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Why do I have condensation on my windows?

4ekWeQhxRn | Posted in General Questions on

I just built a house in upstate South Carolina and moved into it this past spring. It has 2×4 wall construction with OSB, housewrap, and vinyl siding on the exterior. The walls and ceiling are insulated with blown in cellulose. I have an electric heat pump and it is equipped with a humidity control system. My windows are mid-grade vinyl. Since cold weather is setting in I am having some trouble with condensation forming on my windows on cold mornings. Is my indoor humidity too high, or is this a sign that something else is wrong?

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  1. Riversong | | #1

    Yes and yes.

    Your indoor relative humidity is too high (do you have whole-house ventilation?) and you may have either poorly-insulating windows or poor window installation that is allowing cold air infiltration.

    Get a digital hygrometer to monitor indoor relative humidity and keep it below 40%.

  2. J Chesnut | | #2

    It is the combination of vapor in the air and the glass surface falling below the dewpoint temperature.
    In new quality construction condensation should not be an occurrence. How much of a problem your situation is depends on several factors.

    Your midgrade vinyl windows likely have poor U-value and probably facilitating the condensation.
    Also cold air could be leaking through your window frames. Does the condensation show a certain pattern on the windows. You usually see more condensation at the lower part of double hung sashes due to air leakage.

    What exactly is your "humidity control system"? and how much of your windows are getting covered with condensation?

    For us to gauge if you have excessive humidity in your home (which could potentially find its way to the moisture prone OSB through air leaks in your wall assembly) you'd either have to measure your interior RH with a hygrometer or describe to us the number of people living in your home and the amount of hot water you use in your daily routine.

  3. RaterPaul | | #3

    Robert ... let's say that the inside temp is 75 degrees with a RH of 40% and outside is 45 degrees, If every degree of difference in the Delta-T approximates a 2% difference in the RH and discounting the u-value of the glass, dew point is approached at the glazing. What is the answer as the Delta-T widens?

  4. Riversong | | #4

    Wow, RaterPaul! Asked the question three times and I still don't have a clue what you're saying.

    I dunno. Maybe the answer is 42? That's the answer to everything.

  5. RaterPaul | | #5

    LOL... either that or "it tastes like chicken"...don't know what happened...thought I only hit it once ... anyway.
    My point was that with the dew point being reached within the vacuum of the thermopane window, no condensation should appear on the glass unless air leakage is causing the dew point to move to the inside of the glazing... no?

  6. Riversong | | #6

    Thermopane windows haven't had vacuums since the days of Andersen welded glass - they have inert gas (argon or krypton) and desiccant. The inside center-of-glass temperature is dependent on inside and outside temperatures, the number and spacing of glazing layers, and the types and thickness of coatings.

    But the two (or more) panes of glass are connected at the edges with metal spacers that may or may not have a thermal break. So glazing units can easily have dewpoint temperatures at the edges of the glass and the bottom of a window is always the coldest due to the falling of cooled air, often pooling at the sill.

  7. tEMhybnVk5 | | #7

    Simon, it seems like your indoor humidity level might be too high (above 40%). Maybe your mid-grade vinyl window has a poor U-value and/or the installation was done poorly. Also, I would consider the possibility of your window is defective and air would be leaking thru the frame. Make sure you have a digital hygrometer to monitor the indoor humidity and it is below 40%.

  8. GBA Editor
    Martin Holladay | | #8

    As I've often said, there are only two ways to solve this problem:

    1. Lower the indoor relative humidity. (During the winter, this can be done by increasing the ventilation rate.)

    2. Raise the temperature of the glass. (This can be done by installing storm windows or replacing your existing windows with new windows with better performing glazing.)

    Considering the fact that you have a new house, I would suspect moisture from construction materials that haven't fully dried out yet. I suggest you leave your bathroom exhaust fans on for 24 hours a day for a couple of weeks and see if you see any signs of improvement.

  9. RaterPaul | | #9

    "Thermopane windows haven't had vacuums since the days of Andersen welded glass" ... then why does an infrared camera show "footballs" in defective glazing? I am aware that thermopanes currently " have inert gas (argon or krypton) and desiccant", but understood that the "footballs" reflect a break in the vacuum that allows the glazing to "swell" at the center.
    Be that as it may, considering that U-value is the inverse of R-value so that a glazing with a U-value of .25 (pretty good) has an R-value of 4 (very poor thermal resistance), the condensation problem likely is more a result of high indoor humidity and/or air infiltration than poor glazing U-value.
    I highly respect your opinion and although my comment is stated as fact, it is actually a question soliciting your feedback.

  10. GBA Editor
    Martin Holladay | | #10

    Just because glazing spacers are airtight (gas-tight) -- or as nearly so as current technology affordably permits -- doesn't mean that the space between the panes is a vacuum.

    Double-pane IGUs are sometimes convex and sometimes concave, although IGU manufacturers strive to keep the two panes as parallel as possible. If the panes are convex -- a condition you describe as a "football" -- that doesn't necessarily mean that the seals have failed. Changing atmospheric pressure can cause the phenomenon.

  11. Bradleyman | | #11

    I found that the air return in my house was the big culprit for my cause of condensation on my windows. (I had my house tested and it was very tight) I solved this problem by putting in a passive cold air connection into my return air duct to my furnace. Hope this helps.

  12. wjrobinson | | #12

    If you have a automated humidifier as part of your HVAC turn it off by turning off the water supply to it. Most are tapped into a water line and at the tap is a valve... righty tighty turns of the water.

    Get a humidistat for your use so you can observe the level.

    And as others have said... new homes have too much moisture for 1-3 years. So... keep automated system off at least for the first year and then don't set it too high. You may well have plenty of moisture from inhabiting your home and never need the automated system to ever run especially in newer tighter construction. Also depends on where you live... Coastal, is different than mountain top in the Carolinas.

  13. Riversong | | #13


    You're not paying attention. Glazing U-value is relevant only to center-of-glass temperatures and conductivity. You won't generally see condensation in the center of a pane of insulated glass - you'll see it at the bottom where the U-value is determined by the glass spacer and window frame, which is always lower than the center of glass.

    Better quality IG window units have less conductive spacers and frames (and tighter weatherseals), and are less prone to condensation under the same conditions.

    What you call "footballs" in IG units is caused by barometric pressure changes in the local environment. If you're building at 10,000 ft elevation in Colorado, you have to specify high-altitude glass or it will always be bulging since the local atmospheric pressure is lower than where the windows were produced.

    Solar intensity can also effect the internal pressure of an IG unit, depending on the types of glass coatings. Bulging of an IG unit is NOT an indication of seal failure, it's an indication that the seal is intact - internal condensation or hazing IS.

  14. Riversong | | #14

    "at the bottom where the U-value is determined by the glass spacer and window frame, which is always lower than the center of glass."

    Correction: U-value is higher at the edge. More conductive, lower temperature, higher probability of condensation.

  15. RaterPaul | | #15

    Robert ... I'll go sit in the corner if I must...but "footballs" are caused by many more factors than barometric pressure and I would fathom that more times than not, the footballs are reflective of defective glazing than some atmospheric anomaly. The highest natural elevation in my area is the driving range at Oyster Reef Golf course & that is 17' above sea level ... many of the windows shipped here show convection ... a barometric anomaly unless they are defective. Didn't mean for this to become antagonistic or condescending!

  16. aj builder | | #16

    Raterpaul... I am sure you are a great person... but... you definitely are clueless as to what you are talking about.

    Exterior condensation is a whole different topic from broken window seals and fogged up interiors of double pane windows... and I am not going to get into your footballs...

  17. Riversong | | #17


    When I said "you're not paying attention", that was in reference to your ongoing insistence, in spite of explanations to the contrary, that the glazing U-value, rather than the whole window properties, determines interior glass temperatures and condensation potential.

    The rest of this may be a communication problem (though I don't think so). Perhaps you'd care to define "footballs" or use technical language rather than slang. The typical hits on Google for footballs and windows were: "How can I stop youths using my windows as football goals?" and "Blitz Football comes to Windows Phone 7".

    Your first triple post was completely unintelligible and I don' t have a clue what you mean by "many of the windows shipped here show convection".

    But if by "footballs" you mean convexity of the glazing panels, then that cannot be a result of seal failure which would equilibrate the interior and exterior pressures.

    While it's true that loss of argon in a sealed IG unit typically occurs at a faster rate than that at which air molecules can replace it, thus creating a slight negative pressure and concavity of the glass, that is not a sign of seal failure as much as normal leakage. Seal failure is always evident from condensation or hazing between the panes.

    And it does not require an "atmospheric anomaly" to create glass deflection, simply the normal daily fluctuations in temperature and barometric pressure.

    From Cardinal Glass: A difference of 500' in elevation between the location of manufacture and the location of installation will distort the glass equivalent to a 118 mph wind. Taking an undeformed window at ground level and installing it on the 10th floor creates the pressure equivalent of exposing it to a 53 mph wind. A 20° change in air space temperature creates the pressure equivalent of a 175 mph wind. A normal daily barometric pressure change of 0.39" mercury would exert the equivalent of a 105 mph wind.

  18. RaterPaul | | #18

    regarding my "ongoing insistence ... that the glazing U-value, rather than the whole window properties, determines interior glass temperatures and condensation potential.", I stated quite the opposite. "...U-value of .25 (pretty good) has an R-value of 4 (very poor thermal resistance)..." inferred that regardless of the u-value of the glazing, fenestration sucks when it comes to thermal resistance.
    I agree that air leakage and thermal bridging (what you refer to as "whole window properties") combine to most greatly affect interior glass temperatures and condensation potential ("the condensation problem likely is more a result of high indoor humidity and/or air infiltration than poor glazing U-value")..
    My first post (which you refer to as" completely unintelligible" simply stated that 75 degree air with an RH of 40% reaches dewpoint around a dry bulb temperature of 45 degrees ... not difficult to understand. As the delta-t widens, the effects are more significant.

  19. Riversong | | #19

    Since U-0.25 and R-4 are two measures of the same thermal performance, one can't be "pretty good" while the other is "very poor".

    Actually R-4 is excellent thermal resistance for an architectural element that is designed to provide views, daylighting, ventilation and solar gain (the windows the OP described are probably no better than R-2). And a window with a whole unit R-value of 4 should not experience condensation if indoor humidity is controlled.

    One of the things you said in your first post was: "every degree of difference in the Delta-T approximates a 2% difference in the RH". Delta-T is the difference between indoor and outdoor temperatures and has no effect on indoor RH except insofar as it impacts natural ventilation rates.

    If you mean that the RH changes 2% with every degree drop in indoor temperature, that's incorrect. Relative humidity and temperature are not linearly related - RH approximately doubles with each 20° drop in temperature.

    I still have no idea what a "football" is. And insulated glass units don't contain vacuums.

  20. HDendy | | #20

    Any of you have a recommendation for a good digital manometer to check the indoor RH? It seems that would be the first place to start for Simon. I have one of those indoor/outdoor "weather stations", but I'm not sure how accurate it is. I have one of the sensors in my finished attic and the other in my (semi-conditioned) basement, originally to track the attic space out of curiosity/concern how it was performing after I converted it into my office. It has been interesting the see how close the two are tracking to each other.

    Here's an interesting situation, getting back to the original post- I have 1920's original DH single-paned windows with exterior storm shutters. Recently I noticed condensation on the interior faces of the storm glass and in the same room there's a door with single pane glass, no storm shutter, and there was no condensation on that glass. My assumption is that the air between the window and the storm shutter has a higher RH, and dew point. It is a little surprising I guess since the old DH windows are in no way 'tight.'

  21. homedesign | | #21

    It is a little surprising I guess since the old DH windows are in no way 'tight.'

    The Storm window is "cold" and the DH windows leak warm moist air.

  22. HDendy | | #22

    Yes, but why isn't the glass on the door also causing condensation from the indoor air? There is no barrier between the door's glass (single pane) and the outdoor air. There's also one window that does not have a storm window, again, no condensation on the interior. My indoor RH is 33%.

  23. homedesign | | #23

    the inside face of the door glass is warmer than the inside face of storm sash

  24. Riversong | | #24


    You mean hygrometer. A manometer measures air pressure.

    I've used an Airguide Home Comfort Station digital thermometer/hygrometer with high/low memory (no longer available) from Therma-Stor Products which I bought perhaps 20 years ago and has served me well all this time. They now offer a tracking digital humidity meter, the Humidity Alert, which keeps track of accumulated hours of high humidity and has a programmable audible alarm.

    But for $30 you should be able to buy a digital thermometer/hygrometer almost anywhere that will perform reasonably well.

    As for your condensation, the window insulates the storm from the interior air temperature while the door glazing is closer to room temperature. This is the same phenomenon involved with condensation on windows behind shades, shutters or curtains.

  25. HDendy | | #25

    Ahh, of course. I feel silly for not thinking of that before posting. I can't wait to get my hands on an infrared thermometer and do my own field tests. Nothing beats observational learning.
    Sorry for the type-o (hygrometer not manometer, must have had something else on my mind).

    ...Robert, when's your next hygro-thermal class? Maybe I can talk my family into a trip North.

  26. Riversong | | #26

    Hygro-Thermal Engineering: Managing Moisture in the Home will be offered again the weekend of May 28-29, 2011.

    "Robert's presentation on moisture mechanics was the best presentation I have seen on the subject. I would highly recommend this workshop to all builders, architects and building trades people. I believe the issue of moisture as it relates to residential (and commercial) construction is one of the most important pieces of building science a builder today should have a strong working knowledge of."

    - Jay Walsh, Energy Analyst, Energy Star Homes and LEED-H Rater, Center for Ecological Technology

  27. Thomas Jefferson | | #27

    not manometer, must have had something else on my mind

    Man meat?
    Sorry, just giving the moderators more to do.

  28. Anonymous | | #28

    I am a new house homeowner with what I thought were high quality fiberglass triple pane windows - and I too have condensation on the bottom of my windows when the outdoor air temperature is very cold (below 0 F) and the indoor air temperature about 60 F, even though the indoor RH is usually less than 35% because I use an HRV. My house is tightly insulated/sealed and I thought the window installation was very good. It seems like the only other way to get rid of the window condensation is to increase the air temperature to well over 70F. Any other suggestions?

  29. Riversong | | #29

    Nameless New House Owner,

    If the RH is 35% at a normal indoor temperature of 70° and you turn the heat down to 60°, the RH will rise to 50% while the inner glass surface falls from 60.5° to 52° - still well above the 41° dewpoint, but that's center of glass temperature.

    If there is a conductive glass spacer and cool air pooling at the bottom of the sash, then the window can easily reach the dewpoint. There may also be air leakage, either through the window's weather seals or around the frame because of poor installation.

  30. GBA Editor
    Martin Holladay | | #30

    Be sure that you aren't closing any curtains or blinds. Curtains or blinds can cool the surface of your glazing, making condensation more likely.

  31. Glenn Summers | | #31

    HUMMM: Lets see, we have interior window condensation>> Is it possible your HVAC syatem is too large for the interior space ???
    Your cool down is so quick, you don't get the proper de-humidification before the unit gets cold and shuts down.. Nah--- TOO Simple ....................................................

  32. Riversong | | #32

    HUMMM: Lets see, is it possible...Your cool down is so quick...

    HUMMM: Let's see:

    Original Post: Since cold weather is setting in I am having some trouble with condensation forming on my windows on cold mornings."

    I guess not.

  33. J Chesnut | | #33

    Recently I've been made aware of condensation on the exterior window surface of triple-glazed windows in homes meeting Passivhaus standards. Not enough heat going through the glazing to keep its outside above the dewpoint.

  34. Riversong | | #34

    Not enough heat going through the glazing to keep its outside above the dewpoint.

    WHOA, wait a minute. ANY heat going through the glazing, combined with the R-0.17 exterior air film, will keep the outer glass a smidget about outdoor air temperature. And, if it's 1 degree above outdoor air temp, then it's above the dewpoint.

    The outer glass would have to be colder than outdoor air for it to be below the outdoor dewpoint.

  35. J Chesnut | | #35

    Oops, I meant frost.
    Check out the December 17, 2009 entry of this blog -
    Maybe not a phenomena limited to triple glazing.

  36. Riversong | | #36

    Yup, that sure looks like hoar frost, probably from nighttime radiation losses from the outer glass, which has an emissivity of about 0.87 (high-E) after a warm, moist morning breeze brought in the damp.

    But that says nothing about the efficiency of the house. Any decent triple-glazed window will have an outside glass surface temperature about 2° or 3° above ambient on a cold night.

  37. GBA Editor
    Martin Holladay | | #37

    It happens. Night-sky radiation can lower the temperature of a surface below the outdoor air temperature.

    It won't happen unless the window can "see" the sky.

  38. homedesign | | #38

    "Sky Radiation" is very interesting indeed.
    It not Only occurs at night.

    Bill Rose: Long-wave radiant exchange between the sky and exposed surfaces is strong, especially in clear sky areas.
    -Not just night-sky, but "sky" radiation.
    It is important during the DAYTIME as well.

  39. user-659915 | | #39

    "Recently I've been made aware of condensation on the exterior window surface of triple-glazed windows in homes meeting Passivhaus standards."

    We commonly see exterior window condensation here in our humid NC summers when a single-pane window is chilled by interior a/c. This is obviously not the cause of the PA examples. The most likely cause on a triple-glazed window would be a sudden warm moist front coming through after a cool dry period and causing condensation on the still-cold exterior pane: the better-insulated triple-pane glass would be more prone to this effect, at least during the heating season. Any information on the weather conditions around the reported PA events?

    This should in any case be a very temporary condition and quite harmless if the window is properly installed: the exterior of any half-decent window is designed to deal with considerably greater quantities of surface moisture than any amount of condensation could produce.

  40. Riversong | | #40

    It won't happen unless the window can "see" the sky

    So then an eyebrow dormer can protect the window from frost? ;-)

    Maybe an eyelid shutter better yet?

    As for daytime sky radiation:

    The sky temperature may not change a great deal from day to night, but during the day any radiative (long-wave) losses tend to be balanced by solar (full spectrum) gains. So it's at night that radiative cooling to the sky becomes significant. The amount of heat loss is largely dependent on the emissivity of the sky which is itself determined by the amount of water vapor or cloud cover.

    There is negligible heat loss to a cloudy sky, but effective sky temperatures otherwise tend to be between 18°F and 36°F below ambient ground-level air temperature. On a clear dry night, it's possible to lose as much as 70 W/M² or 22.2 BTU/HR·SF. On a humid night, the loss is in the 40-50 W/M² or 12.7-15.9 BTU/HR·SF.

    I did my own crude experiment on Xmas eve in 1989 when I slept outside on a -15°F night. I was wrapped in my down mummy bag, except for my face which was pointed up at a black night sky. Though I was toasty warm, something woke me in the middle of the night and I noticed that my nose was numb (frost-nipped), so I built a fire and prevented the frost from fully bighting my nose. It was that night that made me appreciate mother's admonition to always wear a hat in the winter (because the more you think, the more heat you lose to the sky ;-)

    Robert's corollary to mother's admonition: Either wear a hat in winter or don't think about it.

  41. user-659915 | | #41

    "I did my own crude experiment on Xmas eve in 1989 when I slept outside on a -15°F night."
    This anecdote fully explains Rudolf's red nose, I guess. Were you watching for a star arising in the East or a sleigh coming from the North?

  42. Riversong | | #42

    Actually, I went up to Catamount State Forest (where Boston Township #2 used to be) to get away from the Xmas madness. So what happened? On Xmas day, I was invited by a man who was putting up Wood Duck houses with his daughter around McLeod Pond (an artificial lake at the top of the mountain that used to be the site of the "rich girls' resort") to join his family for Xmas dinner at his 1836 farmhouse (which was originally built by Sam Maxim who operated a foundry as well as a cider mill which supplied some of the "spirits" for the spiritualist gatherings which took place at Spirit Rock behind the farm).

    How do I know all these things about Catamount? I did an "environmental plot study" of the mountain for my certificate in Outdoor Education, which included both the social and natural environment of the 3500-acre hill. I interviewed the three still-living people - all of them old women - who had populated the hill-top community which was nothing but cellar holes when I knew it.

    Catamount had the distinction of having the first schoolhouse (1 room, of course) over which an American flag (hand-stitched from rough-spun cloth) was raised - in support of Jefferson's war of 1812 against the British. But the night it was raised, the Federalists took it down and absconded with it. I, however, ran my fingers over that 16-starred banner some 177 years later at the home of Florence Reynolds, daughter of Elmer Davenport who descended from the initial settlers and wrote a booklet called "The Puzzle of Catamount Hill".

    I first learned of that booklet at the Xmas dinner at the old Maxim farmhouse, and that led me to seek out the people it mentioned. I also spoke with the mother of a friend of mine who had left the mountain when she was still quite young. When I asked her about her memories there, she said she couldn't recall because "I was young when I was born there".

  43. user-969424 | | #43

    I didn't see any mention of the amount of water it takes to build a home. Water is the main ingredient in concrete, drywall mud, latex paints, and it is found in wood products not to mention water infiltration during construction. Once the house is closed up it can take a while for the water to leave the envelope. I not saying this is the sole contributor to high humidity in a new home but it certainly contibutes a considerable amount of water to the interior which is then trapped by tight construction practices.

  44. GBA Editor
    Martin Holladay | | #44

    Actually, one of my responses -- response #10, posted way back in December 2010 -- mentioned construction moisture. I wrote, "Considering the fact that you have a new house, I would suspect moisture from construction materials that haven't fully dried out yet."

  45. user-1021589 | | #45

    So ... Why do I have condensation on EXTERIOR of my TRIPLE glazed FBG windows?

    This issue was touched upon in this post, especially posts 35 through 41, but not in great detail. Here in the Ottawa Valley, Ontario, in JJune through August, this happens every sunny morning. It cant be a sudden warm front each time (if I understod one explanation properly) so what is happening here, and is there any way to address this? Happens on all windows, first and second story, N-S-E- and W. Doesn't occur on double-paned door-windows.

    On the second storey the effect is less towards the top of the windos nearest to the eaves. On the first storey it effects the entire window - and I have yet to install the solar awning over these windows.

    This is a real disappointment from an esthethic perspective, given the cost of these windows - we would like to be able see out of while having our morning cuppa. And of course, the water leaves and unsightly mess on the glass thereafter making the windows appear dirty.


  46. GBA Editor
    Martin Holladay | | #46

    You have condensation on the exterior of your triple-glazed windows early in the morning when there are no clouds in the sky -- for the same reason there is dew on the grass. The outside surfaces of your windows are colder than the air temperature, just like the grass, so that is where condensation occurs.

    How can these surfaces be colder than the air temperature? The explanation is night sky radiational cooling. The window panes start out with some heat, but some of the heat is radiated to the night sky, which is extremely cold. The heat in the window panes radiates to outer space, and there are no clouds to stop the radiation. By earlier morning, the window panes are colder than the air.

    Some of your 2nd-story windows are tucked under your eaves, and the tops of your windows can't "see" the sky, so this radiation can't occur.

    If the phenomenon bothers you, plants some trees between your windows and the sky. Otherwise, enjoy the effect; it's a sign that you have excellent windows. (Double-pane windows don't get as cold, because they leak heat from the interior of the house, which warms up the exterior surfaces.)

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