Mark Meshulam is an expert witness and consultant for window condensation.

Here’s what happened after one of Chicago’s typical cold snaps:
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.

Steam on the Chicago River during arctic blast
Steam on the Chicago River during arctic blast

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.

A steamy Lake Michigan reacts to the cold
A steamy Lake Michigan reacts to the cold

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 window condensation. 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?

Window exhibiting window condensation and frost
Window exhibiting window condensation and frost

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 window 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?

Window condensation on aluminum window

Condensation, including 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 window 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 window 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
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 window 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
Dewpoint vs Relative Humidity at Room Temperature

With that introduction, here are…

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 to prevent window condensation
Recommended humidifier settings to prevent window condensation

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 and contribute to window condensation
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. If you have a “ventless dryer” make sure it has a condenser that captured and drains all the water into the drain and not into the air.
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 in order to reduce window condensation. 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
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.
If your home is lacking a heat source at the base of the window, a great option is to add a baseboard heater or space heater below the window. Be careful not to let flammable materials touch any heat source. See baseboard heaters here:

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 and may become window condensation. 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.
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 and window condensation ensued. Education, and adjustment of the HVAC system can usually fix this problem.
Fix #7: Improve Window U Value

Frost on uninsulated steel windows
Frost on uninsulated steel windows

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 in the way of low temperatures or high humidity before showing condensation. Improve insulating value by replacing windows, or by adding a tightly fitting inner or outer storm window to the existing.

Commercial interior storm window for reduction of window condensation and air infiltration
A good commercial interior storm window can be highly effective for reducing or eliminating condensation and air infiltration

Interior secondary glazing, also called an interior storm window can work extremely well for improving window U factor as well as for reducing cold air infiltration. It is particularly applicable in institutional buildings such as schools and hospitals, in office buildings, in historic buildings and in buildings with curtainwalls. To read more about this excellent option, go here: Building Energy Benchmarking and Window/Curtainwall Retrofit Part 2
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 window 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 to reduce window condensation
Retrofitting injectable foam into window frame to reduce window condensation

For pictures of some pretty amazing window condensation and frost, go here:
Window Condensation and Frost
For more on the phenomenon of window condensation and cold snaps, try this:
Sudden Cold Snap Coincides with Window Condensation Web Traffic

Need a fix for your window condensation problem?

Mark Meshulam, Chicago Window Expert, risking hypothermia to help you with window condensation
Mark Meshulam, Chicago Window Expert, risking hypothermia to help you with window condensation
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39 thoughts on “Window Condensation: Top 10 Fixes”

  1. Excellent article. Thorough and addresses the issues. One additional point that we have experience is that… as the warmer room air near the ceiling contacts the cold glass at the top of a sliding patio door, it changes temperature and cascades down the glass, increasing in speed as it becomes colder and heavier. This convection process can produce what appears to be an “icy breeze” as it reaches the bottom and rolls across the floor. We have found that this condition is most readily apparent to residents in the bedroom and kitchen areas where they are most likely to be found in the bare feet or slippers.

    Charlie Scott
    Deerfield, IL

  2. Hi Mark,
    I’ve watched your articles with interest, and finally would like to add some of the successes we have achieved going into our 6th year of code-compliant Heated Glass applications to address condensation control, occupancy comfort, neutralizing convective currents cascading down glass, and seamless operation of line-voltage powered comfort. As Durango Solariums, (, we were invited by a Swiss company in 2003, to provide our three-decades old proven and reliable sunroom frame and cladding technologies for their problematical high altitude wintergarten conditions, and at that time found some experimental heated glass windows and doors.
    We now have the most experience in North America, with the most code-compliant installations in the U.S. and Canada that have answered the condensation issues as we intercept dewpoint in the most effective, energy efficient manner. We have customers who have never had a drop of moisture on their windows even in steam showers in Colorado when it is below 0 degree F outside, with 100% humidity in the interior.
    Like all new roads, the road is “raught with peril”, as the Iliad and the Odyssey teaches, and there appears to be some real “voodoo” heated glass coming into the market.
    WarmVue has the experience and sophisticated control strategy to use the least energy to provide the appropriate Watts/SF to have frost-free/condensation-free glass.
    There are also some very deep health-risk issues involved with condensation, that could affect standards in certain building environments.
    We gave the only AIA-Accredited course for Heated Glass in the Building Sciences over the last (4) years, and thoroughly understand the technical and project coordination, design, assembly, and performance of this elegant technology.
    WarmVue technology has the proven and reliable history to add to your other solutions for condensation control, and recovery of lost space in buildings due to the effect of cold glass on an occupant.
    George E. Usinowicz
    Architectural Representative
    The WarmVue Partnership
    Carbondale, Colorado
    (970) 749-2602

  3. If you have two separate panes of glass, would you vent the airspace in between the glass to the inside or outside to reduce condensation?

  4. Interesting question.
    The answer is, vent to the side where the lowest humidity is expected when temperatures are coldest.
    In Northern climates, this would be the exterior. In moist Southern climates, this would be the interior.

    Remember that the side you vent the airspace to will freely exchange humidity with the airspace, so you want to vent to the side with lowest humidity in order to keep the air in the airspace as dry as possible.

    The time this matters most is when the surface temperature of the glass drops below dewpoint, because then you will have condensation (not just moist air) in the airspace. That’s bad because it can stain the glass and damage the finishes.

  5. I would like to comment on the paragraph off your article that discusses wall cavities. You suggest filling the air space behind exterior brick with spray foam insulation, but that space serves a very important purpose. Because brick and mortar are not moisture proof materials, there is usually a space left behind the brick to allow any moisture to run down the face of the back-up material and drain out the bottom of the wall cavity through spaces between the bricks for this purpose. If you fill the space, you trap the moisture and encourage rot and mold. You can also cause failure of the brick due to freeze and thaw of the trapped moisture in winter. Air sealing around the windows and making sure the window flashing is properly installed and functioning is a much better solution.

  6. Hi Kim,

    It was not my intention to propose filling the wall cavity, nor would I know how to retrofit such a solution.

    This article, and another now in production, touches upon the potential for benefit by filling the space immediately surrounding the window with foam, by injecting it through the window frame as is shown in the photo above.

    Last week we installed such a fix in an aluminum window after first taking surface temperature readings at 37 points on the interior and also on the exterior of the window. We then filled a portion of the area surrounding the frame while leaving other areas unfilled. This week we will return to take the “after” temperatures to see if there is a difference between the filled and unfilled areas.

    At least in theory, if the foam can isolate the perimeter of the window frame from wall cavity air, the window should stay warmer. We will see. Thanks for writing!

  7. Hi Mark, Sorry for my curiosity, how about the after temperature now, would you please unveil the result, it seems useful. Thanks.

  8. This is probably the most detailed explanation of window condensation that I’ve ever seen. I will definitely refer it to our clients when they encounter this issue. Thank you.

  9. i’m loosing my mind; 2 story home second floor all the windows r sweating like crazy, however, on the first floor there is no condensation at all. my tenant is not responsible enough to put lids on pots or open windows for parts of the day so i’m trying to figure out whether there might be a solution to end it entirely or at least reduce the condensation to a minimum. the water is coming down on the window frame to the carpet. i’m worried this is becoming an even bigger issue. my house was built 100 years ago. i set the thermostat at 73 to 75 cuz my tenants complaints too much but her apartment is warmer upstairs a lot of drafts in between was fixed i used weatherseal to seal in between. i also used plastic and taped the entire window however i still see condensation. i did noticed steam cast iron radiators r not placed under windows and she does have a lot of blinds n curtains. windows r usually very cold. i thought my issue was w the radiator air valve or windows but after consulting with some they said issue could be exterior siding and gutters. i think i need pointing or caulk outside. anyhow i still feel like i need a contractor that can tackle this issue and not guess diagnose it. i told my tenant about a dehumidifier but anyhow i need to figure out whether there might be cracks that need to be repaired. thanks for ur input guys. btw this article is probably the most insightful.

  10. Hi Mark,

    I refer to your interesting article Window Condensation: Top 10 Fixes.
    Regarding the effect of interior temperature on condensation, you said generally a higher interior temperature will result in less condensation. I think you really meant the opposite. Right?


  11. Hi Peter, As the interior temperature of the glass gets warmer, there is less and less chance for condensation.

  12. @David (above) – I believe that venting the window to the *inside* would be more appropriate because it would then allow warmer air to flow around the inner pane of glass, helping to heat it and thus reducing condensation. Have I understood you right, Mark?

  13. Venting, which usually means creating a small amount of air infiltration, does not have a big impact on the thermal properties but it has a large impact on the humidity. In most northern climates, winter condensation is a winter concern. At that time there is generally more moisture in the interior air than the exterior air. The opposite can be true in southern climates where the dry air is on the interior. Venting to the dry side when cooler glass temperatures is the overriding principal.

  14. Thank you for this great article! I love the charts! I live in a condo building in Minneapolis, and have metal window frames (I believe aluminum, but not sure) that are dripping like crazy the past couple weeks ever since the outdoor temps got below 0*F. Based on this article I’m assuming they are cheap aluminum frames without a thermal break.

    I ordered a room dehumidifier, to see if that solves the problem. However I’ve heard that room dehumidifiers might not be able to get the humidity below 40%-50%. Based on your chart for recommended humidier settings, a typical freezing Minnesota winter might require 25% humidity or event 15% humidity. Is that even possible? Is it even healthy? How do I get the indoor relative humidity below 25%? Do you have dehumidifiers you recommend? As far as placement, would it help to place the dehumidifier be right in front of the window?

    We do have someone investigating more permanent solutions for the condo building, but they are very slow and the solution could take years. I need a short term solution so my floors and windowsills don’t get ruined. I’m hoping a room dehumidifier does the trick.

  15. The quickest way to drop the humidity in the living space during a cold snap is:
    1) Turn off the humidifier completely
    2) Wipe off any wet windows with towels and put the towels in the dryer (which must be vented to the exterior)
    3) Turn on all exhaust fans that exhaust to the exterior
    4) Open the windows wide for as long as you can tolerate it – this lets in the dry outside air
    5) Never shower or make soup again

  16. Hi Mark

    Very helpful article.

    Can the placement of aluminium shutters solve the condensation issue?

  17. Shutters on the inside will probably make condensation worse because they will keep the room-side heat away from the window, so the interior window surface will get colder.

    Shutters on the outside could have a minor effect on the exterior by reducing the wind that impinges upon the window. Faster wind speeds remove heat from objects more quickly than still air. But the outside temperature is still what it is, so it will probably reach the window anyway.

    If the shutters were a dark color facing the sun, they could heat up the window behind it which would reduce interior condensation, but only when the sun is on the shutter.

    Thanks for an interesting question!

  18. Thank you very much for the reply Mark!
    I was actually referring to outside roller shutters (aluminium 43mm with polyutherane, gray in color). I believe this can help windows to stay warm and hence reduce condensation during winter season. What do you think?


  19. My living room’s glass window was broken with a blast. It has two layers of glass in between there is a grill. Is it because of condensation between two layers?

  20. I had no idea that my windows influenced the temperatures in my house so dramatically. I looked at how my windows were installed after reading this and it appears that they were all installed improperly. The caulking was all cracked, and there is so much moisture in and around every window. I am going to have to started using all of our ventilation fans and our dehumidifier.

  21. Hi Mark,

    What’s wrong with ventless dryers ? LG and Bosch make condensing dryers that many people enjoy using.

    In a condo application,it’s very difficult to vent out if no laundry was initially set up.Condensing dryers should be a good option in such a case.No?
    Let me know your thoughts,

  22. I have double hung double pane windows. I live in michigan, and we get swings below 0 in the middle of winter. My humidity system actively adjusts the the humidification setpoint down based on the 24h forecasted temp. Overall, that keeps the frost to minimum. I am actually lowering humidity more aggressively than your table in the article.

    However, sometimes I get a little frost on the crack, inside where the double hung windows meet.. the same surface that the window locks are on.

    My question is, would it help to lay a thin layer of plastic across that crack during the winter months? I was thinking it would prevent the colder air from entering and creating the frost to begin with.


  23. Was wondering if the tight sealed clear plastic film you can apply to the interior is a good idea?
    Would provide a much larger dead air space but can it harm thermopane windows?
    Thanks for a great read!!

  24. Further to my last post
    I live in northern ontario
    winter temps to -40
    Thks zgain

  25. If the windows are aluminum or pvc, the sealed film should not hurt the windows. If the windows are wood on the interior and there is a humidity or frost buildup inside the space between window and film, that could hurt the window finish. Try it and see what happens. Maybe do only a few windows and see the performance difference between the retrofitted windows and the non retrofitted. Let us know what happens.

  26. Yes, stopping the air movement will warm the surfaces and reduce the condensation. Since the material should be removeable for summer operation, use a good duct tape or try Mortite, a formable clay-like temporary caulk.

  27. Good point. Dryers that condense the exhaust and drain it into the drain efficiently should be ok. I have seen ones that do not have a condensing feature and they are outrageous!

  28. Has anyone ever heard of or tried using radiant heat wire (the stuff that is put in bathroom floors) to run a single wire around the entire aluminum frame to keep the interior temperature of the frame above freezing?? It seems to me that this would prevent condensation. Any thoughts?

    I have two skylights with aluminum frames that condensate to the point where it drips continuously without rain or snow. On very cold mornings (sub 20) the frames appear crystallized and when the sun rises and hits the exterior the interior warms and all that crystallized water drips.

    Here is the product I was thinking about. It comes with a thermostat that operates with a sensor that could be put on the frame to make sure it is only reading the temp of the frame itself and not the air surrounding it.

    I would appreciate any thoughts on this. Thanks!

  29. Hi Mark- Thank you for writing such an easy to understand and through article. we recently moved and have dove head first into the world of window condensation inside and in between. Our previous home had double paid casement windows and hw baseboard heat and there was never a single issue with those. The home we moved to is only 20 years old but had about a dozen out of 50 windows with blown seals. We had new IG put in those and yet more continued to blow, so we replaced the IG in those too.

    Additionally we had a new propane furnace put in which included a humidifier and on the first cold snap had heavy ice inside all windows- new, old, blown or not. We now have the humidity set below 25% so no more inside condensation or frost, but I wonder could years of interior condensation or frost from the previous owner be the root cause of all the seals going bad? They are popping up like a bad case of dandelions, and I fear we will have to do all 50.

    thank you in advance

  30. There is one fix to above . Get rid of all single and double pan windows get rid of leaky double hung window and windows with aluminum frames
    Install new triple pan windows they are expensive however those triple pan windows fixes the condensation issue
    If you do it correct, make sure the U value of your new window assembly is 0.8 and less. It is hard to find high performing windows and doors however those better doors and good performing windows are well worth their money.
    Remember the Energy lobby loves customer with faulty building envelops; and the building trades will not make an effort to change; you get what you ask for when you shop for cheap building material.


  31. Hi there,

    I have a situation that I can get out of due to how far i have got with implementation and therefore looking for possible ways to reduce and eliminate possible condensation issues that I may have in the future.

    Location: Sydney, Australia.
    Setup: Kitchen glass splash back installed on stand-offs. Gap between the back wall(tiled) and the glass is about 10mm. Planning to silicon around the edges.
    Concern: Someone has raised a point that I will/may have condensation issue behind the glass (esp. around the gas cooktop area)

    Question: will this setup be an issue? if so, are there any remedies to eliminate the issue or reduce?

    help pls.

  32. Hi AJ,
    Condensation happens when surfaces get colder or when humidity increases. If the wall behind the glass is an uninsulated wall, it could become cold enough to condense water vapor behind the glass. If you silicone around the glass edges, that should slow/prevent the migration of cooking humidity from migrating behind the glass. Summary: Seal the glass well and don’t worry about it.

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