Originally Posted by Brian A. MacNeish http://nachi.cachefly.net/forum/images/2006/buttons/viewpost.gif
Ideally, both metal and masonry chimneys should be airsealed for energy conservation purposes (air leakage) and to stop house moisture getting to the attic with the escaping house air. With the metal types, check with the manufacturer as some are still tested with the loose attic insulation shield/firestops and should be left as is. Others were tested for inclusion in the R2000 program (or regular houses, if you wish) where airtightness is required due to the blower door test for certification.
You can insulate around masonry chimneys (after airsealing) with mineral or fiberglass batts…no foam or cellulose directly against the chimney. You can wrap the masonry with a 3.5" fiberglass batt and then blow cellulose up to the batts.
***Don’t you still need a 2" clearance to combustables? ***
Yes, in the framing around the chimney but you’re not going to get that in most older homes…and it may not be cheap to re-build/frame to get it.
See “Keeping the Heat In” from Natural Resources Canada or “Caulking, sealing, and Weatherstripping” from my old gov dept in Nova Scotia. I was involved in the upgrading of the energy brochures.
***Fibreglass is not considered non-combustable, correct? ***
Yes, unfaced fiberglass batts.
You can’t insulate against any metal chimneys that I know of…that’s why the attic insulation shield is required…to maintain the open 2" space required by most manufacturers here in Canada
What is the concern? Airflow through the 2" airspace, or just 2" clearance to combustibles? (and creating hot spots in the metal exterior shell) If its airflow around the chimney, wouldn’t airsealing be a bad thing for the chimney in this case? Yes, if the particular metal chimney system was not tested with an airtight firestop/attic insulation shield.
Funny, Rodney, I just added to another thread about this 2 hours or so ago:
The vent manufacturer requires clearances from “building materials,” not just combustible materials. The vents won’t draft as effectively when there’s a hot-spot created by insulation or other building materials.
"The vent should work better as the insulation keeps the heat in the pipe allowing exhaust gases to stay hotter thus creating a lighter, more buoyant gas and better draft.
The fear with creating a hot spot is that if it is:
- hot enough
- and can emit IR radiation to nearby wood products,
- by pyrolysis over a longer period of time,
- the ignition temperature of the dry wood can drop from 800-900 deg F to the 375-400 deg F range."
From a paper on pyrolysis:
"However, it is possible for smoldering or glowing to exist prior to flaming ignition if the imposed heating causes the wood surface to reach 200°C (392F) or higher for the second regime of wood pyrolysis" *