Brennan Homes Blog

Posted on March 2, 2022

Dampness or frost on basement foundation walls

Categories: Educational

Moisture on a basement foundation wall can occur for two primary reasons. First the moisture could be caused by water leakage or infiltration. This means that the water is on the wall due to a crack that allows water in to the house, water coming over the top of the wall and running down the inside or from some leaky water source like a water pipe or drain line that is leaking on to the wall. The second source of moisture is condensation. Condensation is caused when moisture from the air in the house is deposited on the wall because the wall is colder than the inside air; specifically when the wall temperature is lower than the dew point of the household air at its relative humidity. In laymen’s terms what this means is: as the temperature gets colder outside, the wall temperature also gets colder; as the wall temperature drops the possibility for condensation rises.


So how do you know if the moisture is caused by water infiltration or condensation? There are a couple of clues and tests that you can do help determine the cause. One test is to take a piece of plastic and tape it to the wall when the wall is dry and see if it collects moisture under the plastic (between the wall and the plastic) or on top of the plastic. The piece of plastic does not need to be very big; 6” x 6” will be fine. Just make sure that the plastic is secured tightly to the wall and no air can get between the plastic and the wall. If moisture appears behind the plastic you are likely dealing with some type of water infiltration. If the moisture is on top of the plastic then you are likely dealing with a condensation issue.


In our climate, a northern heating climate, condensation can occur any time of year. In the summer months a usual culprit is too much humidity in the home. When you have warm moist air with a high relative humidity hitting a cool basement concrete wall, condensation can result. One of the first lines of defense in this case is to use a dehumidifier to reduce the humidity in the house. You should also make sure the air is well circulated in the house to not trap damp air in the basement. Consider having your furnace fan run all the time instead of “auto” to keep air moving. In the winter months the condensation is still due to the warm moist air inside the house hitting the cold concrete walls. In the winter months the condensation is still due to the warm moist air inside the house hitting the cold concrete walls. But it could be more related to the wall temperature than the relative humidity of the air. A concrete wall that is above grade whether it has a brick or stone veneer or not a very good insulator and the cold from the outside is conducted through to the interior; making the interior temperature of the wall not much warmer than the outside air temperature. In this case is does not take much moisture in the air to produce either condensation or frost on the interior or the wall.


Fortunately there are ways to deal with condensation and we will offer a couple of suggestions in an upcoming article.

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