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Internal Condensation

Window Condensation: Causes and Remedies


Water or frost on windows is condensation. Condensation is formed when warm moist air comes in contact with cooler dry air just as a bathroom mirror will “steam up” after a hot shower. The inside or outside of your window can sweat or fog because of temperature differentials.


Faulty windows do not cause condensation. Glass is usually the place you first notice condensation because glass surfaces have the lowest temperature of any of the interior surfaces in the house, you will most probably have noticed that your internal house mirrors also cover themselves in condensation too.


The moisture in the air causes condensation. The reason you may observe more condensation in your home is because of modern energy efficient homebuilding techniques and products.

The insulation and construction materials used today are designed to keep cold air outside. This is especially true of new windows. While energy efficient designs and weather stripping keep cold air outside, they also keep warm moist air inside. Older window designs were less efficient, and consequently allowed moisture to escape.

If you didn’t have as much condensation before replacing your old windows, it’s probably because they were draughty. Good windows and insulation all create barriers to the air exchange of a home. When combined with the additional water vapor (moisture) from showers, cooking, or from clothes dryers not vented to the outside, the result is excess moisture and a high relative indoor humidity level.


The key lies in controlling the humidity inside your home. First, let’s understand where the moisture comes from. During the hot humid summer, your house absorbs moisture. The same principle applies to a newly constructed or remodelled home, due to the abundance of moisture from the building materials used in construction.

During the beginning of the winter when you start to heat your home, condensation occurs. After a few weeks, your home will begin to dry out and you’ll see less condensation. Opening a window briefly is a quick temporary solution. The dryer cold air will enter the room while the moist air is allowed to escape.

Other steps to take include:

  • Cracking open a window or door daily to air out your house.
  • Opening a window or running exhaust fans longer in the kitchen, bathroom and laundry room.
  • Opening drapes and blinds, allowing air to circulate against windows.
  • Turning off any humidifying devices in your home.
  • Installing and using a dehumidifier.

If you live in a northern climate, the above as well as the following points may be relevant.


Window condensation should only occur during extreme temperature differences and should be of a fairly small amount. During the winter months, condensation will be seen on the inside of the window. Condensation will present itself on the outside of the window during the summer months.

If you find condensation between the two layers of glass in an insulated window, the airtight seal has probably been broken and the glass will need to be replaced.

If there is too much moisture inside the home, you will find evidence during both the cold and warm seasons. Moisture spots on the ceiling or walls, peeling paint, rotting wood or delaminating plywood, moisture on exterior walls, fungus, mold or mildew growth are signs of a more serious moisture problem. Should you experience these symptoms, an expert heating & cooling contractor should be contacted in order to solve your problem.

  • Adding storm windows or replacing existing single-pane windows with insulated windows.
  • Keeping plants in a sunroom, or in rooms that are infrequently used during extreme cold weather.
  • Adding waterproofing protection to basement floors and walls.
  • Removing radiator pans until sweating has been eliminated.
  • Making sure that open-faced gas heaters are connected to a chimney and using them as little as possible.

How to Cure Window Condensation in Your Home

Causes and Cures for Window Condensation

Each winter sees more and more homeowners vitally interested in solving window condensation. Window condensation is not a happy interest because of bad experiences, which range from irritating to downright expensive.

It may strike you as odd, but the growing condensation problems of the nation are caused by progress. If you have trouble with window condensation, it's probably because you live in a "tight" modern home that you can heat for a fraction of the money it took to heat the house your parents lived in—a home that's cleaner and more comfortable besides! Your condensation problems also result from use of labor saving appliances that make life easier than it used to be.

This article explains the moisture problem of the "tight" home. It offers suggestions for curing condensation problems in existing homes and provides additional suggestions for you who are planning a home. You unquestionably will build a "tight" home, and there are more things you can do to prevent excessive moisture when you build than can be done in a home where the problem already exists.

What Causes "Trouble" Condensation?

A little fog on the lower corners of your windows now and then probably doesn't bother you. By the time you've thought about it a second time it has usually gone away. But we are talking about excessive condensation. Condensation that blocks whole windows with fog or frost. Water that runs off windows to stain woodwork, or in serious cases even damage the wallpaper or plaster. If you have this kind of condensation on your windows, you have good reasons to be concerned and good reason to act.

Don't worry so much about the windows where you can see the effect of excess humidity. You should worry more about what excess moisture may be doing elsewhere in your home. It may be freezing in the insulation in your attic where it will melt and damage your plaster exactly like a roof leak when warm weather comes. Or it may be forcing its way out through siding to form blisters under your exterior paint. That means the most expensive kind of a paint job. It's easy in such cases to blame the paint, or the insulation, or the windows, but it's wrong to blame them. The real villain is invisible. It's water vapor. The best, and usually the only way, to prevent this trouble is to get rid of excess water vapor. Once you've equipped your windows with good storm windows, there isn't very much more you can do to the windows to lick condensation.

What Is Humidity?

Humidity, water vapor, moisture, and steam are all the same in that they are each a form of water. Humidity is an invisible gas. It is present in varying quantities in nearly all air. This moisture in wet air tries to flow toward drier air and mix with it. Scientists describe this force as "vapor pressure." It is often a very powerful force indeed. It can act independently of the flow of the air which holds the moisture. Vapor pressure can force moisture easily through wood, plaster, brick, and cement—right through most of the materials we use to build our homes. That is exactly what happens when moisture seeks to escape from the humid air usually found inside your home to the drier winter air outside.

More Moisture Trapped in Less Space

Certain building materials stop water vapor. Glass is one of these. Also on this list are some varnishes, paints, tiles, and plastic wall coverings. Vapor-seal insulation is designed specifically to stop the escape of water vapor and protect the insulation and your walls from the ravages of water.

Increased use of these "moisture trapping" materials in the last few years has created the modern "tight" home. Moisture created by bathrooms, kitchens, laundries, and occupants no longer flows easily to the outside. The modern insulation and construction that keep cold air outside also keep moisture in, so it is very easy to build up excessive and even harmful moisture levels in such a home. American Builder calls the problem a combination of many causes that build excessive moisture in the modern home.

First, more washing, more bathing, more showers, more appliances, and more gas furnaces all pour more water vapor into homes than in former years. Heating and Ventilating provides builders with reference data on sources of water vapor. For instance, cooking for a family of four adds 4.5 lbs. of moisture a day to a house. Each shower contributes half a pound, a weekly laundry 30 lbs., human occupancy 6 to 8 lbs. per day, dishwashing 1.2 lbs., etc.

All of this moisture must eventually escape from your home. So you see that the modern living of a family of four can easily release 150 pounds, or more than 18 gallons of water per week into the air in your home! And houses with no basements have further moisture problems.

Increased production of humidity is only part of the story. Houses generally have been growing smaller, which means an even greater concentration of water vapor is trapped by modern tight construction. It means more moisture contained in less space.

There is no wonder we've created a condensation problem for ourselves.

How To Reduce Humidity

David Bareuther, Associated Press Building editor, sums up the problem of reducing humidity by saying there are only three ways to reduce humidity:

  1. CONTROLLING SOURCES OF HUMIDITY: For instance, venting all gas burners, clothes dryers, etc., to the outdoors. Use of kitchen or bathroom exhaust fans.
  2. WINTER VENTILATION: Because outside air usually contains less water vapor, it will "dilute" the humidity of inside air. This takes place automatically in older houses through constant infiltration of outside air.
  3. HEAT: The process of heating your home will reduce the relative humidity, providing it's dry heat. It will counter-balance most of all the moisture produced by modern living.

Now before we summarize specific steps for reducing humidity in your home, let's include some basic data about recommended moisture. You can refer to it if you are inclined to test the moisture levels in your own home. The table below is the result of long and careful experiments at the University of Minnesota Engineering laboratories. It shows the maximum safe humidities for your home, not only for the windows, but even more for your paint, insulation, and structural members. In most cases, reducing moisture to these humidities will cure troublesome condensation on windows; if not, you can reduce humidity further without discomfort to you or your family.

If you test humidity in your home, be sure to use an accurate instrument, preferably a good sling psychrometer. Remember, too, that these relative humidities are for 70 degrees F. For higher temperatures, lower humidities are required.

Outside Air Temperature

Inside Relative Humidity
70 F Indoor Temperature

-20 degrees F or below

not over 15 percent

-20 degrees F to -10

not over 20 percent

-10 degrees F to 0

not over 25 percent

0 degrees F to 10

not over 30 percent

10 degrees F to 20

not over 35 percent

20 degrees F to 40

not over 40 percent

7 Practical Steps to Control Condensation

Here, arranged from easy to more difficult, are the steps you should take to reduce condensation on your windows.

  1. Install storm windows or replacement windows with double or triple glazing.
  2. Shut off furnace humidifier and any other humidifying devices in your home.
  3. Be sure that louvers in attic or basement crawl spaces are open and that they are large enough.
  4. Run kitchen or other ventilating fans longer and more often than has been your custom.
  5. Open fireplace damper to allow easier escape for moisture.
  6. Air out your house a few minutes each day. Air out kitchen, laundry and bathrooms during use or just following use.
  7. If troublesome condensation persists see your heating contractor about an outside air intake for your furnace; about venting of gas-burning heaters and appliances; or about installation of ventilating fans.

If the common remedies we suggest (number 1 through 5) don't work, you REALLY have a condensation problem. But the changes your heating contractor may recommend to further reduce humidity in your home should not be very expensive. Certainly they will be less expensive than a big paint job caused by excessive water vapor!

The basic principle of reducing window condensation is extremely simple. When there's too much condensation on your windows, it means that humidity is too high in your home. You should take necessary steps to reduce humidity until condensation disappears. But in practice, window condensation and reducing humidity may become very complicated because a score or more of entirely different conditions may affect the way the condensation problem works out in different homes. Let us just mention a few:

  • The number and types of windows in the home.
  • The type of double glazing system on the windows.
  • The heating system—hot air or water—perimeter or interior wall heating.
  • The type of insulation and vapor barrier.
  • Even the type of soil and quality of drainage.

Because of so many variables, a condensation problem can sometimes be very tough to solve. That's why we recommend that you put an expert to work on your problem if the simpler steps to reduce humidity don't solve your condensation problem. See your architect or your heating contractor first. If they can't help, we suggest that you ask your general contractor or lumber dealer to put you in touch with a qualified expert. They are available both at engineering schools and from the staffs of heating, insulation, wallboard, or window manufacturers.

There are two causes of condensation which are temporary. They will disappear after a few weeks or at most a season of heating. First, there is the moisture that comes from a new construction or remodeling. There's quite a lot of moisture in the wood, plaster, and other building materials of a new home. When the heating season starts, this moisture will gradually flow out into the air in the home. Then it will disappear and not cause any more trouble. Much the same sort of thing happens in milder form at the beginning of each heating season. During the summer, your house absorbs some moisture. After the first few weeks of heating, your house will be dried out, and you'll have less trouble with condensation.

While we have been discussing the control of condensation, we've mentioned just about everything except windows. There's a good reason. There just is nothing much that can be done with windows to cut down condensation. As the building experts have often pointed out, the windows are not to blame for condensation. Both the cause and the cure of window condensation lies in the moisture content of the inside air.


Have you ever noticed the droplets of water that form on the outside of a canned drink when you take it out of the fridge? This is condensation and the reason why it happens is all to do with temperature, air and water vapour.

The temperature on the surface of the can is reduced as air passes over it. As the air gets cooler its relative humidity rises and the water vapour turns into moisture. The air passing over the can is unable to hold onto the moisture which ends up as droplets running down the side of the can’s cold surface.

This is what happens in thousands of households across the nation when the temperature drops inside the home, especially at night time when the heating is turned off. Just like the canned drink, the air reaches the point where it can no longer hold onto to all the moisture that we create in our homes and it migrates to the coldest surfaces - the windows and walls - where it appears as condensation or the more familiar sight of streaming windows.

How to spot condensation?

  • Streaming windows and walls
  • Damp areas on walls
  • Wallpaper peeling
  • Mould growing on window frames, walls and ceilings
  • Soft furnishings and fabrics become prone to mould and mildew
  • A musty damp smell in the property

Condensation is arguably the most common form of dampness and can eventually lead to the growth of mould. It forms on internal surfaces when the temperature drops sufficiently below the temperature of moist air inside the property. You should watch out for it because if left to develop, condensation can lead to an unsightly, musty property. More importantly, it can also aggravate or trigger health problems such as asthma and wider complaints.

Waking up to condensation on your windows is a familiar sight for many people, especially in winter and this is usually the first sign of a condensation problem. If condensation occurs over a prolonged period of time, other signs will start to appear such as damp patches on walls, peeling wallpaper and ultimately black mould growth. These effects can lead to musty smells, damage the fabric of our homes and can even affect our health.

Better Business Bureau—Telltips #421: Condensation on Windows

Condensation is visible evidence of excessive moisture in the air. It may appear as water, frost, or ice on the room surface of windows and doors. The warmer the air, the more water it can hold. This means that the air in the center of any given room will hold more water than the air adjacent to the window or door walls, since this area is always colder. When the warm moisture-laden air moves toward the cooler window or door wall, it becomes cooler and could not hold the moisture it held when it was warmer. Therefore, the moisture is dropped and appears as water on the glass and frames of windows and doors. This occurs more frequently during the winter months, because of the extreme difference between the inside and outside temperatures. If you wish to avoid condensation during the winter months, when the average outdoor temperature drops to 35 degrees or less, it would be wise to maintain a 25 to 35 degree relative indoor humidity.

Ventilation is a very effective way to remove excessive moisture from the air, which is why old poorly insulated houses with single glazed windows often times do not have condensation problems. This is because the air is changed by infiltration around the windows, vents, and other openings. Newer homes which are constructed to meet current insulation standards and energy conservation requirements; or older homes which have been newly insulated through the addition of added attic or basement insulation and installation of prime windows with dual or triple glazed glass, are now so air tight that they present a new problem.

All homes will on occasion have temporary condensation, which is the result of one of three occurrences:

  1. New construction or remodelling. Building materials contain a great deal of moisture. As soon as the heat is turned on, this moisture will glow out into the air and settle on the windows and so on. This will usually disappear following the first heating season.
  2. During humid summers, houses absorb moisture. This will be apparent during the first few weeks of heating, and then the house should dry up.
  3. Sharp, quick, and sudden drops in temperature, especially during the heating season will create temporary condensation problems.


What is Condensation?

Condensation occurs where moist air comes into contact with air, or a surface, which is at a lower temperature.

Air contains water vapour in varying quantities, its capacity to do so is related to its temperature - warm air holds more moisture than cold air. When moist air comes into contact with either colder air or a colder surface, the air is unable to retain the same amount of moisture and the water is released to form condensation in the air or on the surface.

Condensation is generally noticeable where it forms on non-absorbent surfaces (i.e. windows or tiles) but it can form on any surface and it may not be noticed until mould growth or rotting of material occurs.

Conditions for Condensation

In Britain, condensation in houses is mainly a winter problem particularly where warm moist air is generated in living areas and then penetrates to the colder parts of the building.

The moisture in the air comes from a number of sources within the house. Water vapour is produced in relatively large quantities from normal day to day activities - a 5 person household puts about 10 kg of water into the air every day (without taking into account any heating) - i.e.

  • breathing (asleep) 0.3 kg
  • breathing (awake) 0.85 kg
  • cooking 3 kg
  • personal washing 1.0 kg
  • washing and drying clothes 5.5 kg
  • heating - especially paraffin and flueless gas heaters. For every litre of paraffin burnt over one litre of moisture vaporises into air. Every carbon fuel produces some amount of water from combustion.
  • (1 kg of water equates to about 1 litre)

Moisture can also be drawn from the structure of the building into the internal air; from below the floor or through the walls/ceilings.

Problems with the structure of the building can mean that its moisture content is unnecessary high. This can either be due to the method of original construction or as a result of structure failures.

Older houses may not have a damp proof course (DPC) which prevent soil moisture from rising up into the living areas, lack of a DPC can occur in walls or under solid floors.

Buildings may also lack or have insufficient air bricks to allow adequate underfloor ventilation.

Structure failures can range from bridged DPC's (either externally or within the cavity of the wall) to damaged roofing or gutters/down pipes.

The effect of moisture generation is made worse by keeping the moist air in the house - it is theoretically possible to avoid condensation by adequate ventilation. Usually in certain areas of a house (such as bathrooms and kitchens) the warm air contains a lot of moisture, if that air then spreads to cooler parts of the house, it will condense on any colder surface.

Up until the middle/late part of the twentieth century, most house had high natural ventilation as the level of home insulation was low. Conservation then became popular and natural ventilation was greatly reduced by the introduction of double glazing, draught excluders, fitted carpets (which prevent air movement up through suspended wooden floors) and the removal of open fire places with the introduction of central heating.

Houses have become more effectively sealed, keeping any moisture produced within the house and providing better conditions for condensation to occur. Ventilation is only effective if consistent throughout the whole envelope of the house. Condensation is encouraged by poor air circulation where stagnant air pockets form (behind furniture and in cupboards) and the first evidence is often the appearance of mould growth.

Modern life styles mean that many houses remain unoccupied and unheated throughout the greater part of the day, allowing the fabric of the building to cool down. The moisture producing activities are then concentrated into a relatively short periods (morning and evening) when the structure is relatively cold while the building is still warming up.

To control condensation

First of all, you need to ensure that the amount of moisture in the air is not excessive.

Check the structure of the building:

  • Check that the walls are not suffering from rising damp.
    • Ensure that there is damp-proof course, that it is not bridged or damaged. A new damp course can either be installed by removing a brick at a time and inserting a physical DPC, or a chemical DPC can be injected into existing walls.
    • The damp-proof course should be at least 6 inches above any outside concrete to avoid heavy rain from bouncing back up and soaking the brickwork above the DPC - consider lowering the outside surface where necessary.
    • Check that any wall cavities are clear of rumble, debris can accumulate over the years and to remove it normally requires removal of a brick at each corner and racking the cavity clean. Where the dampness is restricted to one area and no other reason can be identified, it is a relatively easy task to check/clean inside the cavity.
    • Check that all airbricks are clear, consider fitting additional airbricks to ventilate under suspended floors (modern practice is to fit a duct across the cavity so that the cavity itself is not vented). Older buildings may not have airbricks, consider fitting them if there are internal suspended floor.
  • Consider applying a surface finish to outside walls to prevent rain penetrating them. Either a clear waterproofing finish which can be brushed on or a paint/textured finish which will cause most of the rain to run down (check that you are allowed to change the outside appearance of your house before you start doing so).
  • Check the roof, make sure that it is sound and directing rain into the guttering, not into the structure of the building.
  • Check the guttering and down pipes, make sure that they are carrying the water away and that there are no damaged/blocked guttering or drainpipes causing the external wall to become soaking wet.
  • Check solid floors to ensure that damp is not coming up through it, if it is, you may need to introduce or replace a damp proof membrane underneath it (potentially a big job) or fit a more suitable floor covering.
  • Check that there are no leaking water pipes or tanks within the house.

Once you are happy with the structure of the building, look at your life style within the building:

  • Try not to breath (NO - just joking !!)
  • After a bath or shower, try to ventilate the room to the outside, not to the rest of the house - just opening a window (and closing the door) will help.
  • Dry clothes out of doors or in a cool area of the premises - this latter suggestion may sound strange, it will take longer but less moisture will be held in the air at any one time.
  • While drying clothes indoors, ventilate the room.
  • When people come in with wet coats, hang them outside the living area to dry. A good reason for a porch.
  • Try to increase the change of air in the premises - increase ventilation. Add forced ventilation/extraction to areas which produce a lot of moisture (kitchen, bathroom). Extractor fans are available with an air-moisture switch so that they operate automatically while the moisture in the air is above a set amount. Other units (more expensive/complicated) are available which remove the moist air but reuse the thermal energy which would otherwise be wasted.
  • Consider changing the fuel you use, electric is the driest, paraffin probably the wettest.
  • Consider using a dehumidifier - domestic types are now available and can remove a surprising amount of water from the air.

If condensation persists after you have sorted out the basic structure of the building and your life style, there are still some other changes to try.

  • In Britain, condensation will almost always occur with single glazed windows. The inside surfaces of these windows can be almost the same as the outside temperature, overnight in winter their temperature can drop below freezing; often the inside window sill will be awash first thing in the morning.
    • Simple secondary glazing consisting of little more than a sheet of glass (or plastic) screwed to the window frame with a seal in between can be fitted. This is relatively cheap (especially if purchased second hand - remember that window frames tend to be a standard size, so the chap down the road having double glazing fitted, may have secondary glazing which will fit your windows). Fixed secondary glazing must not be installed on all opening windows in a room as some ventilation is essential. DIY kits are available which allow the secondary glazing to be temporary removed or opened to allow the original window to be opened for ventilation.
    • Alternatively new double-glazing windows can be considered. Although much more expensive than simple secondary glazing, there are additional benefits; existing wooden or metal windows will need continuous maintenance and repair - with new double glazed windows, you get new window frames which will probably be low maintenance or maintenance free.
    • Although secondary or double-glazing is unlikely to eliminate all condensation, they should reduce it to an acceptable amount.
  • Some decorative materials always have cold surfaces, (i.e. ceramic tiles, mirrors etc.) and are well known for the formation of condensation. Unfortunately we tend to use tiles in the kitchen and bathroom, two rooms where high humidity are likely. There is not much you can do where this occurs other than keeping the room (and so the tiles) evenly heated throughout the day or improve ventilation.
  • Some wall surfaces can also be a problem.  Where the wall is papered the situation may be made worse if there are many layers of paper, (this just acts like blotting paper) so strip off all the layers and re-paper the wall.
  • Things can also be improved by lining the wall with thin expanded polystyrene (normally available from your wallpaper stockist) before you hang new wallpaper.
  • Painted walls can also have a cold surface. If you do not want to paper it, consider lining it with wooden panelling or another material such as cork tiles. Alternatively a wall can be insulated by fitting a false wall with a layer of insulation behind and the front either being panelled or covered with plasterboard so that the new surface can be papered. However, remember that with all these 'covering up' methods, they possibly just hide and do not cure the problem. If the wall is suffering from rising or other damp problems, with the passage of time, the damp will cause damage to the lining (wet rot to the timber etc.) and this will not be seen until it is really serious.
  • Ceilings under the roof should not suffer too much from condensation providing adequate roof insulation is fitted. If there is no or little roof insulation, additional insulation should be installed (for some groups of people, there are financial grants in the UK for such work - check with the Local Authority or advice centre for details). Additional insulation

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