What do the Professionals Say about Condensation?

 Window condensation is a frustration for homeowners, especially if the windows are new.  Immediate concerns are of mold, water damage, peeling paint and reduced energy savings. To help our customers prevent window condensation, we need to understand why condensation occurs and how to prevent it.   

The following information has been collected through multiple sources including but not limited to Multiple Window Manufacturer's, the NFRC, and several State Construction Solutions Websites.

The quickest, and simplest first step is to make sure that there is proper air exchange to balance the humidity levels.  Space heaters, Beds or other furniture next to the window, with closed doors are the #1 VISIBLE cause of window condensation when the windows themselves are current and properly installed.

 Humidity and Condensation

Condensation occurs when there is a reduction in the rate of air exchange (i.e. closing windows in winter) and there is excess humidity in the home.    Humidity is the amount of moisture or water vapour in the air. People and pets produce moisture when they breathe or perspire. Even indoor plants produce moisture. We add water vapour to indoor air through everyday household activities: cooking, showering, bathing and doing laundry. Every time you run the dishwasher, you add 5 lbs of moisture to your home!

 We need humidity for our comfort and health. But too much or too little humidity can produce a host of difficulties in homes. Some of the problems are just nuisances like condensation on windows and musty smells, others can be more serious such as wet stains on walls and ceiling, moldy bathrooms and allergic reactions. Humidity hassles often occur during the colder months when windows are closed, and indoor air circulation is reduced. 

Humidity Levels

Experts have developed rules of thumb to help homeowners make decisions regarding humidity levels in their homes.  The limits should be used as guides only. Acceptable or comfortable humidity levels will vary from season to season, from house to house, and even between rooms in the same house. To measure the indoor humidity in a home, you can purchase a small inexpensive tool called a hygrometer that measures relative humidity (RH). The recommended indoor relative RH when the outdoor temperature is above 10° C is 30 to 50 per cent. When the outdoor temperature is below 10° C, the recommended RH is 30 per cent. 

Controlling Humidity

The first step of preventing condensation is controlling humidity. Begin with installing new energy-efficient windows with a high performance rating. High efficiency windows may not solve all condensation concerns, but properly installed, sealed and insulated windows are the first step towards eliminating humidity issues. Although excessive humidity shows up as condensation on the cold surface of a window, the window is probably not the source of the condensation problems.

Further reducing humidity in homes is not difficult. Begin by remembering to open or close doors and windows on a periodic basis to exchange air in the house. Exhaust fans may have to be installed in bathrooms and kitchens, and seal up the house with weather stripping and caulking.   Humidifiers, both stand alone or attached to furnaces, can be sources of excessive moisture and mold, so ensure they are not overused. 

Another Professional's way of saying it :


· Condensation is the result of a simple law of physics. Vapor will turn to liquid when it comes in contact with any surface that is at its dewpoint or lower.  The dewpoint is determined by the air temperature, its pressure, and the amount of water vapor in the air (relative humidity).

·  No way to completely eliminate condensation.

·  Average home produces 20 gallons of water per day into the air of the home just from washing, bathing, cooking, etc.

·  The window is only a "symptom" of a home that’s too humid.

·  Aluminum windows have cooler surfaces than wood or vinyl windows, so they’re the first to show condensation.

·  Even with vinyl or wood windows, condensation can be apparent when a home is not properly ventilated.

·  Condensation can also appear on outside surfaces when a higher performing glass type is used. This glass can allow the outside surface to reach the dewpoint, because it reflects the heat back indoors and off the glass surface.  As outside air temperature increases, the outdoor condensation should diminish. You will mainly see this challenge in the morning hours. This is the same phenomenon that is observed on your lawn, outdoor furnishings, or your car windows when left out for the evening.

Solution Ideas for indoor condensation

Glass : ·  Low-E Glass &  Heat Mirror will have a warmer interior surface, which will help reduce the forming of condensation. 

Frame : ·  Vinyl Windows & Wood Windows have a warmer frame surface, which will help reduce the symptoms of condensation

Other Considerations

·  CAUTION :  A home that is too humid can show condensation on the windows, the floors, even the walls.

·   Suggest ventilation systems in heating/cooling equipment or wall vents, fans, etc. to help minimize the humidity of the home.  

Managing Condensation and Mold

 Follow these easy steps to reduce the risk of excessive condensation and mold growth.

1.  Check all window and patio doors for smooth operation and sealed air tight.

2.  Use exhaust fans regularly in rooms with high humidity (bathrooms, pool rooms, etc.

3.  Use a ceiling fan to continually circulate air.

4.  When practical, open all windows and doors when experiencing excessive moisture inside your home.

More Tips to prevent window condensation:

• Install an air exchange unit or a humidity control device to maintain the proper level of humidity

• Insulate water pipes

• Ensure hot air from registers does not flow directly on to the interior surface of the glass

• Turn furnaces with humidifiers off until condensation reduces

• Ensure the exhaust from the clothes dryer is vented outside

• Cover crawl spaces with 0.15 mm (6 mil) polyethylene

• Ensure gutters drain water away from the house

• Avoid drying firewood in the house

• Waterproof damp basement walls and floors

• Use exhaust fans while cooking

• Open a window when doing laundry

• Close the bathroom door and open a window after baths and showers

• Ventilate the home at least once a day

Allowing Your Window and Patio Door to Properly Drain

Mother Nature can be unforgiving to your home. Thankfully, with the newer vinyl window styles, you can rest assured that no water will penetrate the inside of your home. Most new window and patio doors are specially designed with a "weep" drainage system to protect the inside of your home. Proper maintenance of your weep system is required to assure proper drainage.         

1.  The weep system is designed to allow the water to drain to the outside as water builds up. It's normal for the water to build up on the sill or the outside track. It's important to keep the sill or track clean of any dirt or debris.

2.  Periodically inspect the weep holes to make certain they are clear of any dirt or debris. Use a soft bottlebrush to clear openings. It is especially important if you anticipate excessive rain in the near future

WHO is the NFRC :

 NFRC administers an independent, uniform rating and labeling system for the energy performance of fenestration products, including windows, curtain walls, doors, and skylights. For more information on NFRC, please visit our Web site at www.nfrc.org or contact NFRC directly at 301-589-1776.

What the NFRC has to say :

In today’s market, architects, builders, and consumers have the opportunity to choose among many different windows. In colder climates that have a heating season, many home and building owners may have concerns regarding the formation of condensation on the inside of their windows. Condensation on window surfaces can reduce visibility, obscure the view through the window, and, in some cases, damage curtains, walls, carpets, and even the window itself. 

What is Condensation?

Condensation, which can appear as a light coating of water, water droplets, frost, ice, or some combination of the four, forms on any surface when the temperature (°F) of that surface is less than its dew point temperature. For example, if the temperature of the glass in a window is 50°F and the dew point temperature for the glass is 55°F, condensation will form on the surface of the glass.

The dew point temperature of any surface is directly related to the amount of moisture that is in the air, which is called the relative humidity. It is also related to the temperature of the air in the room, which is known as ambient air temperature. As the relative humidity in a room increases, the dew point temperature also increases, which means that a surface is more likely to show moisture even at warmer temperatures. As the relative humidity starts reaching levels near 100%, moisture will form on almost any surface, no matter what the temperature of that surface. For example, bathroom and kitchen areas typically have higher humidity conditions at certain times of the day. On the other hand, surfaces in living or working areas where the relative humidity is low have lower chances for the formation of condensation.

How To Stop Condensation on Windows

To increase the resistance of windows to the formation of condensation, it is important to maintain the surface temperature of the window above the dew point. To accomplish this, manufacturers must reduce the amount of heat that transfers through a window, which is called the thermal transmittance or U-factor of the overall window product. The higher the U-factor of a window, the higher the potential that condensation will form on a surface of the window unit.

NFRC recognizes three parts to a window: the center- of-glazing; the edge-of-glazing; and the frame. Heat from inside the house will conduct its way through the parts of the window that are the least energy efficient, causing those parts to have lower indoor surface temperatures. To reduce the potential for condensation, each component of the window should be thermally efficient.

You can reduce the potential for condensation on each window component by looking for the following improvements:

Center-of-Glazing – Going from single-glazed (one lite of glass) windows to multiple-glazed windows or insulating glass units reduces the potential for condensation. Choosing energy-efficient, low-e coatings in multiple-glazed or insulating glass units further reduces the potential for condensation.

Edge-of-Glazing – Similar to the center-of-glazing, going from single-glazed to dual-glazed or insulating glass units reduces the potential for condensation on the edge-of-glazing surface, and using high- performance glass further reduces the chance for condensation. A third step for reducing the potential for condensation is the use of warm edge spacer systems that reduce conductivity through the edge.

Frame – Going from highly conductive metal framing systems to thermally broken metal frames or thermally improved framing materials (like wood or vinyl) reduces the chance for condensation formation.

Determining Condensation Resistance With NFRC 500 

NFRC has developed a standardized methodology for determining the potential formation of condensation on a window – called Condensation Resistance. Condensation Resistance is reported on a scale of 1 to 100. The higher the number, the better a product is at resisting condensation. The results are based on a set of standardized conditions (O°F outside temperature, 70°F inside temperature) and three levels of relative humidity – 30%, 50%, and 70%. Surface temperatures for the window are normalized and recorded for the three window areas (center-of-glass, edge-of-glass, and frame). The Condensation Resistance of the window is then determined by the lowest rating obtained from the three component areas of the window.

It should be noted that NFRC 500 only reports condensation formation on the inside surfaces of windows, and that in the real world, environmental conditions vary from the standardized environmental conditions used to determine Condensation Resistance. This standard (NFRC 500) is not meant to predict condensation; rather it is meant to be a tool for rating and comparing window products and their potential for condensation formation.

NFRC has additional information for selecting energy efficient windows on its Web site www.nfrc.org. Of special interest, see the NFRC Certified Products Directory, which lists hundreds of manufacturers and thousands of products authorized for certification by NFRC. If you need further information, contact our offices in Maryland (301-589-1776) or Kansas (785-862-1890).

A - U-Factor measures how well a product prevents heat from escaping a home or building. U-Factor ratings generally fall between 0.20 and 1.20. The lower the U-Factor, the better a product is at keeping heat in. U-Factor is particularly important during the winter heating season. This label displays U-Factor in U.S. units. Labels on products sold in markets outside the United States may display U-Factor in metric units. 

B - Solar Heat Gain Coefficient (SHGC) measures how well a product blocks heat from the sun. SHGC is expressed as a number between 0 and 1. The lower the SHGC, the better a product is at blocking unwanted heat gain. Blocking solar heat gain is particularly important during the summer cooling season.

C - Visible Transmittance (VT) measures how much light comes through a product. VT is expressed as a number between 0 and 1. The higher the VT, the higher the potential for daylighting.

D - Air Leakage (AL) measures how much outside air comes into a home or building through a product. AL rates typically fall in a range between 0.1 and 0.3. The lower the AL, the better a product is at keeping air out. AL is an optional rating, and manufacturers can choose not to include it on their labels. This label displays AL in U.S. units. Labels on products sold in markets outside the United States may display AL in metric units.

E -Condensation Resistance (CR) measures how well a product resists the formation of condensation. CR is expressed as a number between 1 and 100. The higher the number, the better a product is able to resist condensation. CR is an optional rating, and manufacturers can choose not to include it on their NFRC labels.

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