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Diane in Pittsf...
Please help resolve problem with condensation in chimney chase

Our 2-story cape in Pittsfield, Mass. was built in 2013. The house has spray foam insulation. We have a zero-clearance Kozy Heat wood burning insert, with a fuel pipe that rises through the second floor and exits the house through a dormer roof.

Condensation freezes in our chimney chase, thaws, and drips down the stove pipe. We thought we had corrected the problem by installing a pitched ceiling plate, attic insulation shield and insulation, but unfortunately it is still occurring. The water has found it’s way through a gap around the pitched ceiling plate and is now running down the sheetrock. Neither our contractor nor mason have a resolution.

The chimney chase only contains the stove pipe for the Kozy Heat insert. The pipe passes through a ceiling that has spray foam insulation and exits the house through the dormer roof on the second floor (so there is no attic). There is a 4” space around the stove pipe where it exits the house, and there is also a pitched ceiling plate with an attic insulation shield surrounded by fiberglass insulation at a depth of about 12” above sheetrock.

The chase is constructed of 2x4’s with Zip panels taped at the corners and seams, covered with wire lathe and cultured stone, and the chase cap is stainless. There is no ventilation allowing for air circulation in the chase.

We’re wondering if we should try any of the following options, or something else? And if so, what?
• Remove the pitched ceiling plate, attic insulation shield and insulation during the spring/summer and dry out the inside of the chase
• Add ventilation to the chase area to allow moisture to escape – how would this be done and by what type of contractor?
• Insulate the chase above the roof to make it part of the insulated envelope?
• Other options??

Re: Please help resolve problem with condensation in chimney chase

I think I know what is going on here but I am not getting a clear picture of the installation of the flue pipe. Does the pipe exit the back of the stove and go through the wall into the chase or does it go through the ceiling first and then go into the chase somewhere above the room the stove is in, or does it exit the top of the stove and then into an elbow where it goes through the wall into the chase?

If it goes through the wall, then why do you have a pitched ceiling plate and if it goes through the ceiling, why do you have an external chase? Or is the chase external? If it is internal, then is the rock on the interior side of the house?

Does the stove have to be operating for this to occur? Is the flue pipe the type that uses the outer shell to bring in combustion air for the stove so it doesn't use the conditioned room air for burning?

I think that fundamentally, the issue is that warm conditioned room air is getting past the pitched ceiling plate, rises up the chase to the attic where it gets past the insulation and the plate that is around the flue there and exits the attic.

The chase itself is significantly colder than the house so that room air will get colder, when it gets colder, it relative humidity (RH) will rise until it gets past the dew point which is the temperature at which RH is 100%. The moisture condenses against the pipe and drips down.

The cure for this will be very difficult, especially if the outer shell is used to bring outside air into the stove for combustion. The first step would be to provide some type of vapor shield to keep room air from getting into the chase. The the chase should have a small vent near the bottom to allow outside air in. The chase MUST be insulated to the house. If the chase is on the interior side of the house, all the walls to the interior must be treated as exterior house walls and insulated as such. The chase wall against the exterior would not be insulated.

But even if the chase is done right, if the flue provides outside combustion air to the stove, then the outside for the flue will be very cold and condensation will occur on it and some of that will get on the nearby walls and ceiling.

Diane in Pittsf...
Re: Please help resolve problem with condensation in chimney chase

Thanks for the insight Keith3267.

The flue pipe comes out the top of the stove, passes through the second floor and through the roof on the inside of the house. The pipe goes through a dormer roof, so there is no attic.

The stove draws air for combustion from through a small vent from outside the house that is basically level with the stove.

There doesn't seem to be a correlation between the drip and use of the stove: when the air in the chase area above the roofline warms up after a freeze, either by sunshine/warmer temperatures or heat from the stove, condensation thaws and drips.

Can you please explain: "the chase MUST be insulated to the house" - how? Right now the chase section that passes through the second floor is insulated on the outer wall, but only has sheetrock on the 3 inner sides. So I think you are saying that we need to add a vapor shield (what kind??) and insulate these 3 inner sides.

What type of vent should we put into the chase? The chase is built from zip panels with wire lathe and a cultured stone façade.

Many thanks for your response and for any additional info you can provide.

Re: Please help resolve problem with condensation in chimney chase

I am getting a clearer picture now. Unfortunately the chase was not built correctly. The exterior wall of the chase should not have been insulated and the three inside walls should be insulated with vapor barriers on the interior side of these walls. The chase should be considered "outside".

The plate that the flue goes through in the ceiling also needs to be sealed.

What is happening here is that the flue is made of steel and steel conducts heat quite well. The flue should have an inner sleeve and an outer sleeve with some type of non combustible, high temp insulation between them. The outer sleeve gets very cold when the stove is not in use and should stay cold even when the stove is in use.

Unless it is raining or foggy, the outside relative humidity (RH) should be below 100% so there should be no condensation on the flue above the roof line. When it is over 100%, you need a water tight seal around the flue on the roof and I'm guessing that you have that and don't have an issue there.

But the outer sleeve of the flue conducts the outside temperature down into the chase. You are getting warm water laden air into the chase from the house because it is uninsulated and there is no vapor barrier. Now you are probably thinking that you warm house air is pretty dry, maybe 20% RH, the the amount of water that air can absorb is dependent on temperature.

There is a relatively little used term called Absolute Humidity (AH) that measure the amount of water in the air in grams of water per cubic meter. The AH does not change with temperature, but with a given AH, the RH does change, going up as temperature drops. When the temperature drops the where the AH becomes 100% RH, that temperature is called the dew point and that is where condensation or fog occurs.

The outside sleeve of the flue is well below the dew point so when the warm moist inside air hits the shield, it condenses. And this is the root of your problem.

The cure is to remove the interior walls of the chase, remove the insulation between the chase and the outdoors. If the stone is on the exterior side of the house, the chase can be vented by either removing a small stone and putting in a screened vent near the bottom and another near the top or by drilling a series of small upward slanting holes through the grout into the chase, top and bottom. It does not take very many.

Then insulate the floor and interior walls of the chase, put a vapor barrier over the insulation and re-rock the walls. Put some kind of seal around the stove pipe where it goes through the ceiling so that room air below won't rise up into the chase. Now cold dry outside air will enter the chase at the bottom, get slightly warmed up, decreasing its RH even further and absorbing any moisture that may be getting by the vapor barriers and exiting the upper vents, keeping the flue pipe dry.

You may see some condensation on the stove pipe in the living room now because it will be cold and any room air hitting it will cause condensation to occur on it, but that won't damage the sheet rock.

The best solution though would be to have the stove pipe make a 90 degree bend a foot or two above the stove and go out the wall there, then another 90 degree bend and up through an an external chase (faux chimney). The bottom of the external flue can have a removable plate for cleaning the flue pipe and the crud stays outdoors. Then you can recover the space used for the inside flue in the upstairs room. This is how mine is done. My external chase is just sided with wood siding.

Diane in Pittsf...
Re: Please help resolve problem with condensation in chimney chase

Thank you Keith!

Re: Please help resolve problem with condensation in chimney chase

Ok there is some mis information in keiths response. With a zeroclearance unit you can never have a 90 degree bend. And many times the chimney has no insulation just an air space between the inner and outer flue. There are some that are insulated and some that have 3 walls as well. You need to check the installation instructions to see if the chase needs ventilated to start off. then if the chase is insulated then all of it needs to be including the chase cover. There is no problem with insulating the entire chase it is actually very desirable it just needs to be done correctly.

Re: Please help resolve problem with condensation in chimney...


Thank you for the info.  Very helpfull.  My question is about this same topic.  

I am building a home with an exterior chase.  The exterior wall of the great room is spray foamed with open cell in 2x6 wall.  The chase is exterior to that wall and opens at the bottom to accept the Napoleon NZ6000 zero clearance wood fireplace.  I will insulate the three walls of the chase to the firestop which is about 6 feet below the great room ceiling.  But remember that the entire great room fireplace wall is sprayed to the ceiling and through the 3' attic space in the scissor trusses.  The fireplace vents through a triple wall Hart & Cooley TLC chimney so very safe.   At the top of the chase is a 2'x4' stainless steel cap with a 1/2" larger hole that the 10" pipe will pass through with a collar.   Before I "close it up", I'm concerned about condensation buildup in the area above the firestop.  The stainless steel drips right now, but right now there is no firestop or fireplace, just a hole straight up to the top.  Here's my question:  Do i need to insulate above the firestop OR simply vent the chase from just above the firestop and also at the top OR both?

Stuck in a dilemma,


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