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Moisture in Windows

Guide to Condensation on Windows
Know the Facts, Questions & Answers

Today homes are built with superior vapor barriers and insulation that tend to keep the majority of moisture inside the house, instead of allowing it to filter to the outside as it did in older homes. Information below will help you to understand the causes of condensation in today's homes and will give you suggestions as to how you can eliminate and control condensation as much as possible.

  1. What caused condensation?
  2. Why does frost or condensation form on new window?
  3. What is humidity in general?
  4. What causes excess moisture or high relative humidity which can cause condensation?
  5. Why can two identical homes, side by side, react differently to condensation?
  6. Why condensation is more noticable the first part of the heating season and directly after winter thaws or winter rains?
  7. Why frost and condensation may appear  after the installation of new energy efficient windows?
  8. What can be done to control condensation and high relative humidity?

Q. WHAT CAUSES CONDENSATION?
A. The source of condensation is humidity or invisible water vapor which is present in all but the driest air. When this water vapor comes in contact with a surface that is below what is called the "dew point temperature" the vapor becomes liquid and is called condensation. This process of changing water vapor to liquid occurs on bathroom mirrors and walls after someone takes a hot shower. It also can occur on glass surface of the windows during the winter if the inside air contains enough water vapor. This condensation can occur at any normal temperature, provided the moisture concentration (relative humidity) is high enough. Water on the inside surface of the windows is condensation  and the solution usually doesn't come from the windows itself.

Q. WHY DOES FROST OR CONDENSATION FORM ON THE WINDOW?
A. The water vapor in the air tries to flow towards drier air and mix with it. This process is termed "vapor pressure". It is not an instantaneous movement, but a definite movement from an area of high vapor pressure to where the air is drier. In cold weather, vapr pressure is very strong in the house because the cold outside air holds very little moisture: This vapor pressure can force moisture easily through most of the materials we use to build our homes: wood, drywall, even cement and brick susceptible. Moisture in the bath, kitchen, and laundry areas is absorbed into the warm inside air and then rushes to mix with the dryer outside air. The most obvious indication of this - is condensation or frost on the inside surface of your windows: The moisture comes in contact with a surface of the windows but can't get through the glass or frame. It, therefore, condences to form either frost or water, depending on the temperature of the surface.

Q. WHAT IS HUMIDITY IN GENERAL?
A. When air will hold no more moisture, it is said to be saturated. Relative humidity is a percentage of moisture in the air in relation to complete saturation. EXAMPLE: 100% relative humidity would be rain. 50% relative humidity indoors in the wintertime would be excessively high and dangerous. 10% inside relative humidity would be comparatively dry air. Warm air can hold more water vapor than cold. Even though inside and outside humidities could be the same in the winter months, the inside air would be holding far more water vapor since it is warmer. Condensation can even occur in warm weather. Some examples are as follows:

  1. Condensation forms on the glass of ice water since the surface of the glass is down to the dew point temperature of the inside air. 
  2. Dew (condensation) forming on grass during the cool nights in summer months.

Q. WHAT CAUSES EXCESS MOISTURE OR HIGH RELATIVE HUMIDITY WHICH CAN CAUSE CONDENSATION?
A. With energy conservation the main concern today, modern homes are being built with the increased use of "moisture trapping" materials such as increased insulation in walls and ceilings, use of vapor barriers over insulation: better performing windows and doors so there is less air infiltration and air exchange moisture, adds to the problem of excessive condensation.

  1. Uncontrolled furnace humidifiers. 
  2. Damp basement walls and floors.
  3. Excessive boiling when cooking.
  4. Laundry hung up to dry.
  5. Bathing - taking showers.
  6. Large number of plants watered daily.
  7. Unvented appliances such as automatic clothes dryers and all gas appliances (water vapor is one of the products of gas combustion).
  8. Crawl spaces without adequate vapor barriers.
  9. Drapes, blinds, shutters, or similar devices, applied to the interior of windows, can contribute to condensation.
  10. Air Circulation - a window may be in the area in which there is poor circulation of warm air.
  11. Orientation and exposure - If windows are exposed to prevaliling winds, then they will be slightly colder then the other windows in the home or some windows in the home may be protected by othere buildings, trees, etc, which form a windbreak, while otheres are totally exposed to the wind and therefore may cause condensation.

Q. WHY CAN TWO IDENTICAL HOMES, SIDE BY SIDE, REACT DIFFERENTLY TO CONDENSATION?
A. Condensation can differ from house to house and family to family. Identical, side by side homes, could show different degrees of condensation, or one could be condensation free. Normal living processes generate water vapor. Cooking, bathing, and laundry all contribute to water vapor contents in the house. A family of four is said to generate 4 gallons of water into the air in the course of the day. A new baby in the house causing additional laundering could bring more signs of condensation that did no exist last year. Or the same thing could occur when another person of any age joins a household. Each individual family has its own lifestyle which contribute to different degrees of water vapor in the air.

Q. WHY IS CONDENSATION IS MORE NOTICABLE THE FIRST PART OF THE HEATING SEASON AND DIRECTLY AFTER WINTER THAWS OR WINTER RAINS?
A. During the summer, your home has absorbed a great deal of moisture. Condensation will be more noticeable the first several cold spells directly into the heating season. This is because the house is still moist and it will take several weeks of coninuous heating to be dried out. Condensation will usually dissipate as the heating season progresses. A home picks up considerable amounts of moisture during winter thaws and rains. If a cold spell sets immediately after a thaw or heavy rain, the relative humidity in a home will be at an extreme high which can cause frost or condensation, untill the humidity level is reduced by moisture transmision to the cold outdoors. This is one of the reasons why frost or condensation can be more noticable immediately after a rain followed by cold weather.

Q. WHY FROST AND CONDENSTAION MAY APPEAR AFTER THE INSTALLATION OF NEW ENERGY EFFICIENT WINDOWS?
A. Before replacement windows are installed, most homes have fairly loose windows that have excessive drafts around them, which automatically reduce the humidity levels within the home. Actually, in many older homes, it is impossible to optain high humidity levels due to the moisture lost around loose windows. After the installation of new energy efficient windows or doors, drafts are reduced to a minimum and the house is made far tighter than it ever has been in the past. The interior moisture can not easily escape to the exterior, thus causing higher humidity levels which could not be obtained before.

Q. WHAT CAN BE DONE TO CONTROL CONDENSATION AND HIGH RELATIVE HUMIDITY?
A. Any or all of the steps below can alleviate a condensation:.

  1. Shut off furnace humidifier and any other humidifying device in your home. 
  2. Use kitchen exhaust fans while cooking, or at least close kitchen door to rest of house and open window slightly for ventilation. During and after taking a shower or bath, the bathroom should be ventilated with the use of ventilating fans of by opening windows slightly for ventilation. 
  3. Windows in laundry rooms should be open for ventilation when laundring.
  4. Large number of plants should be concentrated in a sun room or other seldom-used room during critical cold weather.
  5. Basement floors and walls should be treated with efficient waterproofing.
  6. Opening windows slightly throughout the house for a short time each day will allow for humid air to escape and for drier air to enter. The heat loss will be minimal.
  7. It is recommended that curtains and blinds should be open during the day time to provide movement of air at the surface of the window.
  8. Make sure that all heat vents are in open position and are not covered with plastic covers, furniture or carpet.
  9. Never hang up clothes to dry indoors in a house with extreme humidity.
  10. Open fireplace damper to allow moist air to escape.
  11. Provide vents to outside in all major gas appliances. Also, vent all clothes dryers.
  12. In crawl spaces, provide proper vapour barrier and wall insulation to prevent moisture from escaping from walls ino your home.
  13. Run a dehumidifier if necessary.
  14. Install Hear Recovery Ventilation system fo constant air exchange in the home.

If condensation persists, see your heating contractor about an outside air intake for your furnace, about ventilating of gas-burning heaters and appliances, or about installation of ventilating fans or roof turbo-fans in the attic.