How to deal with concrete block chimney
Zins
| Posted in Green Building Techniques on
Rennovating a home that has a large 1:12 roof. In the middle of this roof is a 7’x3′ chimney stack for a woodstove, the stove vent also goes through it.
What are some approaches to dealing with bridging and moisture penetration from it?
Some kind of membrane and foam? I don’t know about temp issues.
Planning an unvented flat roof. EPDM, 6″ ISO, air barrier, deck, OCM, vapor barrier. Probably R50 or so.
Hoping to get the the rest of the home reasonably well sealed and insulated.
Thoughts?
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Unless you meet certain requirements, you need at least a 2" clear air space around the chimney, which many people want to fill with insulation but it's not safe to do so. But you can make that space airtight, by installing sheet metal across the opening and sealing it to the framing and the masonry. That will have the greatest impact with the least effort.
Beyond that, you could wrap the exterior portion of the chimney with framing and insulation (maintaining a 2" air gap), and/or you could do the same on the interior.
Thanks, Michael. This is very helpful. Are you aware of any articles here that talk about this more extensively? I've had trouble finding good information.
The basic principle sounds simple enough. Frame a 2" gap around the stack, insulate and seal.
What about doing that on the exterior, well sealed and insulated (with 2" gap) , to limit the worst of the and for the interior portion applying some kind of masonary facade? Could that be applied directly to the block our would I need to leave an airgap again?
The built-in search function doesn't work very well but this is what Google found: https://www.google.com/search?q=greenbuildingadvisor.com%3A+insulating+chimney&rlz=1C1ONGR_enUS1048US1050&oq=greenbuildingadvisor.com%3A+insulating+chimney&aqs=chrome..69i57.4499j0j7&sourceid=chrome&ie=UTF-8#ip=1
When in use, masonry chimneys can get very hot so it's important to leave a 2" air space all the way from the combustion source to open air. You could frame around and insulate the chimney above the roof plane, and seal the top with a full sheet metal cap, which is good practice for large chimneys to keep them from absorbing stormwater.
7 feet x3 feet is a huge and it is in a prime location.
Is the plan to use the stove?
You would have a very different home without the chimney.
Walta
Zins,
The presence of a very low R-value chimney may inform what improvements it makes sense to do to the rest of your roof. As Allison Bailes shows in this article on attic access hatches, these "holes" in your building assembly have a disproportionate effect on lowering the overall insulation value, lately negating the gains more insulation elsewhere yields.
https://www.energyvanguard.com/blog/attic-stairs-a-mind-blowing-hole-in-your-building-envelope/
How I dealt with a masonry chimney through an unvented roof with 6" of polyiso insulation:
I had a bit of a different situation than you, as it was an 8:12 pitch with asphalt shingles, a layer of new plywood sheathing, 6" of polyiso, and then air barrier over original sheathing. Flashing the chimney at the roof plane was done normally by the roofers, with sheet metal flashings & counterflashings. Where the chimney passed through the insulation layer, we ripped 2x stock to 6" height on the sides, and 6" parallelograms on the top/bottom edges, and basically built a little "box" around the chimney, with ~3" clearance all around. I didn't feel comfortable leaving edges of foam exposed in that area, so the wood "box" covered the foam edges and ensured the clearance would be maintained.
My situation was a bit more complicated, because the chimney was on an exterior wall, so it actually transitioned the air barrier a bit lower after being boxed on on the interior (we framed a 2x4 wall, on the chimney side of the wall we had a 2" air gap, then 2" of rigid mineral wool boards, then the 2x4 wall). Where we transitioned through the air barrier, I bridged the gap with some sheet metal that I bent up against the chimney, screwed in place with masonry screws, and then sealed to the chimney and all the seams with high-temp silicone.
How I would have liked to deal with the chimney if I had the time and budget: Tear the darn thing down, install a class-A metal chimney from a system that has off-the-shelf pieces for transitioning air barriers and insulation layers if a wood-burning appliance is desired, and use direct-vent gas appliances if necessary.
My take is that a masonry chimney can be dealt with if necessary, but think carefully about whether it's actually necessary. If it is an important aesthetic or architectural aspect, work with what you've got. If it's purely functional, evaluate whether you have the time / energy / budget to remove & replace with something easier to deal with.
One note: The flashing / sealing details will be tricker on an almost-flat roof, and I'm not sure how that's typically handled. Probably some sort of fabricated curb / metal flashing thing.
jonny_h,
Good advice.
As the owner of a 3 x8 chimney I am going with tear it down below the roofline and replace with metal insulated chimney pipes .
Roofer won't take responsibility for leaks at the chimney and two pipes are a lot easier to water seal than all that chimney perimeter, oh, and the top surface can leak, making it look like your brand new roof is leaking, ask me how I know.