Mark Meshulam is an expert witness and consultant for window condensation.
This article originally ran on December 9, 2009.

This week Chicagoans experienced our first brutal blast of winter. Temperatures dropped from an average of 30°F to almost 0°F in a single day. Windy City winds dropped the wind chill to minus 20°F.

Our bodies endured the new arctic reality with shock. The forgotten pastime known as shivering violently was revisited like a psychoanalytic catharsis. Stinging fingers and cheeks became new unwelcomed sensations. Heavy coats, hats, gloves and scarves came from nowhere to engulf our citizenry. Steam emanated from our mouths. A hopeful few sprayed cans of deodorant skyward, hoping to accelerate global warming.

Our buildings reacted to the first big chill with shock, too. Windows that behaved passably for months were unveiled this morning looking as if they were scenes from a winter wonderland, coated with frost, ice and beads of water. Patches of snow pack clung to our mullions.

How did our world go topsy turvy so quickly? Why was it raining and snowing inside our homes?

The answer is science, my friends. The result of a simple equation with only four variables:

• Exterior temperature
• Interior temperature
• Interior humidity
• Thermal performance of the windows

If you tweak any of these variables beyond their designed limits, sooner or later condensation, then frost will appear on your windows. Interestingly, we can control three of the four variables. Exterior temperature alone is in the hands of the Almighty.

What is condensation?

Condensation on aluminum window

Condensation occurs when water vapor (a gas) in our air undergoes a temperature drop, such as when moist air moves across a very cold window. As the temperature of the water vapor descends, it eventually reaches its “dew point”, the point at which water vapor changes into liquid water. The liquid water forms on the cold surface, and can accumulate there in large volumes as more and more moist air circulates past the window.

The effect of interior temperature on condensation

If room-side heat is allowed to flow across the cold window surface, it will heat the window, causing the window’s surface temperature to rise. As it rises, it will eventually cross the dew point, then condensation will no longer form on the window surfaces, and condensation that is present will begin to revert to a vapor state, also known as evaporation.

Relationship of dewpoint to temperature with RH held constant

And so, if all other variables are held constant, generally a higher interior temperature will result in less condensation. This will only work if the room-side heat can actually reach the window surface. Blinds and curtains can have the effect of blocking room-side heat, and in extreme conditions must be left open.

The effect of interior relative humidity (RH) on condensation

Generally, as the interior humidity goes higher, so goes the dewpoint. Holding the interior temperature constant at 70°F, compare these two cases:

• If the RH is 25%, condensation will form at 32°F
• If the RH is 95%, condensation will form at 68°F
  And so, if you want to reduce condensation on your windows, reduce the interior humidity.
  

  Dewpoint vs Relative Humidity at Room Temperature

With that introduction, here are…

Chicago Window Expert’s

Top 10 Fixes for Window Condensation

Fix #1: Control the humidifier
As the outside temperature goes down, so should your humidifier setting. Here are industry recommendations:

Recommended humidifier settings

Fix #2: Use bathroom exhaust fans
Showers pump a lot of moisture into the air all at once. Run the fan during and 1/2 hour after the shower to expel the moisture before it reaches the windows. At this point we are NOT recommending going unwashed. Recent revelation on this point: if privacy permits, crack the door open to allow more air supply into the room. This will make the exhaust fan much more effective. If this isn’t practical, cut a louver into the door (preferably at the bottom), or cut 3/4″ off the bottom of the door. Door undercuts are a well-known way to allow air to move from room to room.

Fix #3: Use kitchen exhaust fans

Steam from cooking can humidify the air

While we do recognize the occasional need to eat, we wish to point out that cooking also places substantial humidity into the air in a sudden way. Run the exhaust fan during and for a short time after the oatmeal is cooked. If you have a “ventless” range hood, don’t kid yourself, it won’t help with this problem. It will just dump the moisture right into the living space.

Fix #4: Use laundry exhaust fans and NO “ventless” dryers
Washing machines and dryers add moisture to the air, so run exhaust fans if you have them. If not, open the window a bit. Always duct the dryer to the exterior. “Ventless” dryers are a sham and the “inventor” of this stupid idea should be publicly flogged.

Fix #5: Direct the heat to the window
Best practices dictate that the heat source, such as a baseboard heater or supply register be placed beneath the windows to warm them. Sadly, most buildings I see do not fully implement this idea. Even where such design exists, too often the depth of the window sill prevents the convective air flow from contacting the coldest part of the window, the bottom.

Tight window treatments can promote window condensation

A corollary to this postulation (I love saying that) is: Arrange your window treatments so that they allow warm air flow between window treatments and window. One of the grand ironies of physics is that while draperies do a great job of keeping heat away from the window, they do not keep room-side moisture away. A phenomenon known as “vapor drive” continues to push moisture into that space, just so that you can be surprised in the morning when you open your drapes and find penguins living there.

Fix #6: Be aware of construction moisture
Construction products such as paint, drywall joint compound, and even fresh lumber and concrete will continue to release moisture into the living spaces. It can take up to a full year for a building to achieve its “service equilibrium”, so don’t freak out when you frost up after expensive construction.

We window purveyers are also aware of a perverse situation in which we retrofit new energy efficient windows into an existing building and they immediately frost up, causing consternation to the buyers and even reduced cash flow to the purveyers. The reason for this cruel twist of fate is that the previous windows were so leaky that they allowed the evacuation of large amounts of moisture right through the windows, keeping room-side humidity low. The new windows tightened the envelope, and extreme humidity levels ensued. Education, and adjustment of the HVAC system can usually fix this problem.

Fix #7: Improve Window U Value
Non-thermally improved windows with single glazing just won’t cut it any more. Not only will you pay hard earned cash to heat your local suburb, the poorly insulated windows will tolerate little moisture before showing condensation. Improve insulating value by replacing windows, or by adding a tightly fitting inner or outer storm window to the existing.

Frost on uninsulated steel windows

Fix #8: Tighten window air infiltration
With all the talk of R values and U Factors out there, poor little air infiltration has been grossly neglected. Fact is, a window that allows the passage of excessive air will bring to waste most of your insulating efforts. On a cold, windy day, run your hand all around the window and try to find air movement. The feeling of a light breeze on your hand can mean big energy losses. That same breeze can chill the inner window surfaces and promote spots of greater condensation and frost. Seal breezy areas with caulk. If it is at an operable portion of the window, replace gaskets or add foam blocks to fill the hole. Or call us and we will do it.

Fix #9: Fix the perimeter caulk
Breaches in perimeter caulk can bring cold air to the interior plane of your window, exactly where you don’t want it. Make sure your outside caulk is doing its job.

Fix #10: Understand the wall cavity
If you were Superman and had X-ray vision, you could get rich and hire someone to read this article for you. But in addition to that, you could look inside the walls immediately surrounding the window and you would discover a startling fact: there might be empty space in there! If you have a masonry wall, there is almost always a space behind the brick. This space could be quite large, extending unobstructed behind all of the brick. Other wall designs also have such a “wall cavity”.

The reason this is important, is there can be huge amounts of cold air circulating in the cavity. This cold air can chill your window frame or blow around the frame into the room, bringing condensation and energy losses. To fix this, first understand it, then develop a reasonable plan of attack. Recently we have become involved in remedial methods that involve injectable foam, retrofitting compartmentalization into walls, and sealing interior joints. If this sounds like a solution for you, give me a call!

Retrofitting injectable foam into window frame

Need a fix for your window condensation problem?

Mark Meshulam, Chicago Window Expert, risking hypothermia to help you with window condensation