ICC insulated chimney and a vented chase
Hi,
We’re detailing our woodstove installation in Nova Scotia (Climate Zone 6)
We’re working with a local chimney shop to order parts and approve our plan. But we’re doing the installation ourselves on our 16×20 building.
It’s 2 storey with a non-vented cathedral ceiling with above the roof foam insulation.
We’re seeking advice here about our chase. We plan to make our chase airtight but we are not sure if we should insulate it inside the building.
Here’s a shortened list for our installation (I’ve included a rough drawing)
– Jotul F602 Woostove
– Double Wall (sealed) Stove Pipe
– Insulated Round Support Box
(We’ve called the manufacturer and they approved our idea of wrapping the chase with a smart vapour barrier (Intello or Majrex) from the cathedral ceiling to the floor where the stove pipe meets the chimney. They give the option of a regular Round Support box or an Insulated Support box that allows to seal to the vapour barrier. It looks like it’s the only option to make our chase airtight.
– Insulated ICC chimney
(Enclosed in a continuous chase from the interior to the exterior) A chimney going through a living space has to be enclosed and we wanted to continue the chase above the roof for better flashing details with our standing seam roof.
– Vented flashing (Since the chimney doesn’t run through a vented attic, it’s required to have a vented flashing above the chase)
– Storm collar and Rain Cap
We’ll also have a Radiation Shield Tube, a Roof Radiation Shield and a Cathedral Ceiling Radiation Shield to meet the shielding and fire stopping requirements for Canada and per the manufacturer recommendations.
We can’t stuff insulation in the chase so we are limited to wrapping the insulation around the chase.
My boyfriend thinks that an uninsulated chase might be a problem in the summer with warm moist air entering through the vented flashing and condensing on the walls of the chase if the building is air conditioned.
Should we worry about condensation in our chase? Or does the stack effect in our tall chimney chase helps keep it dry?
We’re not sure if insulation is needed if the chase is airtight.
And if so, should we use rigid foam to seal the exterior of the chase inside the building without using a smart vapour barrier except at the support box ? Should we choose mineral wool in combination with the smart vapour barrier ? Does it matter ?
We’re not too worried about heat loss through the chase, but we’re confused around best practices.
Thank you for making it through this long post!
-Jeanne
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Replies
Hi Jeanne,
My understanding is that there should not be any insulation in contact with the metal chimney, the insulation can trap heat against the chimney which may lead to durability issues. I have not confirmed this with any of the chimney manufacturers, you may want to have this discussion with the chimney manufacturer you are working with.
As far as insulating the exterior of the chimney chase as it moves through the home, I don't think that would be an issue as long as the insulation is outboard the box and clearances are maintained inside, between the box framing and chimney. I might think about using something that is less likely to burn or off gas in the event of a fire, like a rigid mineral wool product. That being said, I don't think it's critical that the chase be insulated below the roof. I've seen a lot of woodstove chimney pipes left uncovered once they enter the home through the roof.
Air sealing is something that should be done with chimney chases. You're drawing shows you are planning on having two heat or radiation shields installed, one between the two floors and a second where the chimney pipe enters the unvented roof assembly. The shield(s) needs to be sealed to the air barrier and the chimney sealed to the shield. We typically make the shield to chimney air sealing connections using a high-temp caulk (RTV sealant) listed for the rated temperatures of the chimney.
Hope this helps,
Randy
Hi Randy,
Thank you for your answer! We’ll definitely keep insulation out of the inside of the chase to maintain the clearances. Our plan for the chase is the following:
- Drywall lining the inside of the chase (2” away from the chimney)
-2x4 and Mineral wool
- Intello or Majrex wrapped around the chase and sealed to the fire stop at the floor level
- Drywall, mudded and taped
The Insulated Round Support Box (the connecting piece between the chimney and stovepipe) we are planning to use has this description in the catalog :
“The Insulated round support includes an insulation wrap for the chimney length inside the support and an insulated firestop to be sealed to the vapor barrier.”
We’ll keep your comment in mind about the high temp caulk during our installation if we see that we need it at the Cathedral Attic Insulation Shield level.
Let me know if you have any thoughts on our planned chase construction
Thanks,
Jeanne
Jeanne,
My own chimney follows a similar path. It is double wall from the wood stove to the ceiling, and then insulated pipe from there though the second floor, up into a lantern / chimney, and through the roof. The benefit of leaving it exposed is huge. When the stove is going the insulated pipe adds a lot of heat to the upstairs. If you can avoid enclosing it in a chase I would recommend going that route.
If you do build the chase, give some thought to the sequencing of the construction. The difficult part will be fastening the inner drywall lining.
I would make the air barrier connection at the top where it goes through the roof.
The rest of the chase bellow should be part of conditioned space.
Akos,
Agreed. That's the way I've always done it. There is no advantage to either insulating the chase, or having it air-sealed - except where it penetrates the building's air-barrier.
Thank you Akos for your post! We’ll give some thought to keeping the chase in the conditioned space.
We’re considering sealing the air barrier to the Attic Cathedral Ceiling Shield (fire stop)
Hi Malcom,
Thank you for your answer! But I have no other choice than to enclose the chimney in a chase on the 2nd floor. It’s code here to enclose it when it passes through an occupied living space.
I like the double dormer. It’s interesting!
Our reasoning behind insulating the chase is that we could get better sealing (from the air entering the chase through the vented flashing) with the Insulated Round Support (at the floor connection) than with the Attic Cathedral Ceiling Shield (at the ceiling)
The manufacturer offers an insulation wrap for the Round Support but not for any other parts in our installation. We don’t know how we can do the same at the roof level.
Since our continuous chase through the roof has to be vented, it complicates a bit the installation when it comes to air sealing and insulating!