Condensation Explained

Windows do not cause condensation.

Excess interior humidity is the source.

Windows don’t

manufacture water!

Everything you need to know about condensation.


A little fog on the corners of your glass shouldn’t bother you.  And it probably doesn’t.  But “problem” condensation, fog or frost that covers whole windows, definitely bothers you.  And it should.  The visible effects are just the tip of the iceberg.   What is happening elsewhere in your home?  Take your attic.  Excess moisture may be freezing in the insulation where it will later melt and damage your plaster.  Or perhaps it’s forcing its way out through your siding, where it will form blisters under the exterior, or it may be promoting mildew in your home. 

   The culprit is too much water vapor.  Not the window, not the insulation, not the paint.  The only answer is to get rid of excess water vapor.  Water vapor, or humidity, is one of the invisible components of air.   This humidity tries to flow toward drier air, since this air has a lower vapor pressure.  In other words, the higher vapor pressure in moister air forces the moisture to areas which are drier.

            In cold weather, the air outside is generally drier than within a warm house.  What happens, then, is that the moisture is forced to the outdoors.  The most obvious indication of this is condensation on your windows:  the moisture comes in contact with the cold surface of the glass.  It therefore condenses to form either frost or water (depending on whether the temperature of the glass surface is below or above 32°. 

            The “tight” construction of modern homes traps the moisture in many ways.  Certain varnishes, tiles, plastics and acrylics—which are increasingly used in construction—do not allow moisture penetration.  Thus the moisture created by kitchens, laundries, bathing and human bodies (as well as cat, dog and other pet bodies) can’t flow easily to the outside.  Insulation and construction materials that are designed to keep cold air outside also keep moisture inside, further increasing the moisture level in your home.       

 Forced Air issues

Forced hot air system return vents, especially located on the floor below a window, will draw warm, moist air across the interior surface of the window glass.  Condensation may occur if this air is too humid and the glass temperature is cool enough to create a dew point.  Condensation can occur on any window in any room, even rooms where several other windows are installed and they remain unaffected. Reduce the occurrence of condensation by:

  1. Diverting the return air to a different register away from the window
  2. Close off the return, thereby shifting the return air to a different register
  3. Install a dehumidifier in the suspect room
  4. Turn off or lower the humidistat on your furnace



A room can actually have a "convection current", whereby warm air rising out of the heating device below the window crosses the window above. This window amy be cool enough to chill the air which then causes the cooled air to drop. As the air is heated again and rises, it again crosses the plane of the window where it again is cooled and drops downward.  This"draft" is not a "draft, it is physics.  Warm air rises, cool air drops. Divert the warm air register to keep warm, moist air from directly or indirectly striking the inside glass surface.  Warm air holds more moisture than cold air.  A single vent pointed at a window may cause condensation to occur on that window without affecting other windows in the same room. 


Order windows with the Super-Plus+E or the Premium Plus+E Home Comfort Energy Packages and what happens: the glass is warmer in the wintertime, cooler in the summer.  By raising the glass temperature, the dew point is changed.  There is an upfront cost up for either glass package, but the rewards are many.  Less fuel use, less risk of conductive draft and a reduction in the risk of condensation occurring on the glass.




In simplest terms, condensation is simply moisture build-up on a material that is below the dew point temperature of the surrounding air. If the glass goes below that temp - you have condensation. If it doesn't then you don't.


Independent Resources

  • Clearing the Air About Condensation, Tom Feiza, Home Tips
  • Home Moisture, Home Energy Guide, Minnesota Department of Commerce Energy Information Center
  •  Indoor Air Quality – Temperature and Humidity, Environmental Health & Safety, Bowling Green State University
  • Council Notes on Moisture Control, University of Illinois Small Homes Council, Building Research Council

How to further decrease the chance of condensation occurring on the interior of your window




Energy Star rated energy efficient windows will further cut down the creation of moisture on the glass by raising the interior glass surface temperature in wintertime.  However, if a home  continues to have excess humidity, new windows will not eliminate the humidity; only diminish the possibility of condensation occurring on the glass.

ENERGY STAR qualified windows, doors, and skylights save you energy and money, increase the comfort of your home, and protect your valuable possessions from sun damage. They are also better for the environment because lowering your energy use means less air pollution from power plants.


·        Save money and energy. Replacing single-paned windows with ENERGY STAR qualified windows or choosing ENERGY STAR over the typical clear-glass double-paned alternative can save a significant amount of money on your energy bill.

·        Say goodbye to winter drafts,reduce the risk of sweating and lower the inside temperature of the glass in the summer sun. Thanks to a host of new technologies including Super Spacer, ENERGY STAR qualified windows, doors, and skylights keep your home cooler in the summer and warmer in the winter, making you more comfortable year round.

·        Protect your home's interior. Many ENERGY STAR qualified windows, doors, and skylights act like sunscreen for your house, protecting your photographs, artwork, furniture,  carpets, and wood floors from sun damage.

·        Buy with confidence. Every ENERGY STAR qualified window, door, and skylight is independently certified to perform at levels that meet or exceed strict energy efficiency guidelines set by the U.S. Department of Energy.

·        Ask for ENERGY STAR. To purchase the most efficient window for your home, ask for products that are ENERGY STAR qualified in your Climate Zone. To learn more, see the window purchasing tips on the following

·        Seal and insulate with Home Sealing. Sealing your home's envelope is one of the most cost-effective ways to lower your home's energy bills and improve your comfort.

What's in a pane - or two - or three?

All energy efficient windows have at least two panes, but not all double-paned windows are energy efficient. Twenty years ago, double-paned meant energy efficient; today, advanced technologies have enabled the development of triple glazed windows that are much more efficient than traditional clear-glass double-paned windows. For maximum energy savings, count on ENERGY STAR.