Understanding Water Damage Caused by Condensation
- 17 hours ago
- 2 min read
When investigating the cause of water damage inside a home, one must always consider condensation. This past winter, we were asked to determine the cause of water damage that was revealed when a ventless gas fireplace was removed from a house. The damage appeared as a water stain on the inside of wooden sheathing on an exterior wall. The wall was inside the gas fireplace enclosure.
Key observations include:
The wood sheathing was directly exposed to the ventless fireplace.
There was no gypsum board (drywall) finishing the exterior wall.
There was no evidence of stormwater leaks or exterior water intrusion.
The water stains were localized inside the fireplace cavity, not on other parts of the wall.
Based on these observations, we concluded that the cause of the water damage was the ventless fireplace in combination with a flawed vapor barrier.
What Is a Ventless Gas Fireplace?
A ventless gas fireplace operates without a chimney or vent pipe. Instead of expelling combustion gases outside, it releases them directly into the room. This design offers easy installation and efficient heating, but it also introduces moisture and combustion byproducts into the indoor air.
Because ventless fireplaces burn gas inside the living space, they produce water vapor as a natural byproduct. This moisture can increase indoor humidity levels, especially in poorly ventilated rooms or during cold weather when windows remain closed.
How Does a Ventless Fireplace Cause Water Damage?
Condensation happens when warm, moist air comes into contact with cooler surfaces. In ventless gas fireplaces, combustion produces water vapor that mixes with indoor air. When this moist air touches the cooler wooden sheathing on an exterior wall, it cools down and releases moisture in the form of water droplets.
Factors that increase condensation risk include:
Lack of ventilation: Without proper airflow, moisture accumulates inside the fireplace cavity.
Cold exterior walls: Exterior walls tend to be cooler, especially in winter, making them prime surfaces for condensation.
Absence of drywall or insulation: Without gypsum board or insulation, the wood sheathing is directly exposed to moist air and temperature fluctuations. In other words, the layer of drywall forms a vapor barrier to keep moist air away from the cool exterior wall.
High indoor humidity: Activities like cooking, showering, and using the fireplace itself raise moisture levels indoors.
This condensation can soak into the wood, causing stains, warping, and eventually structural damage if left unaddressed.
If you would like professional help to determine the cause of water damage or similar insurance claims. Tice Engineering would be happy to help. Click on the "investigative engineering" or "about" links above.
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