bldginsp
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# Posted: 18 Oct 2016 02:14pm - Edited by: bldginsp
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I'm pretty sure rock wool is considered non-combustible, so theoretically the flu could contact it. But always good to have clearance around the flu to allow air to circulate a bit to avoid heat buildup next to the flu.
DaveBell- its true that confined air is a good insulator, that's basically what all insulation does. But if the air can move, or if there is any draft, you lose the insulating effect. What insulation does is hold the air and prevent it moving.
In a situation like this I'd first seal up the masonry as much as possible with trim, caulk, mortar, etc. then put a layer of rock wool in there, leaving space around the metal flu. Most double and triple wall metal chimneys require 2" clearance to combustibles, so you could use fiberglass if it's held back.
Also, if you put the woodstove on the hearth in front of the fireplace you might not have the required 16" of masonry in front of the stove. Also, the hearth in front of the fireplace is generally built on wood. If your stove is made to go on such a hearth, as most are, you are fine, but if the stove you are using is an insert that is not inserted, it could be a problem. Inserts are rated to be placed on the inside of the fireplace, where the hearth has no wood beneath it.
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