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Moisture in the Air


Have you ever noticed condensation on your windows?  For example, in the fall when we turn on our heating systems, condensation often occurs on windows. This occurs when invisible water vapor in the air condenses on the cool glass.  Windows and metal window frames tend to be the coolest surfaces in our homes so moisture forms there first.

Moist air is always trying to reach dry air.  This is called vapor pressure and cannot be stopped by porous building materials like wood, plaster, stone or brick.  It is stopped by certain paints, plastic and glass.  That is why you see condensation on your windows first and why it should alert you to the fact that condensation could be occurring where you cannot see it - within your walls.

Water Drops

Condensation is the visible result of excess humidity.  Warm air holds more moisture than cool air, but when it meets a cool surface, it cools and can't hold as much moisture.  This excess moisture changes to water and becomes visible as condensation.  Condensation requires a cool surface and moisture in the air.  This moisture is measured as a percent of the total amount that the air will hold at a specific temperature.  The percentage is known as relative humidity.  Cooler air holds less moisture.  And dew point is the temperature at which the air is fully saturated with invisible water vapor and vapor starts to become visible water.

Often the problem is really bad in the fall.  This is when we have cold snaps.  The exterior temperature changes quickly and so the window glass temperature drops quickly thereby producing condensation. Windows fog up in the early morning because the overnight temperatures cool the glass below the dew point.  As the outside temperature rises the glass warms and condensation disappears.

Why do windows fog on one side of the home?  Typically this occurs on the inside of storm windows.  The prevailing wind creates pressure on the side of your home facing the wind and creates a negative pressure on the downwind side. When the temperature of the glass inside your home drops below the dew point of the inside air, visible moisture forms on the glass.  The combination of a high level of moisture in the air and a cool glass surface triggers the condensation process.  A higher level of condensation occurs when there is more water vapor in the air and the glass surfaces get even colder.

Don't blame the windows!  They are not the problem; they are just a cool surface delivering the message that excessive moisture is trapped inside a tight home.  The windows tell you that you have a moisture problem.  New homes present special problems.  In addition to being built tight and allowing little air exchange, they trap construction moisture.  When a home is built, much moisture is trapped in wood, dry walls, concrete etc.  It takes at least one full heating season to dry out a new home. Newer homes are also wrapped with a plastic vapor barrier to stop moisture and air leaks.

For many, window condensation is a temporary annoyance. However, in some homes, condensation continues and can become a serious problem.  Water can damage the wood surface of windows.  In the winter, ice may form on windows. To solve the condensation problem, the level of invisible moisture in the air of your home must be reduced and the surface temperature of the glass must be raised.

Since homes are now made tighter, are well insulated and no longer "breathe" on their own, steps must be taken to ventilate the house, allowing moisture and humidity to escape. There are several ways to reduce moisture levels.  First, limit the sources of moisture.  Showers, cooking and similar activities add lots of moisture to the air.  A typical family of four releases over 2.5 gallons of water per day into the air of their home.  For instance, cooking for a family of four adds about 4.5 pounds of moisture a day, each shower contributes .5 pounds, and a weekly laundry 3.0 pounds. A family of four can easily release 150 pounds or more than 18 gallons of water per week into the air in their home. Damp basements, plumbing leaks, plants and pets all compound the moisture generation problems.


Moisture (in pounds) produced by the average family of four per day:

It could total as little as 20 pounds (2 gallons) or as much as 50 lbs. (5 gallons) in a single day.  Now consider that it only takes 4-6 pints of water to raise the humidity level of a 1000 sq. foot house from 15 to 60 percent.

Some ways to limit moisture sources:

  1. Cover any bare crawl space soil with a plastic vapor barrier. Correct grading and drainage problems. Seal basement floors and walls.

  2. If your furnace has a built-in humidifier, turn it off as well as the water supply to the humidifier.  Try leaving the humidifier off year round unless your home becomes extremely dry.

  3. Install a dehumidifier.

  4. Store firewood outside.

  5. Control or cover other sources of humidity such as radiator water pans, fish tanks, and a large number of plants.

  6. Run full loads of clothes and dishes in the washers to reduce the number of uses.

  7. Take shorter baths and showers.

  8. Cover pots and pans when cooking.

  9. Fix dripping faucets and cover sweating cold water pipes and heating ducts with insulation blankets.

  10. Increase the amount of ventilation. 

  11. Use ventilation in bathrooms and vent to the outside. Vent kitchen exhaust fans to the outside. Evaluate attic ventilation - all bath and kitchen exhaust fans must exit the attic.

  12. When the air is dry outside, open the windows and "air out" your home.

  13. Consider keeping a window slightly open all winter or whenever condensation starts to occur.  Select a window on the downwind side of your home (for 2 story windows - select a 2nd floor window as warm moist air rises).

  14. Consider forced ventilation with an exhaust system or heat recovery ventilator.

  15. Vent the clothes dryer to outside. Don't hang wet clothes inside to dry.

  16. Ventilate the attic - because of vapor pressure, the moist warm air from your home can go right through your ceiling into your attic.  If the attic is not ventilated, humid air will condense on the cold underside of your roof.  This condensation can start to rot the roof boards, cause ice dams, or damage your plaster, paint and attic insulation. There should be no hints of moisture in the attic.  In general attics should have one square foot of free-air ventilation for each 150 square feet of attic floor space or 1 square foot of ventilation for 300 sq. feet of attic space, if there is a continuous vapor barrier.

  17. Never use un-vented fossil fuel burning devices like kerosene heaters indoors - these heaters create carbon dioxide and water vapor and can create dangerous carbon monoxide.

  18. Check gas-fired appliances to be sure they are drafting properly up the chimney - back drafting releases carbon dioxide and water vapor into the home.

  19. In a weather-tight house install an air-to-air heat exchanger.

What is the correct humidity level?

This depends on the type of window, the outside air temperature and the inside air temperature. When the glass is cold, the humidity level or dew point must be lower to prevent condensation. For a home with double glazed (double glass) windows and an interior air temperature of 70 degrees, the indoor humidity should not exceed the following levels.  If it does, the windows will fog.

So what is the ideal amount of relative humidity in the air?  Based on keeping the indoor temperature at 70 deg F a guide is

Other Options for Controlling Moisture:

Energy efficient doors and windows help.  They prevent cold air from entering around them.  Spacers between glazing in double or triple glazed windows are used for energy efficiency because they prevent cold air from migrating through them.  Special metallic coatings have been developed (low E) which reflect radiant heat and restrict its flow through glass.  In cold weather it keeps heat inside, in hot weather it keeps heat outside.

Using energy efficient windows will keep the interior glass surfaces warmer, reducing the interior cool surfaces where moisture can condensate.  However, they do not block the existence of condensation.

If you have high performance glazing or specially insulated glass, the interior glass temperature will be warmer because there is less heat loss. With warmer glass temperature, you can raise the humidity level.  If you have triple glazing, you can also raise the humidity level. The benefits of double and triple glazing include saving energy, reducing the amount of condensation, and reducing outside noise.  They will also decrease down drafts from windows which allow radiators to be positioned more freely.

Moisture on the inside of a storm window or outside pane indicates that the prime window is allowing air and moisture to leak out to the storm window where it condenses. If there is moisture on the inside of a window pane, it is sign that airborne water is trapped in the house due to poor air circulation and exchange.  Moisture sometimes accumulates between a single pane and exterior storm windows. Storm frames are made with breathing holes that permit condensation to escape.  If these breathing holes become plugged or puttied,  moist air becomes trapped and condensation appears.  It is important to keep these breathing holes clean.

To prevent or reduce condensation problems inside your walls and protect your insulation, the side of the insulation exposed to the high vapor pressure (arm side in the winter) must be covered with material that will impede the natural drive of moisture to flow through the inside surfaces of exterior walls toward the lower vapor pressure outside.  Such material must have a high resistance to moisture and flow and is usually called a vapor retarder.

Humidifiers - many people feel the need to add moisture to their air with a humidifier.  Besides contributing to your condensation problems, humidifiers can also cause respiratory problems.  In fact the EPA is concerned by the raised concentration of particles in the air that can be inhaled when humidifiers are used.  Unless humidifiers are kept very clean, they can encourage growth of molds and bacteria in the reservoirs and in over-humidified rooms.  These molds and bacteria can cause an allergic reaction in people who are healthy.

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