A chimney/flue passing through a second floor ceiling into an attic space:
Should it be insulated and/or airsealed around the ceiling penetration? This has been a debated topic in my office for a few days now. I’m just looking for some more ammunition for my arguement. Any thoughts welcome.
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.
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
Brian,
I’ve been on both sides of the fence over this one. I have gotten opinions from various people that insulation/airsealing can create hot spots and a potential hazard. Others say not to worry about it. I look at it from 2 sides: 1 as an Energy Efficiency advisor, the other as an inspector. I’m just trying to make sure the solution is safe first, energy efficient last.
Quote:
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:
Originally Posted:
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.
My response:
"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:
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" *
So…all that being said, metal chimneys: I got that one. Masonary chimneys is where I have the issue. Here’s my thoughts:
From an energy efficiency standpoint: Non-combustable insulation and airsealing should be done for heat loss and moisture reasons, etc, etc.
From an inspectors standpoint: 2" clearance to combustables required (which I understand most pre-existing homes don’t have, and it is not feasable to re-frame). Also the issue with insulation creating potential hot spots. So, better off leaving well enough alone.
My opinion is safety over efficiency, which in this situation is the case, is it not? Insulating/airsealing a masonary chimney may cause a problem. Or am I just beating a dead horse?
Methinks the horse is and has been dead for many years or else the government publications would not be allowing these methods. I’ve seen it done since the 1970’s-80’s and haven’t heard of any problems.
I used to teach my own expanded version of “House-as-a-System” and house pressures for all the WETT courses here back in the 90’s when a Deputy Fire Marshall was the main trainer. We went through this discussion then…he’d seen yearly reports on the causes of fires and this was not a problem with chimneys in good shape (no mortar missing) with intact liners.
Now steel chimneys were a different thing… quite a coincidence happened one year when he told me about a $330,000 large rural farmhouse fire loss. The homeowner installed his own steel chimney serving a kitchen wood stove up through the first floor kitchen ceiling/unused second floor kitchen chamber floor…quite common here in the Maritimes. An insulator then retrofitted the house with cellulose and filled the ceiling/floor cavity over the kitchen…but the owner did not install an insulation spacer and…the insulator didn’t notice this. So about 3 months later during very cold weather…a major fire and total loss. Turns out I knew the insulator and eventually got a call from him to see if I could help him on the owner installation issues!!! I’m not a fire investigator/specialist so declined!
From talking to the fire marshall though, I thought that there was something fishy about the fire and loss due to a few things he said…but that is not my field so didn’t press my points. In the end, the 2 insurance companies split the loss!! I have heard from another wood stove dealer in the province about cellulose blown against a steel chimney serving a wood stove resulting in a fire! Be careful in your work!
Thanks Brian. I’m gonna bury the horse now.