Will packed Roxul make a good seal?

[TradEddie]

I need to seal the flue above my insert, I have no issues with my insert, but I have a significant downdraft of cold air coming into the room when it’s not running. The space and access restrictions mean that while I can make a two-part block-off plate to cover most of the area, those plates will not meet in the center and I will still need to seal a large gap between the liner and the plates. Will packed Roxul around the liner be enough to create a good seal?
I’ve looked at all the block-off plate posts and wikis, but those seem more concerned with retaining heat when running than sealing air when not. I don’t run the insert on weekdays, so this could be a big heat loss. My old slammer had a much better seal at the surround, but I'd like to do more than that.

TE

 

[weatherguy]

Yes, roxul is the best way to do that, thats what I used and it made a big difference. Make the BOP too when you have time, that will make a difference too.

 

[mellow]

Yep, works great as a soft block off plate. If curious you can see mine in the link in my sig.

 

[TradEddie]

Well, roxul went in tonight, and the block-off plates are held in by friction until I can run a drill without waking anyone. Can't feel any draft at all now, with temps below 30, so it's already an improvement. I've been keeping a close track on my overnight interior temperature drops, hopefully I can actually see a difference in the AM.
No pics of this, it's not my neatest work.

TE