Insulation Until Blockoff Plate

stuff some unfaced fiberglass insy up in there till you get it... it won't look pretty when you pull it out, but it'll do.

Um, I think I've read here before atleast 100 times that regular fiberglass insulation won't hold up to the temps. DO NOT use fiberglass insulation instead of a block off plate.

I did some digging just to make sure what I said was correct. I guess this topic is like Chevy/ Ford Husky/Stihl

read these threads........

https://www.hearth.com/econtent/index.php/forums/viewthread/41863/

https://www.hearth.com/econtent/index.php/forums/viewthread/48300/

https://www.hearth.com/econtent/index.php/forums/viewthread/57417/


I wouldn't do it!

Rather than fibreglass insulation, you can use mineral wool - I have roxul (with no blockoff plate). I also plan on installing one at some point but for now the set up is working. I'm sure it would be better with the blockoff plate, though.

The two main reasons most cited for not using fiberglass seem to be 1) possible outgassing; and 2) melting in the event of a chimney fire.

Any outgassing likely results in a fraction of whatever normal levels of toxins we are exposed to everyday - so yes, you can reduce your potential exposure, but there are hundreds of other ways to reduce minor risks that we don't bother taking. I have come to believe this one falls into the category of the irrelevant, but that is a personal choice.

Fiberglass should not be used in any application designed to provide protection from a chimney fire. But most of the applications discussed, as the one here, do not have that purpose. So again, I believe the fact that fiberglass melts at 2100F is irrelevant here. Even if rock wool is superior, that does not mean fiberglass is unacceptable.

From a practical point of view, however, consider that any spun fiber insulation will not stop air flow, it will only slow it. So while either rock wool or fiberglass is better than nothing, they will not fully serve the purpose of a blockoff plate in terms of either safety or heat loss. I think the mistake that can be made here is in assuming insulation, of whatever type, will serve a function for which it is not suited. For heat loss it's a toss up, but if you desire your insulation to have any safety purpose at all, there is no doubt that fiberglass is inferior to rock wool (which is used commercially as fire-safing).

Sealed blockoff > rock wool > fiberglass > nothing?