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Community and Q&A

Chimney chase condensation

Boro | Posted in GBA Pro Help on

Hello All,
Classic problem I believe.

I have a wood framed chimney chase which houses two 8″ insulated, stainless steel metalbestos wood stove pipes and my main DWV stack. On certain days, when the inside/ outside temps, and the relative humidity are just right, I have condensation build-up and dripping from the underside of my chimney chase roof.
Construction detail:

6×10 timber rafters 36″ (on center)
2 x 6 rough sawn decking (not T&G)
Grace Tri-Flex
3 layers of 3″ Polyiso foam board
2×6 Sleepers 36″ (on center, screwed to rafters below)
3/4″ T&G OSB decking
Grace Ice & Water over entire roof
Standing Seam metal roof

Chase Construction built on top of roof (with exception of metal roof)
2×4 and 2×6 construction
3/4″ OSB wall sheathing
Sheet Flashed around perimeter
Grace ice & water over exterior walls (which I think is best to remove at this point)
1 – 2″ Closed cell spray foam on interior chase walls
3/4″ plywood cap w/ ice & water
Stainless Steel Chase cap (3 penetrations)

The condensation is dripping only from the exposed underside of the stainless steel cap. This is where the manufactures clearances from combustibles (2 inches) dictate the wood hole be 4″ larger than pipe.

So my question is:
1. Should I turn the chase into a vented, uninsulated air space by continuing insulating across/ at the roof line (or maybe even below to the attic floor) and cut vent strips/ holes in the chase wall?
The concern I have with this option is will the relatively warm pipe still cause condensation even when vented?

2. Or should I continue insulation on the underside of the chimney chase cap with a combination of fire resistant spray foam and rock wool?
My concern here is of course the lack of venting directly under the cap.

I lean towards option 1 but would appreciate any further input.
Thanks a bunch,

GBA Prime

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  1. GBA Editor
    Martin Holladay | | #1

    One of your questions is, "Will the relatively warm pipe still cause condensation even when vented?"

    Actually, the warm pipe lowers the risk of condensation. Heating up the chase will reduce the condensation; a cold chase increases the chance of condensation.

    You wouldn't have condensation if you didn't have warm, humid air inside your chase. The probable source of that moisture is an air leak in your ceiling. I don't know how much effort you have made to air-sealing your ceiling, but every bit of effort helps.

    Since most ceilings aren't airtight, your best bet to solve this problem is to ventilate the chase. Ideally, you want to allow exterior air to enter the chase at the bottom and leave the chase at the top. You don't want the chase to be airtight or well sealed. Let the wind blow through that chase.

  2. homedesign | | #2
  3. Boro | | #3

    So the idea should be to never let the inside of the chase be cooler than the outside temp. Something that could occur with its sealed and partial insulated condition.
    I imaging this would happen in the morning when the stove pipe is cool and the morning temp is rising. The chase would hold the overnight low temp for some time and condensate.
    Thanks for the help. This site is great.


    It's difficult to do at this point but we usually put a galvanized fire stop / flashing at the roof penetration at the base of the chase. We seal this to the metal flue pipe and metal radon pipe with the red gooey fire caulk but hold our insulation well back from the flue per the manufacturers recommendation. This serves to keep humid air that may work it's way into the attic from moving up into the chase and condensing and it helps minimize water damage in the event that a hurricane might toss a tree against the chimney chase. It's a bear to get a good fit with galvanized flashing when you have three pipes to contend with but if you plot it out carefully you could probably get three separate pieces of steel fit from three different orientations to over lap and get the job done. Just remember that the light red caulk is very crumbly and you will need to get the gooey dark red stuff to have a chance of it sticking to steel flashing long term. You will also want to have some of that peel and stick aluminum tape for areas not too close to the metalbestos pipe.

    Your chase will then be un-vented but not taking on moisture from the house. You can pop in a vent to the exterior at the bottom if you want to (I generally allow the down-hill side to have a bit of ventilation directly above the roof to allow any water that might get blown up under the cap in a hurricane to escape) but so long as it is isolated from the warm moist attic air you should be fine.

  5. GBA Editor
    Martin Holladay | | #5

    Q. "So the idea should be to never let the inside of the chase be cooler than the outside temp?"

    A. Not really. The idea is to keep the air in the chase dry (something that can be accomplished by increasing the flow of exterior air through the chase) and, if possible, to keep any condensing surfaces warm. If the metal flue is hot, it can raise the temperature of the interior surfaces, including the metal cap, and make them warm enough to avoid condensation. But in general, you can't depend on the warmth of the flue, because the operation of any appliances in your house is intermittent. So I think that trying to heat the air in the chase deliberately isn't the best strategy. That's why I advised you to keep humid air from the house away from the chase -- Michael's suggestion of including an air barrier at the roofline is obviously a good one -- and to ventilate the air in the chase with exterior air.

  6. TUGdo7DovU | | #6

    I just learned that has an approved insulation (look for SUSI) that can be in direct contact with the chimney wall, i.e. within the normal 2-inch air zone. Although its use is limited to a short distance in three of their shielding products, it offers the possibility of stopping leaks and condensation at cold surfaces. I plan to use this product.

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