Good morning everyone,
I’m located in zone 4, and predominantly a “cooled” environment (HVAC is cooling the house more than warming it throughout the year). We are adding a small woodstove and installing the wall thimble for the flue to exit horizontally.

Looking at the thimble, there is a lot of air space around where the flue passes through, and my concern is how that “air cavity” might adversely affect thermal transfer in seasons when the stove is not in use. Let’s presume that we have sealed up the thimble and flue appropriately, and that the penetration is completely airtight. 

Is there any consideration to using something like Rockwool’s ROXUL Safe product (link below) cut in such a way that there is a space for the thimble sleeve to pass through? (see the attached “pattern” image)
ROCKWOOL ROXUL Safe® a semi-rigid, mineral wool batt insulation designed to provide enhanced fire protection | ROCKWOOL

The building code and installation instructions reference a minimum distance to combustible materials…if the ROXUL Safe is considered non-combustible, does that mean it would be suitable to use next to the flue to insulate this void?

In the end, it is a minor detail to insulate the void. I will err on the side of caution if I can’t find a definitive answer on this (I have also reached out to the mfg, but have yet to hear anything) and will be content in making sure the thimble and flue penetration is airtight…but it would be nice to take care of this if it is an option.

Thanks!