Why do I have condensation on ceramic tile floors when the heating system is off?


Why do I have condensation on ceramic tile floors when the heating system is off?

Asked by Suzanne Salvas

I have a geothermal system, which is turned off for the summer. The cooling system is on but not running at this time. My home is ICF (Insulating Concrete Forms) and I have a panel deck as the floor base. Last summer both the kitchen and bathroom (tile floors) were slippery with condensation. This year only the bathroom floor is very wet. The room is also very cool. I have tried opening windows, turning on the electric fireplace but nothing dries it out. What could be causing this? I thought it might be the membrane, but it doesn't affect both rooms and it didn't do this prior to the installation of the central air conditioning system and air exchanger last year.

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Florian Speier's picture


You are experiencing a typical problem of buildings that stay cool without the use of air conditioning. Your ICF building has a high thermal mass, which is a great thing because it can balance heat swings, allowing you to stay cooler during hot summer days without the use of air conditioning.

The condensation is a direct result of bringing fresh outside summer air inside your home. Opening the windows actually makes it worse.

Let me explain this.

The weather forecast for Fryeburg, ME

Along with the temperature, the weather forecast often talks about humidity, expressed in a percentage.

While I am writing this article, the humidity forecast for the next forty-eight hours for Fryeburg, ME cycles between about 40% and 98%.

  • This is called relative humidity.
  • "Relative" because it is relative to the amount of water the air, at that temperature, can hold before it needs to drop the excess water in the form of rain, dew or condensation.
  • Hot air can hold a lot more water than cooler air. If you take warm summer air and cool it down, the relative humidity will increase. It's still the same absolute amount of water in the air, but the capacity of the air to hold the water gets diminished as you cool it. At some point you will reach the dew point. This is the point when relative humidity equals 100%, and any further cooling of the air means it will need to drop some of that water.

Inside your home

The same thing happens in your house.

  • You let warm summer air in, and as it gets cooled down in your cool house, the relative humidity rises.
  • Some air will come close to your especially cold floor tiles (they take longer to warm up during the day and are therefore cooler than other surfaces) and that air will reach the dew point and water will condensate.

The math is surprising: If the outside air is 90 degrees and humidity only 55%, the dew point is 72 degrees. This means that any surface 72 degrees or colder will get condensation.

Opening windows makes it worse (during summer)!

Trying to air this out by opening windows makes the problem worse, as the summer air you bring inside the home carries more moisture that in turn will condensate.

Your new air exchanger does the same thing: it brings in more fresh air.

  • Before you had the air exchanger, you probably only vented by opening windows.
  • And you may not have opened your windows that often in summer, because you wanted to keep the cool air inside.
  • As a result, the amount of warm summer air that got into your house was small enough that it did not matter.

The effect is very well known to owners with uninsulated basements. If you get moisture issues in summer, opening the windows will make it worse.

  • However, if you open the basement windows in winter, the basement actually dries out.
  • The effect is reversed, because the winter air (even if it feels wet due to a high relative humidity) gets warmed up in the basement, and the humidity drops.

Two potential solutions

Let's look at a solution for you. There are two things you can do:limit the amount of incoming air, or dry the air before allowing it into the house.

  • To limit the air, you need to switch off your air exchanger.
  • If you want to dry it, you will need to switch on the air conditioning at a minimal level, because the AC actually dries the air, too, in addition to cooling it.

For more information:

Read "Our aluminum-frame windows are harboring condensation and mold. Can you recommend window solutions for a damp environment?" a Q&A answered by Steve Saunders.