Renovation & Design

Sealing chase at top of building enclosure should stop moisture

Question: I have a question about what to do about moisture collecting on the underside of the metal cap of my false chimney chase. The entire chase has insulation and vapour barrier, but it would be considered a cold zone. Where it penetrates the roof, there in a piece of R10 rigid foam board, which is screwed and tuck taped to the framing, basically sealing off the upper portion above the roof line.

The problem happens every winter, when a small amount of moisture-laden air enters the chase above the roof line. This moisture collects on the cold chimney cap in the upper part of chase, which turns to frost. When the temperature warms to about -5C and the sun is out, of course the frost melts and you can hear water droplets falling on to the rigid foam below.

I know its not a lot of water, because I can access the bottom of the chimney chase, and I see a small puddle which eventually dries up. But, I want to fix this once and for all. 

First I would remove the metal cap and spray foam on the underside to stop the frost from forming. Secondly, I would put a static vent on the cap cover to allow the moisture to escape.

Do you think a regular roof vent would work if I screw and silicone it down to the chase? I should mention that the chimney is for esthetics only, and is part of the house design.  Thanks, Arny.


Answer: A small amount of moisture from frost buildup inside your open chimney chase may be a minor inconvenience, but can be difficult to prevent without air sealing additional areas. While insulating the underside of the metal cap may help somewhat, concentrating on sealing the chase before it leaves the building enclosure may be more successful than trying to vent the top of the chase, itself.

A small amount of moisture dripping onto a piece of waterproof foam insulation in the attic, or at the roofline, a couple of times each winter should not be a major concern. Regardless, I commend you on trying to find a simple solution to this problem. To fully address this issue, you must understand the forces that create this stack effect in your home.

The stack effect is the name given to the natural airflow inside a building enclosure, due to warm air rising. Inside your chimney chase, which is a true stack, cold air may leak into the bottom from the basement. The heat generated by the gas fireplace, and the central heating system in the home, will warm this air, forcing it up the inside of the false chimney. If there are any air gaps at the top of this area where it enters your attic, which is likely, the warm air will continue upwards. It will start to cool as it makes its way through the cool attic and if it still has enough heat energy it will leak into the chase above the roofline. This is where you are seeing the results, frost and condensation.

To prevent the stack effect from occurring, ideally the chimney chase should be well sealed at the bottom and the top. This will prevent air leaking into the cavity, while the sealed top will stop air movement, because it will have no where to escape the sealed enclosure. Adding a vent to the chimney cap, or chase above the heated home, may actually increase air movement, causing more frost buildup. It may help vent the small amount of water dripping onto the rigid foam once the weather warms, but that water may increase in volume.

A better solution is to try and stop the air movement before it leaves the heated building enclosure. Installing another couple of layers of rigid foam, or blown-in high density foam, inside the chase at the floor of the attic should work better. This will require crawling into the attic and cutting open the chase, but may yield a better result.

Adding a passive vent to the top of your false chimney chase may do little to prevent your frost and dripping issue. Sealing the chase at the top of the building enclosure, which is the attic floor, should go much further in preventing warm air intrusion, which is the root cause of the melting and dripping frost.


Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the past president of the Canadian Association of Home & Property Inspectors — Manitoba ( Questions can be emailed to the address below. Ari can be reached at 204-291-5358 or check out his website at



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