Winter Condensation in Your Home

Sometimes the water you see came from inside your home.


It is a perfectly natural reaction; you see some water dripping out of your bathroom fan or notice some mildew in the corner of a closet and assume that you must have a leak coming in from the outside of your home somewhere.  Sometimes that may be the case but other times, especially if you are noticing these things in the middle of winter, that moisture you are seeing may actually be water that came from inside your home.  The source is the water vapor that is in the air all around us and the reason you are now seeing it is because of condensation.

What is condensation?  Here is an over-simplified description.  All air contains some water vapor.  The air in a home tends to be especially humid due to things like hot showers, boiling pots of water in the kitchen, people and pets breathing, watering indoor plants, etc.  Warm air is able to hold more moisture than cold air (like the warm air in your Minnesota house in January versus the cold air outside).  When warm humid air comes in contact with a colder surface the air right around that surface cools and suddenly it can no longer hold the same amount of moisture that it could when it was warm.  The excess water condenses to form droplets of water on the cooler surface.  It’s the same phenomenon that forms fog, dew on the grass and the ‘sweat’ on the outside of a cold can of beer on a warm summer day.  

So, let’s look at some of the ways you might experience this in your home.

One of the most common manifestations is the condensation that forms on the inside of windows.  This is because the glass in your window is generally going to be the coolest surface inside your home during winter months.  When the warm, more humid air comes in contact with the cooler pane of the window glass the water vapor is squeezed from the cooled air and forms droplets on the glass.  The cooled air then sinks and is replaced with new, warmer more humid air and the process continues.  

Another common demonstration of condensation is the aforementioned bathroom exhaust vent in winter. After a hot shower has run the warm humid air in the bathroom is vented out a pipe through the much cooler attic.  When the fan is shut off some warm moist air remains in that pipe.  The moisture in that air condenses on the inside of the pipe and, if it is cold enough, can freeze forming frost on the inside of the pipe.  This can build up over time and then one day the attic temperature rises a little, the frost melts and you see all of that water running out the bathroom fan.  It looks like a leak, but all of that water came from inside the house.  You would be surprised how many calls we get on this every year.

The other issue I mentioned before is the damp, clammy spot that you sometimes see on outside walls or in poorly insulated areas of a home that can lead to the formation of mildew and mold.  This is another example of a warm humid air reacting with a cold drier air.  Humid air will always try and move towards drier air, independent of other air currents, in an effort to equalize itself.  This is force is known as ‘vapor pressure’ and the moisture will actually try and force itself through the drywall and other building materials creating these damp spots.

The last example I will mention here is also the most concerning and potentially can cause the biggest problem and that is when heat loss into the attic is so great that the space warms up to the point where frost starts to form on the underside of the roof decking.  When warm, humid air finds its way into the attic and comes in contact with the colder plywood it gets squeezed and the moisture condenses on the cooler underside of the roof deck.  This can build up over time and can form a thick layer of frost over the entire underside of the roof.  Then, one sunny day that roof deck warms up and all of that frost can melt at the same time and inside your home it will look like you are getting leaks everywhere, in every room even though it’s still winter, there likely isn’t snow on the roof and it’s not raining.  If this is the problem you are having you need to consult with a specialist to figure out how to get your attic cooled down.  In the winter your attic should ideally be the same temperature that it is outside and all of that heat you are paying for should be kept in the conditioned space where people live.

For all but the last problem the solution is the same; control indoor winter humidity levels (and maybe let your bathroom fan run a little longer).  Yes, winter air is dry and some people are very sensitive to this, but if running a humidifier is causing these kinds of problems in your home you really need to evaluate and choose the lesser evil.  The Minnesota Department of Public Safety makes the following recommendations based on keeping the inside temperature at 70 degrees:

  • If outside temperature is 20 to 40 degrees, humidity indoors should not be more than 40 percent.
  • If outside temperature is 10 to 20 degrees, humidity indoors should not be more than 35 percent.
  • If outside temperature is 0 to 10 degrees, humidity indoors should not be more than 30 percent.
  • If outside temperature is 10-below to 0, humidity indoors should not be more than 25 percent.
  • If outside temperature is 20-below to 10-below, humidity indoors should not be more than 20 percent.
  • If outdoor temperature is lower than 20-below, inside humidity should not be more than 15 percent.