Understanding Condensation

Introduction

Condensation. The fog that suddenly appears in cold weather on the glass of windows and patio doors. It can appear on the interior or exterior of the window. I can block out the view, drip on the floor, or freeze on the glass. It's annoying.

While it may seem natural to blame the windows, you shouldn't. Interior window condensation is really an indication of excess humidity in your home. Exterior condensation is a form of dew. The glass simply provides a surface on which the oisture condenses visibly.

The important thing to realize is that if escessive humidity is causing window condensation, it could also be causing problems - sometimes hidden - elsewhere in your home.

Some sign of excess humidity:

Condensation on windows
A "damp feeling" in the home.
Staining or discoloration of interior surfaces.
Mold or mildew on surfaces or a "musty smell."
Warped wooden surfaces.
Cracking, peeling or blistering interior or exterior paint.
Sweating pipes.

What to do? We compiled Understand Condensation to answer questions you may have about condensation, indoor humidity, exterior condensation and ventilation. We'll start with the basics, and offer solutions and alternatives along the way.

As you read through, and learn about the causes and cures for condensation, remember: It's not the windows; it's the humidity.

Condensation & Humidity

Q: Just what is Condensation?
A: Fog on windows is a form of condensation. So is the water that forms on the outside of a glass of iced tea in the summer. It all comes from water vapor in the air. It can appear on the interior or exterior of the window glass.

Q: What Causes it?
A: When warm, moist air comes into contact with cooler curfaces, the excess moisture in the air condenses. That's because the cooled air next to the cool surface can't hold as much moisture as the warmer surrounding air.

Q: What does condensation on windows mean?
A: Window condensation can be a danger sign. It may mean that excessive indoor humidity could be doing unseen damage to other parts of your home.

Q: What is humidity, anyway?
A: Humidity is water vapor, or moisture, in the air. Usually it's invisible. But sometimes, as in the form of steam or ground fog, it's concentrated enough to be seen. Nevertheless, all air contains a certain amount of moisture, visible or not.

Q: Where does the moisture come from?
A: There are many things that generate indoor moisture. The normal perspiration and breathing of a family of four adds about half a pint of water to the air every hour. Cooking three meals a day adds four or five pints of water to the air. Each shower contributes another half-pint. In fact, every activity that uses water (like dish washing, mopping floors, doing laundry) add moisture to the air. The truth is, daily living activities of a family of four can dd more than 18 million gallons of water a week to to the air in their home. And the more water vapor in the air, the higher the relative indoor humidity.

Q: What's relative humidity?
A: Air can hold only a limited amount of water vaopor, and that amount depends on the air tmperature. When air at a certain temperature contains all the vapor it can hold, it's said to have a relative humidity of 100%. Thus, when it holds only half as much water vapor as it could, the relative humidity is 50%.
Cooler air is capable of holding less vapor than warmer air. So air at 30°F and 100% relative humidity actually contains less water than air at 70°F and 100% humidity.

Q: How can I tell if there's too much humidity in my home?
A: Check for damp spots on ceiling and room-side surfaces of exterior walls, particularly closets. Look for water and ice on windows. Even water-filled blisters on outside paint surfaces indicate excessive indoor humidity.

Q: What about relative humidity and my health or well-being?
A: Most health experts agree that relative humidity can affect your well-being. They suggest maintaining indoor humidity levels between 30% and 50%. According to the World Health Organization, at levels higher than 65%, the incident of upper respiratory illness might occur in people suffering from asthma and allergies. Lower moisture levels (below 20%) may induce dryness or itching of the skin and aggravate certain skin conditions.

Q: Is condensation more prevalent in any geographical region?
A: Yes. Condensation is more apt to occur in climates where the average January temperature is 35°F or colder.

Q: What does excess humidity do to my home?
A: Excess humidity contributes to the deterioration of any home .It can pass through walls and freeze in the insulation. In spring it melts, damaging your ceiling and walls. Or, excess humidity can force its way out through siding to form blisters under your exterior surfaces.

Q: You mean moisture can actually go through walls?
A: You bet. It's because of a force called "vapor pressure." Moisture in wet air tries to flow toward drier air to equalize itself. This flow acts independently of air currents. In winter, inside air is much more humid that colder outside air. So the vapor pressure, or equalization process, actually forces the inside moisture through cement, wood, plaster, and brick, toward the outside.

Q: What happens then?
A: Because certain varnishes and paints block the flow of moisture, condensation ca occur between the inside and outside walls, or under exterior paint surfaces. I can rot a home's wood frame and blister the paint.

Q: Does condensation occur only in winter?
A: Usually. However, condensation can occur whenever water vapor in the air comes in contact with a surface whose temperature is lower than the dew point (the temperature at which air becomes saturated and produces dew). Exterior condensation may occur on windows glazed with High-Performace glass. It is so efficient at keeping radiant heat indoors it creates a lower exterior glass surface temperature for longer periods of time than regular insulating glass.
Also, during hot, humid summer days, for example, condensation can form on the outside of windows when conditioned air inside the house makes the surface temperature of the glass cooler than the dew point.

Q: Does condensation depend on whether my home is new or old?
A: Generally, yes. Years ago, before all the concern about energy efficiency, homes were built less weathertight than they are today. Insulation concepts were very basic. Walls and ceiling were built with much more porous materials. Water vapor could easily flow in and out of walls.
Today's homes are much "tighter." Windows and doors are built to reduce air leakage substantially. Weatherstrippng, modern insulation, vapor barriers, and construction techniques intended to keep out cold air all can lock moisture inside. As a result, moisture created by bathrooms, kitchens, laundries, and occupants no l onger flows to the outside, unless provisions for mechanical ventilation have been made. So it's very easy to build up excessive, even harmful, moisture levels in today's homes.

Controlling Indoor Humidity

Q: If I increase the relative humidity in m home in winter, can I lower the temperature and save energy?
A: Not really, although there is a relationship between how warm you feel and relative humidity . That's why it feels hotter in summer when the humidity is higher. In winter you might feel colder initially in a room at 70°F and 20% relative humidity than in one at 65°F with 35% relative humidity. However, the body soon adjusts to such slight differences, and if you feel cold at 65°F, the humidity level really doesn't matter - you'll still feel cold.

Q: What if I do increase the humidity in my home in the winter?
A: Even though increasing the relative humidity won't mean changing your thermostat setting significantly, higher humidity in the home in winter might mean greater comfort. Some people think they feel more comfortable breathing humidified air.
At one time it was believed that humidifiers helped people stay healthy during the winter months. Recent tests have shown that for usually healthy people this is not the case. In fact, humidifiers may actually cause some health problems.
Humidified air can help reduce static electricity and shrinkage in furniture and woodwork caused by dryness. Weigh the advantages and disadvantages. You may decided to sue a humidifier. But remember, too much humidity causes condensation - and that's not good.

Q: So how does humidity affect window condensation?
A: Excessive humidity is generally the cause of window condensation. As the outside temperature drops, the window glass also develops a lower temperature. When moist air molecules in the warmer room come in contact with the cooler glass pane, the water molecules condense and form water droplets.
That's how condensation occurs. Determining when condensation will occur and preventing it depends upon the type of window, the relative humidity level inside your home, and the temperature of the air inside and outside.
However, experts have a general guide we can follow to help us control humidity and window condensation: As the outside air temperature drops, you should also decrease the humidity level within your home. For example, if your indoor air temperature is 70°F when it's 20°F outside, your relative humidity level should be 35-40%. However, when the outside temperature drops to -20°F, your inside relative humidity level must also drop - to a 15-20% range - in order to minimize condensation.
By the same token, if you lower your room temperature to conserve energy, you must also decrease the level of relative humidity to minimize condensation.
According to another commonsense guideline, you should maintain as high a relative humidity factor as you can for comfort. Then reduce the humidity level when condensation occurs. In many homes, this simply means turning off the humidifier.

Q: How can I reduce indoor humidity in winter?
A: There are a number of different suggestions:

Reduce the amount of moisture you put into the air by turning your humidifier down or off when the weather turns colder.
Vent all gas appliances, clothes dryers and exhaust fans to the outside. Your attic and crawl space should also be ventilated to the outside with the earth in your crawl space covered by a vapor barrier.
Make sure you run the exhaust fans in the kitchen when cooking and in the bathroom when bathing or showering. Run your exhaust fan until your mirror in the bathroom is clear or the temperature near your stove si back to normal. Be careful not to run the exhaust fan too long and overheat the motor.
Avoid storing firewood i your house or basement.
If you have a forced air furnace, make sure your home is properly ventilated by installing a fresh air intake. Or if your home is extremely "tight" and you have excessive moisture, it may be helpful to install an air-to-air heat exchanger. (More about ventilation in the next section.)

Q: How do I measure indoor relative humidity?
A: It's important to remember that relative humidity levels quoted in weather reports indicate outdoor humidities in outdoor air. So they have little bearing on your hoes indoor relative humidity.
To get an accurate reading, you can use humidity measuring instruments called hygrometers. (The sling psychrometer is one type of available.) Otherwise, keep the commonsense guideline in mind and watch your windows for symptoms of excess humidity. When excessive moisture collects on the inside glass in a living room or bedroom you're approaching the humidity danger level.

Q: Will reducing the humidity in my home during the winter help control condensation?
A: It's the most practical way. Condensation indicates excessive humidity. Eliminate the excessive humidity, and you eliminate the condensation.

Ventilation

Q: How can I ventilate my home?
A: There are basically two types of ventilation: interior and structural.
As a temporary solution to an acute problem, open a window in each room for just a few minutes. Remember, inside air continually gains humidity through daily living activities.
Opening windows allows the stale, humid air to escape and fresh, dry air to enter.
During a shower or bath, for example, turn on the exhaust fan so steam can go outside instead of remaining n the home; let the fan run until the mirror clears. Be careful not to run the exhaust fan too long and overheat the moror. If necessary, open the bathroom window to speed up the process.
Structural ventilation is slightly more complex, but will save you costly repair bills in the long run. Consult a heating and ventilating contractor or another expert in this field.

Q: Should structural ventilation include attics?
A: Definitely. Many homeowners cover all attic louvers in winter in hopes of saving fuel. If the attic is properly insulated, this practice can only do harm: The indoor moisture penetrates ceilings, then condenses on the cool underside of the roof and can even form frost. If the attic were ventilated, moisture would be transferred to the outside air.

Q: What harm can attic condensation do?
A: A lot. Moisture condensing in attics produces mildew, or rotting conditions. Or it drips down to the ceiling below, damaging plaster or paint. Thermal insulation also becomes wet and provides less resistance to heat loss.

Q: Are some kinds of attic ventilation better than others?
A: Yes. The best type of attic ventilation for your home is a system that provides maximum air flow through the attic to the outside. For this reason, a combination of continuous eave vents and ridge venting is one of the most effective types. To be on the safe side, consult a local heating and ventilating contractor. He'll help you determine whether your attic ventilation is adequate.

Q: Just how much attic ventilation should I have?
A: That's a difficult question to answer, because the size and number of vents depends on the location of the home, wind direction, physical size of the building, quality of workmanship and kinds of building materials used.
Consult a local heating and ventilation contractor. They will be able to tell you exactly how much ventilation your attic should have.

Q: What about the crawl space? Should it be ventilated too?
A: Yes. It's important that the crawl space beneath a house also be ventilated. It cn evaporate gallons of water each day. If the space si sealed, that water penetrates the floor above and causes more indoor humidity problems.
Providing foundation vents in the crawl space reduces the humidity, and a vapor barrier (like polyethylene film) on the ground prevents moisture leakage into the house above.

Q: How much ventilation should I have in the crawl space?
A: At least one vent should be located near the corner and placed as high as possible, but not higher than the bottom of the joists. Plus, if you have a ground cover, one 8" x 16" vent for each 350 sq/ ft. is needed, with a minimum of four vents.
Again, it's wise to consult your local heating and ventilating contractor, for number and proper placement of vents.

Windows & Condensation

Q: Do windows cause condensation?
A: Wndows are not a cause of condensation. They provide a ready medium on which the vapor can condense. But the primary cause of condensation is excessive moisture in the air. Windows are simply indicators of that moisture.

Q: What causes condensation on windows and patio doors?
A" Remember: cool air can't hodl as muc moisture as warm air can. So when the warm, moist air of the room comes into contact with the cold glass surface, some water vapor which the cooled air can no longer hold is deposited on the glass. Condensation can also occur when the flow of warm room air over window surfaces is restricted.

Q: Are windows the only place condensation is forming?
A: Possibly not. There's a point between exterior and interior walls that's just as cold as the inside window surface. Chances are, if you can see condensation on your windows, it's also forming between the walls. Roomside surfaces of exterior walls are normally warmer, but occasionally condensation occurs on cold spots such as nailheads and in the corners of outside walls and closets. That's because insulation is weaker and circulation restricted in those areas.

Q: Why do I see condensation on my windows and patio doors first?
A: Condensation is generally seen first on windows and patio doors because hey tend to have the lowest temeprature of any of the visible surfaces in the house.

Q: Does the amount of condensation depend on window type?
A: Sometimes. Angle bay or bow windows, for example, usually experience more condensation than other window styles. This is because inside air circulation around those window types is usually more restricted And, since they hang away from the insulated house wall, abys and bows could be a few degrees cooler in temperature. To diminish excessive condensation, it's smart to insulate between the window head and platform, as well as between the seat and platform. In extremely cold climates, additional insulation above and below the window platform may also be necessary. As a secondary measure, placing a common electric fan near the window to promote air circulation may be helpful.

Q: How do drapes and window shades affect window condensation?
A: Drapes and other window coverings don't cause window condensation, but they can contribute to the problem by restricting the flow of warm room air over the glass surface. Therefore, condensation is more apt to occur when drapes are closed and shades are pulled down. Today's heavily insulated drapes are tighter shades can contribute to the problem even more.

Q: There is a 1" to 2" wide strip of condensation all the way around the window. What is this?
A: Chances aer good that you have "organic glass" in your window which simply means that the two panes of glass are held together - and apart - by a metal spacer. Because metal is conductive it will transmit cold from the outside to the inner glass...cooling that 1"to 2" strip closest to the glass edge to the point that condensation forms. In effect, the center of the glass stays warmer than the glass close to the edge, allowing condensation to form near the edge. It doesn't mean that the window is leaking air or not working. Quite the contrary, it is working extremely well.

Q: What causes condensation on the inner surface of combination-type storm windows?
A: All operating windows leak some air between the window frame and sash. (Because, in order for them to open and close, they are not caulked shut.) So when warm household air seeping in around the sash becomes trapped by the colder storm window, condensation forms on the inside-surface storm pane Providing outside ventilation to the combination storm window usually lessens this problem.

Q: What damage can excess window condensation do to window?
A: It can cause the paint to peel from the sash or stain the inside. Water can run down into and cause dampness around the window frame. It can sometimes cause paint to peel on the outside of the window.

Q: Are ther any cases where window condensation is only temporary?
A: Yes, there are primarily three: new construction or remodeling; the beginning of each heating season; and quick changes in temperature.

Wood, plaster, cement, and other building materials used in new construction and remodeling produce a great deal of moisture. When the heating season starts, this moisture will gradually flow out into the air in the home. It will usually disappear during the first heating season and not cause any further trouble.
At the beginning of the heating season, there may be a certain amount of temporary condensation. During the humid summer your house will have absorbed some moisture. After the first few weeks of heating, it will dry out, and you'll have fewer condensation troubles.
Sharp, quick drops in temperature can also create temporary condensation problems during the heating season.

Q: Is there anything I can do to my windows to lessen the formation of condensation on them?
A: Installing storm windows on single-pane windows will help some. If your windows have double-pane insulating glass, consider adding an exterior mounted removable glazing panel (RGP) on your windows. If window replacement is a possibility, install insulating glass.

Q: How much can these actions lessen the formation of condensation on my windows?
A: It all depends on what you do. Here is the amount of relative indoor humidity permitted by various glas types before condensation forms:

Single pane 13%
Double-pane insulating glass 37%
Triple glazing* 54%

*Double pane glass with Removable Glazing Panel
(Percentage refers to center of glass at 70°F inside, 0°F outside, outside wind velocity of 15 mph, no air movement inside, and uniform heating conditions. Under these conditions, condensation will form more quickly at the edges of the glass.)

High Performance Glass & Exterior Condensation

Q: How does High Performance glass work?
A: Through a special metallic coating bonded to the inner airspace surface of this double pane glass. It's almost invisible, but this revolutionary coating actually detects radiant heat and restricts its flow through the glass.
During the cold weather, High Performance glass keeps heat inside. In warm weather it keeps heat outside. You stay comfortable 365 days a year, no matter what your climate.

Q: We've been experiencing a layer of moisture on the outside of our windows. What is this?
A: What you are seeing is dew. The same condensation you can see on car windshields, lawns and streets many mornings. Under certain conditions, a higher incidence of exterior condensation may occur on windows glazed with High Performance insulating glass than on windows with regular insulating glass. This happens because High Performance insulating glass is more efficient at keeping radiant heat indoors in the winter that it creates a lower exterior glass surface temperature - for longer periods of time - than regular insulating glass. At certain times of the year, under certain atmospheric conditions, it is possible for High Performance glass windows to develop a layer of moisture on the outside surface.

Q: What causes it?
A: High Performance glass often have microscopically thin metallic coatings bonded to an inner surface of the sealed pane of the glass. This metallic coating restricts the flow of heat through the sealed glass. In heating situations, it restricts heat from getting out of the house. In cooling situations it restricts heat from getting into the house.

Q: Why doesn't it happen all the time?
A: Condensation like this happens only when the exterior surface temperature of the glass falls below the dew point of the air. When humidity levels are higher, the glass needs to be only a few degrees less than the air temperature to cause condensation to form on it. Most of these situations occur during Spring and Fall, when cool nights follow warm days. The exterior pane of the High Performance insulating glass unit is colder because the coating has kept radiant heat from warming it.

Q: How can a window be colder than the adjacent air?
A: The answer is night sky radiation, a phenomenon that has been exploited for thousands of years to collect water and to cool buildings in the desert. The clearer the night, the colder an exposed surface becomes, sometimes dropping several degrees below the adjacent air temperature. When this happens, condensation forms.

Q: Is it anything to worry about?
A: Not really. It usually only happens when interior and exterior temperatures and exterior relative humidity reach a critical point where condensation is inevitable. Depending on where you live this may be only a handful of times in the Spring and in the Fall.

Q:Will this happen on all windows?
A: It can, but it will last longer and is most often seen on windows that face west because the morning sun does not reach them to help speed up evaporation. It is less likely to happen on windows with High Performance glass.

Q: Can I do anything to prevent all this from happening?
A: Remember, this is probably only going to happen a relatively few days of the year...and then only under certain conditions. There is no practical way to prevent this natural atmospheric phenomenon.

Q: Does it mean my window are leaking air, or not working properly?
A: No in fact just the opposite is true. This is actually the best example of the efficiency of a low-E, Argon-filled glazing. Because the outer glass surface is so well insulated from the heat inside, there is no compensation for nighttime cooling. Thus, condensation can occur. With ordinary windows, exterior heat losses are offset by the warmth passing through the glass from inside the house. This heat loss means tha tthe outer surface of the glass remains at least as warm as the adjacent air. But remember, the main reason for having High Performance windows is to reduce heat flow.

If you see the condensation on the outside it is evidence that your energy is being saved, that your conservation efforts are working, and that your window investment is paying off.

Building a Low Condensation Home

Q: We've been experiencing a layer of moisture on the outside of our windows. What is this?
A: What you are seeing is dew. The same condensation you can see on car windshields, lawns and streets many mornings. Under certain conditions, a higher incidence of exterior condensation may occur on windows glazed with High Performance insulating glass than on windows with regular insulating glass. This happens because High Performance insulating glass is more efficient at keeping radiant heat indoors in the winter that it creates a lower exterior glass surface temperature - for longer periods of time - than regular insulating glass. At certain times of the year, under certain atmospheric conditions, it is possible for High Performance glass windows to develop a layer of moisture on the outside surface. Q: I'm building a home. What steps can i take to prevent excessive condensation?
A: Include these in your planning:

Choose wood or vinyl-sheathed wood rather than metal windows and patio doors. This will reduce the likelihood of condensation on frames and sash. Wood is one of nature's best insulators; metal is a poor one.
Wood helps insulate your home against outside cold and keeps heated air inside. Metal frames without thermal breaks, on the other hand, readily conduct heat. So their inside surfaces are cold in winter. When humid inside air makes contact with the cold metal, condensation - and often freezing - occurs.
Make sure yoru contractor or builder uses a high quality, kiln-dried lumber in construction.
Make sure the attic and crawl space aer cross-ventilated. The crawl space should be covered with a vapor barrier. This will prevent water vapor from rising from the soil into your house.
If you have a forced air furnace, install an outside fresh air intake.
Make sure your clothes dryer and all gas appliances have outside vents. Water vapor i one of the by-products of gas combustion.
If you have a basement, do whatever is necessary to prevent soil moisture from leaking into it- depending on the soil and drainage conditions of your lot.
Be sure to place heat vents below glas areas of windows and patio doors.
Use exhaust fans in the kitchen, bathrooms and laundry. They'll not only help reduce excessive moisture but clear away stale air as well.
Select windows and patio doors with double-pane insulating glass. Adding removable glazing panels can help, but for convenience choose a high performance glass.

Summary

Remember the best way to avoid condensation is to reduce the humidity of the inside air. Some solutions to window condensation problems pose a different kind of problem - that of energy consumption Some ventilation techniques (e.g. opening window so air out a home or running fans more frequently) may add slightly to a home's energy bills. It is up to the reader to determine the most practical solutions to the specific problems involved.

Be sure that the attic and crawl space are properly ventilated. Add a vapor barrier to cover the earth in the crawl space. Don't close off ventilation louvers in the winter.
If yo uhave single pane windows, install storm panels. If you have windows with double-pane insulating glass, add removable glazing panels. If window replacement is a possibility, replace with a high performance glass windows and patio doors.
Eliminate any controllable sources of moisture in your home. Turn your humidifier down or off when the weather turns colder. Avoid storing firewood in your house or basement.
In winter, provide more controlled ways for moisture inside to get out. Kitchen, bathroom and other ventilating fans should be vented to the outside. Make sure you run the exhaust fans in the kitchen when cooking and in the bathroom when bathing or showering. Run your exhaust fan until your mirror in the bathroom is clear or the temperature near your stove is back to normal. Be careful not to run the exhaust fan too long and overheat the motor.
In winter, remove screens and keep draperies and window shades open as much as possible.
If troublesome condensation persists, see your heating a contractor about an outside air intake for your furnace (required by some state building codes on all new residential construction), about ventilation of gas burning heaters and appliances, or about installation of ventilation fans. Also, check on what impact an air-to-air heat exchanger could have on your humidity levels.
When you're planning a home, take the necessary steps outlined in this booklet to prevent condensation problems. Again, work with your heating and ventilating engineer. Your gas or electric company may have a residential heating engineer you can consult.