to seal the top cap to the chimney. Insulation around the top of the top of the chimney just absorbs moisture and falls down.

A sheet metal blockoff plate sealed well (high temp silicone) with mineral wool above / resting on it, is what to use. The sides and back of the insert can also be insulated with mineral wool the the front surround added. Good air sealing isn’t necessary there as you already sealed the block off plate.

Thank you for this. I have some aluminum sheeting in the back shed. Would that be okay to use? Also, I found this stuff as an alternative to the wool that is a fraction of the costs, will this work instead of the mineral wool. Here it is:

24" x12" x1" (Thick)Ceramic Fiber Blanket Fireproof Insulation Baffle Rated to 2400F, High-Temperature Resistance for​

 

[EbS-P]

The ceramic is nastier to work with and requires you to use a mask/respirator when handling. It should not ever be left exposed. Not a big deal here.

I would want more than 1” thick, but I think the difference would not be noticeable between 1 and 4. I got a pack of this (see link) and used probably 1/3 of it For each stove. I stuffed it up around the insulated liner then a full piece on top of the block off plate. I still had some leftover. I use that to start garden seeds now. R value to R value the rockwool is a better buy. You can use the ceramic if you want. Any insulation is better than none.

Your friend has shared a link to a Home Depot product they think you would be interested in seeing.

Owens Corning R13 Thermafiber Fire and Sound Guard Plus Mineral Wool Insulation Batt 15 in. x 47 in. 1196588 - The Home Depot

Thermafiber Fire and Sound Guard Plus mineral wool insulation is designed to provide excellent thermal, noise control, and fire resistance in residential and light commercial construction applications.

www.homedepot.com

 

[SantaCruz22]

Thank you for this. I have some aluminum sheeting in the back shed. Would that be okay to use? Also, I found this stuff as an alternative to the wool that is a fraction of the costs, will this work instead of the mineral wool. Here it is:

24" x12" x1" (Thick)Ceramic Fiber Blanket Fireproof Insulation Baffle Rated to 2400F, High-Temperature Resistance for​

Awesome!! Thank you so much for your kind detail. I'll do as you recommend.