What Causes Condensation on Windows?

  • Condensation

  • A sign of a deeper problem

  • Why do my windows condensate?


    Water droplets forming on the inside of your window this winter may do more than block your view of the falling snow---they could indicate costly moisture damage occurring in your home. Homeowners can control condensation and prevent long-term damage to the home’s wood structure and paint by regulating the humidity of the home, improving air circulation, and installing insulated triple-pane or double paned windows.



    Windows do not cause condensation, they just happen to be the place where moisture is most visible. You should use the amount and severity of window condensation as a sign that moisture damage may be taking place inside your walls or ceiling cavities. Moisture can eventually lead to rotting wood, deteriorating insulation, and blistering exterior paint.


    Moisture problems are best controlled by lowering the humidity in the home. Increasing ventilation or controlling moisture output at the source, such as venting the kitchen range to the outside, will lower a home’s humidity. And, in new homes, the natural moisture emitted by building materials raises humidity levels through the first heating and cooling cycle.


    Sometimes new windows can be a part of the solution. An old window with damaged seals or deteriorated surfaces may fog over more readily because the window surfaces are especially cold. Installing new windows with three panes of glass or two-paned windows with features such as blinds between the panes permit warmer glass on the inside, reducing the chance of condensation and creating a more energy-efficient home altogether.


    If you are experiencing condensation on your windows this winter, chances are that condensation is also occurring in other parts of your home that you cannot see. Consider lowering the humidity levels by venting rooms with high humidity levels such as the kitchen and bathrooms directly to the outside.

    INSULATING GLASS- a combination of two or more panes of glass with a sealed air space between panes. Available for a variety of products and in a variety of glass thicknesses, types, sizes and assembly thicknesses.


    PANE- a framed sheet of glass within a window or door.


    GLAZING- the process of applying or installing glass into a window or door sash. Also refers to type of glass used in the process.


    ARGON-FILLED- a type of insulating glass that delivers particularly high energy efficiency. The sealed air space between panes of glass is filled with argon -a colorless gas- which causes the air space’s insulating value.


    LOW-E GLASS- low emissivity glass has a transparent material on its surface which acts as a thermal mirror. Low emissivity glass is used to increase a window’s insulating value, block heat flow and reduce fading.



    It should be pointed out that a little fogging or frosting that comes and goes on the lower corners of our windows is nothing to be concerned about. Condensation that blocks the whole window with fog or frost, though, presents more of a problem. Runoff can cause staining on woodwork, wallpaper and plaster. Heavy condensation of this type on your windows is symptomatic of moisture damage that can be occurring throughout your home.

    Household humidity is a modern problem. That’s because more and more of us are living in moisture-trapping homes that are more economical as being cleaner and more comfortable. So… were we unwise or even a little hasty in hoping for the energy-efficient “tight” home? Definitely not. This information explains the moisture problems of the modern home and offers practical and easy to follow suggestions for solving the problem of excess indoor humidity.



    Outside air is usually drier than the air in your home. The moisture in wet air is compelled to flow toward and mix with the outside drier air, and it does so with a force of nature that known as vapor pressure. A relentless force, vapor pressure can even go upstream to airflow as it forces moisture through wood, plaster, brick, cement and most of the other materials we use to build our homes. This phenomenon can force excessive indoor humidity through siding to form blisters under the exterior paint.



    Because, in achieving today’s “tight” homes, builders and remodelers have made increasing use of moisture-trapping material such as glass, less permeable varnishes and paints, tiles and plastic wall coverings. These materials, plus the application of vapor-seal wall insulation, combine to keep moisture inside where it can condense and cause fogging on cooler window surfaces as well as dampness on other interior surfaces. Houses with no basement have additional moisture problems.



    Life-style changes are elevating water-vapor levels in today’s homes. There is more washing, more bathing, more showering and there are more appliances and gas furnaces. How much water vapor do day-to-day family activities generate? Cooking for a family of four adds 4.5 lbs. of moisture a day to a household. Other contributors, to name just a few, are: each shower; 1/2 lb., weekly laundry; 30 lbs., human occupancy; 6 to 8 lbs. Studies show that a family of four can easily release more than 18 gallons (150 lbs.) of moisture per week into household air.




    Your family needs a certain level of indoor humidity for health reasons. How can you tell if your home humidity level is acceptable for both health and home? As stated previously, the wintertime build up of a little fog or frost on the lower comers of your windows indicates your humidity is still within the proper range.

    One way to test the humidity inside your home is by using a psychrometer. The following table shows the maximum safe humidity levels for a home that is heated to 70’ F. Lower humidities are required for higher indoor temperatures.

    Outside Air Temperature Recommended Maximum Indoor Relative Humidity For A Household Temperature of 70’ F.


    -20’ F. or below .…………………………………………..not over 15%


    -20’ F. to -10’ F. .………………………………………….not over 20%


    -10’ F. to 0’ F. …………………………………………….not over 25%


    0’ F. to 10’ F. ……………..……………………………..not over 30%


    10’ F. to 20’ F. ……………………………………………not over 35%


    20’ F. to 40’ F. ……………………………………………not over 40%




    1. Shut off all household humidifier units (including the furnace humidifier).


    2. Ventilate regularly. Air out the entire house for a few minutes each day. Ventilate kitchen, laundry and bathrooms during use.


    3. Run exhaust fans longer and more often.


    4. Open the fireplace damper.


    5. Be sure attic and basement crawl-space louvers are large enough and that they remain open.


    6. Install windows that are double glazed.


    7. Vent humidity producers such as clothes driers and all gas burners to the outdoors.




    If the simple steps above don’t solve your problem, ask your heating contractor about installing an outside air vent for your furnace… or the addition of more or larger exhaust fans.


    Remember, windows do not cause condensation but excessive humidity levels can. The reasons for excessive window condensation and humidity can be complicated. Sometimes the diagnosis requires the service of a qualified expert who can make recommendations based on an overall evaluation that includes: the number and type of windows, the type of double glazing system used, the heating system (hot air or hot water, perimeter or interior wall heating), the type of insulation and vapor barrier, and the type of soil and quality of drainage. One thing is clear. Whatever is required, homeowners cannot afford to let humidity go unchecked.


    The indoor humidity can be checked with a humidistat, available as a dial gauge or a more accurate wet and dry bulb thermometer.

    In today’s tightly sealed homes during cold weather some condensation can be considered normal.


    Possible offenders are:


    • Humidifiers working with warm air furnace


    • Kitchens


    • Fish Tanks


    • Obstructed attic vents


    • Moisture collected in carpets, furniture and drapes


    • Bathrooms


    • Laundries


    • Damp Basements


    • House Plants


    • Fireplaces


    • Improperly vented driers


    Controlled ventilation and elimination of excessive indoor moisture can keep humidity within bounds.


    1. Turn off or set back furnace humidifiers until sweating stops, remove pots of water on radiators or kerosene heaters.


    2. Use exhaust fans or open windows slightly in kitchen, bathroom and laundry during periods of high moisture production such as cooking, taking showers, washing

    and drying clothes. Clothes driers should be vented to outside. Do not hang clothes to dry indoors.


    3. Keep basement as dry as possible by waterproofing floors and walls.


    4. Make sure attic vents are unobstructed.


    5. Place all house plants in one sunny room where the door can be kept shut and avoid over-watering.


    6. Opening windows slightly for a brief time will allow humid air to escape and drier air to enter. The heat loss will be minimal.


    7. Use a dehumidifier, properly sized, to reduce the amount of humidity (moisture) in the house.





    “I never had condensation before you installed vinyl replacement windows......Why do I have it now???”


    Your old windows were not energy efficient-they permitted moisture laden air to escape around the perimeter of the sash and around the window frames if not properly caulked.


    By adding new insulation, roofing, siding, certain paints and acrylics, and vinyl replacement windows, you make an old home more energy efficient, thus trapping heat and moisture that formerly escaped.


    Vinyl replacement windows are among the most energy efficient methods of fenestration available. Moisture and heat that escaped through your older much less efficient windows is now trapped, it cannot escape, and the moisture appears on the glass in the form of condensation. The danger is not the water on the glass, but the condensation you cannot see—soaking into your insulation; underneath your paint; into wood studs; and promoting mildew in your home.


    Your new energy efficient vinyl replacement windows have shown you that the excess interior humidity (moisture) must be reduced to avoid future problems in your home.


    The attached report on the causes and cures for the excess humidity in your home should be carefully read to identify for you what problems may exist in your home.

    “On my old windows, I had no condensation. Since I got new windows in my house, I find that in cold weather, frost builds up on them and later, water runs down on the sill. Did I buy bad windows? What can I do about it now?”


    Maybe, maybe not. Various factors affect the formation of condensation on windows: the windows themselves, the indoor climate and the outdoor climate. Obviously the outdoor climate did not change because of the new windows. That leaves us with the indoor climate and the windows. The homeowner knows the windows were changed but what most people do not recognize is that consequently, the indoor climate changed as well.


    Why is that? The old windows were not airtight and therefore, a great deal of air exchange with outside occurred. In winter, outside air is quite dry, so its infiltration reduces the relative humidity inside the house. When the humidity level inside is low, the potential for surface condensation is also reduced. Then new windows are installed, which are certainly more airtight than the old ones. The exchange of dry cold air does not happen as much. So the moisture level in the house increases, even though the indoor activities have not changed. These factors are interconnected: acting on one makes the whole system react. In other words, by performing better (being more airtight) than the old ones, good new windows appear to be more sensitive to condensation. Understanding this opens the door (or the window) to solutions.


    Today, most windows are made differently than they used to be. For example, factory-sealed glazing, while reducing the maintenance of glass substantially, has a feature that conducts heat and cold considerably at the edge of the glazing. This metal component (called “spacer”) often relates to interested condensation potential at the perimeter of the glazing. The detailing of the joint between the glazing and the sash, and the sash and the frame, as well as the installation of the windows, and all contribute to making the window more vulnerable to condensation.


    The solutions will require some changes in the house. A look at living habits that generate moisture is in order. First, the humidifier: it may have been needed with the old leaky windows, but this may not be the case anymore. Turn it off for a few weeks and monitor how the inhabitants and the windows feel. Examine other sources of moisture such as cords of wood drying in the basement, a non-vented clothes dryer or clothes being dried inside. Unventilated bathrooms, a dirt floor in a crawl space or a wet basement. Minimize their input to the moisture load inside: do not dry large quantities of wood inside, vent the clothes dryer outside, in the bathroom use an exhaust fan connected to a timer or the light switch, install a thick plastic film on a dirt floor, use a sump pump if necessary, have an eaves trough system to drain roof water away from foundation walls, etc.


    Moisture removal can also be improved by ventilation. Install a fresh air intake in the basement and use an exhaust fan frequently to contribute to air exchanges with the outside in a controlled way and in a controlled path. (If the home has a combustion heating system, exhaust fans should always be used in combination with fresh air intakes to avoid backdrafting of exhaust combustion gases.)


    Examine heating habits as well. Turning the indoor temperature down at night may be good for the heating bills but it results in colder windows, more prone to condensation. The heat delivery at the window is also important. The heat registers should be free of obstructions (remove added-on air deflectors, move furniture away from registers and remove plants and shelves from the top of water radiators). Blinds should be left open during the day, to get rid of the dew formed during the night.


    The fan of the forced air heating system could run continuously to cause convection at the window surface. As a last resort, you can try a remedy used for old windows: install a thin plastic film on the inside of the window, tightly sealed at the edge to the trim or the interior finish. Try it on one window (after condensation residue has dried out) before deciding to apply it all over the house.


    So, what is the point of getting tighter windows if later, extra ventilation has to be added. Here are a few reasons: Comfort- tighter windows minimizes cold drafts into the rooms. Air quality- a controlled flow of air should come in and out of the house irrespective of the outdoor climate. Mechanical ventilation allows for this control while natural leakage through cracks does not.


    Durability of materials: Condensation of warm humid air exfiltrating through leaky windows can cause damage to the walls underneath the windows when it melts. Being more airtight, new windows reduce this occurrence. Tighter windows are more energy efficient as well.


    Dear Sir: My son has a small two-bedroom ranch house, approximately 30 years old, and is having a humidity and mildew problem.


    The windows were installed some time ago by the previous owner. They appear to leak, which may be some of the moisture problem as they form condensation on the inside considerably. The window type is somewhat loose fitting.


    The attic insulation (orignally) appears to be a paper and glass type that is roughly 2 to 2.5 inches thick. This was installed by the original builder by just laying it in place after the rock lattice was installed, so the insulation slabs were not stapled to the ceiling joists to form a solid vapor barrier.


    The reason that I mention this is because there are signs of moisture and mildew appearing at the ceilings.


    We will also add a ridge vent and soffit vents to help ventilate ‘the attic space as there are only two small louvers (8 x 12 inches) at each gable end. We would appreciate your comments.


    Condensation on the windows in the main portion of your son’s home is probably a result of a too high humidity level in the home. When late fall rolls around, a home is usually quite humid because of the dampness of the basement and other interior spaces through the spring and summer months and also because the furnace may not yet had time to dry out the inside of the home.


    Even the best-installed windows will show signs of condensation when the exterior temperature drops suddenly.in the winter, high interior humidity (and, consequently, condensation on w’indow’ glass) can be caused by excess moisture created by cooking, bathing or doing laundry.

    Therefore by using the kitchen, laundry and bathroom exhaust fans you should be able to rduce the indoor humidity during the heating season to the point where the glass remains free of condensation, for the most part. In the meantime, opening windows widely on dry days for a brief period of time right as evening approaches will help to bring less humid air inside the house.


    The mildew on the ceilings is another story. True, fans and lower indoor humidity will help to some extent, but you should also look for other potential causes of this sign of excess moisture. The additional attic ridge and soffit vents that you plan to install are a good start in controlling attic condensation, one of the potential causes of your ceiling mildew.


    I’d climb up in the attic with a flashlight and check for dark splotches or crystallized water droplets on the rafters and underside of the roof sheathing; sure signs of condensation.


    Also, carefully examine the existing insulation by looking and feeling for signs of dampness.

    Carefully check the areas over the kitchen and bathroom(s).


    If there is dampness or signs of previous dampness in any of the above places, you’ve got too much moisture and not enough air circulation in the attic.

    Add your vents and remove any damp insulation. Place heat lamps and/or electric fan-type heaters in the attic to dry out any damp areas. The plaster will take a while to dry, but you should be able to reduce the moisture level considerably in a week or nvo.


    Do the same treatment downstairs for the ceiling areas too. Washing the mildew spots with chlorine bleach will kill the fungi that cause the growth.

    Once you manage to get everything dry, you should re-insulate with vapor barrier-faced fiberglass such as have now. Install the vapor barrier down toward the living space, and friction fit it between the joists. On top of this, consider adding another layer of unfaced insulation either fiberglass bats or blown in cellulose or fiberglass. Eight to ten inches total is not too much.


    What is humidity condensation?


    It’s common knowledge that too much humidity can cause extensive and expensive damage throughout a home. It would help, of course, if we could see this harmful humidity build-up as it is occurring in the air, but we can’t. Humidity is as invisible as the air that carries it.


    Fortunately there is one reliable visible warning sign. We see it when the build-up of water vapor condenses to form fog or frost on the window panes. Understanding Condensation is the best way to stop it from affecting your home.



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