Window condensation & frost causes and cures

By: Matt Grocoff , Contributing Writer
In: Old Houses, Old House Construction, Home Improvement Tips, Green Renovations, Technology, Historic Preservation

Humidity levels in your house are a year-round issue.  While wet or frosted windows are probably not a sign that you have bad windows, it may be a warning sign that you need to take some action.  Chances are that whatever is causing condensation is also making your house less comfortable and costing you more to heat and cool.

Condensation appears when the window’s surface is colder than the room temperature and drops below the dew point.  This is the same reason your mirror gets foggy when you take a shower in the winter or your can of soda gets sweaty in the summer.

The most effective ways to manage moisture levels are 1) reducing moisture at the source, 2) stopping air leakage, and 3) ventilating properly.

Reduce Moisture

The first line of defense is to reduce the amount of moisture coming from your house.  Check out the list below of sources of home moisture.  Just your family’s breath and sweat can add 1.5 gallons of water per day.  One solution would be to stop sweating or breathing.  But, it’s not recommended.

You can reduce the interior moisture levels by installing high-efficiency shower heads and faucet aerators.  A high quality 1.5 gallon per minute shower head will save 15,000 gallons of water per year for a family of four.  Water efficient dishwashers and clothes washers will similarly reduce humidity levels.

Make sure your dryer is properly vented to the outside.  Venting to the inside not only adds nearly a gallon of water per load, it spills unhealthy air into your living space.

Typically you should not need to use a humidifier in the winter because of the amount of moisture that is generated during your daily activities.  If you find your house too dry, consider a whole-house ventilation system such as an energy or heat recovery ventilator (or maybe several house plants).

Stop Air Leakage

Moisture on the inside of the storm window may mean that the your main window is leaky and carrying moisture to the cooler storm where it condenses.  Weatherstripping and caulking your old windows will help stop these air leaks.  Read “Save Big Bucks By Restoring Your Old Windows & Making Them More Energy Efficient“).  Ultimately, this will save your windows and save you money.

If the moisture is on the inside of your single pane windows consider adding high-quality Energy Star rated storm windows.  This will slow down the heat loss from your house and keep the inside window warmer and keep away the condensation.


You can control how your house is ventilated by opening and closing windows, using bath and kitchen fans or installing a energy recovery or heat recovery ventilator.  Energy or heat recover ventilators (ERV or HRV) which run continuously can help maintain proper moisture levels.

Running bath and kitchen fans will help exhaust excess moisture and help prevent mold from these high-moisture source points.  It’s a good idea to put a timer switch on exhaust fans so that stay on only as long as necessary and are automatically shut off when they are not needed.


When the heating and the outside temperature is below 15º, the relative humidity should be below 30%. In very cold weather, the humidity may need to fall below 30% to prevent condensation or frost on windows.

The University of Minnesota has developed guidelines for the minimum recommended humidity levels for houses. Based on a 70ºF interior room temperature, engineering studies established the following guidelines:

Outside Temperature Inside Humidity

20º to 40ºF Not over 40%

10º to 20ºF Not over 35%

0º to 10ºF Not over 30%

-10º to  0ºF Not over 25%

-20º to –10ºF Not over 20%

-20ºF or below Not over 15%

These guidelines do not guarantee that condensation will not appear on mirrors or windows. Factors such as closed blinds or drapes may require you to decrease the relative humidity in your house below these guidelines.

Moisture source and estimated amount (in pints)
Source: Minnesota Extension Service, University of Minnesota
Bathing: tub (excludes towels and spillage) 0.12/ standard size bath shower (excludes towels and spillage) 0.52/ 5-minute shower

Clothes washing (Automatic, lid closed, standpipe discharge) 0 +/load (usually nil)

Clothes drying: vented outdoors 0 +/load (usually nil) not vented outdoors or indoor line drying 4.68 to
6.18/load (more if gas dryer)

Combustion - unvented kerosene space heater 7.6/gallon of kerosene burned

Cooking: breakfast (family of four, average) 0.35 (plus 0.58 if gas cooking)
lunch (family of four, average) 0.53 (plus 0.68 if gas cooking)
dinner (family of four, average) 1.22 (plus 1.58 if gas cooking)
simmer at 203°F., 10 minutes, 6-inch pan (plus gas) less than 0.01 if covered, 0.13 if uncovered
boil 10 minutes, 6-inch pan (plus gas) 0.48 if covered, 0.57 if uncovered

Dishwashing: breakfast (family of four, average) 0.21
lunch (family of four, average) 0.16
dinner (family of four, average) 0.68

Firewood storage indoors (cord of green firewood) 400 to 800/6 months

Floor mopping 0.03/square foot

Gas range pilot light (each) 0.37 or less/day

House plants (5 to 7 average plants) 0.86 to 0.96/day

Humidifiers 0 to 120 + /day (2.08 average/hour)

Respiration and perspiration (family of four, average) 0.44/hour

Refrigerator defrost 1.03/day (average

Saunas, steambaths, and whirlpools 0 to 2.7 + /hour

Ground moisture migration 0 to 105/day

Seasonal high outdoor humidity 64 to 249 + /day

Matt Grocoff, Esq. LEED is founder of Thrive - Net Zero Consulting Collaborative, host of Greenovation.TV, a contributor to The Environment Report on Public Radio, the green renovation expert for Old House Web, and a sought after lecturer. His home is America’s oldest net-zero energy home and was called “Sustainable Perfection” by The Atlantic, honored as one of USA Today’s “Seven Best Green Homes of 2010″ and Preservation Project of the Year.  He has been featured in hundreds of publications and news shows including Washington Post, Detroit Free Press, Preservation Magazine, Solar Today, Fox Business News, Huffington Post and more.  Join him on Google+, Twitter and Facebook