Window Condensation Resistance

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Condensation Resistance
Window Condensation Resistance
Other Names:
Window Condensation Resistance Rating, CR Rating
A window rating that measures the amount of condensation it can occur.

Homeowners who live in heat dominated climates should keep an eye on a window's condensation resistance rating when deciding on the best replacement window option. When condensation forms on a window it can damage the home furnishings as well as the window itself. It also reduces visibility and adds to moisture build-up within the house.


[edit] Condensation

Condensation is formed when a light coating of water, including frost, ice, water droplets or a combination of these elements, forms on the window surface. This occurs when the exterior temperature drops significantly below the temperature of the home's interior together with a high level of relative humidity. At this "dew point" condensation forms on the glazing surface. Rooms that typically have more humid conditions, such as bathrooms and kitchens, tend to have a higher rate of condensation along the windows. Areas where relative humidity remains low have fewer problems with condensation formation.

[edit] Preventing Window Condensation

Window manufacturers attempt to build window units which will reduce the formation of condensation. To do so, they must find strategies for reducing the amount of heat that transmits through a window -- the U-Factor. This helps to ensure that the surface temperature of the window will have a higher temperature than the dew point and the window unit will resist condensation formation.

[edit] Rating Condensation Resistance

As heat from the home's interior conducts through the various window sections it is directed to the parts that are the least energy efficient. This causes those sections to have a lower indoor surface temperature. When manufacturers create thermally efficient component parts, the energy efficiency of the total window unit is increased.

Ratings for condensation resistance are broken down into three sections by the National Fenestration Ratings Council (NFRC), the agency responsible for assigning window ratings. These component parts include Center-of-Glazing, Edge of Glazing and the Frame.

[edit] Center-of-Glazing

When measuring the condensation resistance capabilities of a window unit the NFRC will review the ability of the glazing to resist condensation. double pane replacement window units and energy-efficient, low-e coatings help to reduce condensation build up on the window.

[edit] Edge-of-Glazing

There is a potential for condensation build-up along the edge-of-glazing surface. Warm edge spacer systems lower the rate of conductivity through the window's edge. In addition, dual pane glazing and high performance glazing both help to reduce the chances for condensation forming along the edge of glazing.

[edit] Frame

The framing material can add to a window's condensation build-up. High conductive metal framing systems tend to increase condensation . Thermally improved framing materials and thermally broken metal frames, such as wood or vinyl, decrease the propensity for condensation formation.

[edit] Testing

The NFRC measures and rates the condensation resistance properties of windows using their NFRC 500 tool. This standardized methodology rates the window on a scale of 1 to100 with higher numbers providing better condensation resistance. The NFRC 500 only reports on condensation formation which occurs on the inside surfaces of windows. Due to varying environmental and climate conditions the standard (NFRC 500) does not predict condensation. It is a tool that homeowners can use to rate and compare the potential for condensation formation on varying window products.

[edit] Optional Rating

The condensation rating is an optional National Fenestration Ratings Council measurement which provides the customer with data that will allow him to make an informed decision when he is reviewing alternative replacement window options. The condensation rating is not included in the Energy Star Program. It is listed on a window's label for code compliance purposes and to promote window comparisons.

[edit] Variants

Condensation rates for a replacement window are dependent on a wide variety of environmental factors including humidity and climate. The condensation resistance ratings are intended for purposes of window comparison.

[edit] National Fenestration Ratings Council

The National Fenestration Ratings Council (NFRC) rates replacement windows' and new construction windows' condensation resistance. The NFRC is a non-profit agency which acts as a third-party authority to providing window ratings according to a window's ability to protect the home from various weather elements as well as to reduce solar heat gain and offer quality thermal insulation.

NFRC ratings are posted on labels which are attached to windows manufactured by participant window manufacturers. The labels allow the manufacturers to provide concrete data about windows' energy performance to homeowners who are looking for replacement window options. The Energy Star program, a program affiliated with the United States Department of Energy, uses NFRC ratings to identify windows which may be eligible for tax credits.

The NFRC rates a window's U-Factor, Solar Heat Gain Coefficient, Air Leakage, Visible Transmittance and Condensation Resistance. However, the Energy Star program only utilizes the U-Factor and Solar Heat Gain Coefficient portion of the ratings program as part of its mandate to identify eligible Energy Star program windows.

[edit] U-Factor

A window's U-Factor is an indication of the window's capability to prevent heat from escaping from a building. The lower the U-Factor, the higher the window's capacity for maintaining the home's heat. Recommended window U-Factors for most climates is 0.30.

[edit] Solar Heat Gain Coefficient

The solar heat gain coefficient relates to the rate at which a window blocks the sun's heat. It is recommended that homeowners look for a lower SHGC which is more successful at blocking unwanted heat gain than high SHGCs.

[edit] Visible Transmittance

The VT Visible Transmittance rate measures the quantity of light that transmits through a window. Windows with high VT ratings have an increased potential to use the sun's light in an energy efficient way than those with low VT ratings.

[edit] Air Leakage

The ALAir Leakage rating indicates the rate at which air enters a home through window openings -- generally through cracks or other spaces in which the glazing is held in place such as within the window frame or through gaps between the window frame and the wall's rough opening. AL rates are indicated as numbers of between 0.1 and 0.3 with windows with lower AL ratings better able to prevent air leakage than windows with high AL ratings.

[edit] Condensation Resistance

Condensation Resistance measures a window's facility for resisting the formation of condensation along the window's frame or glazing. Condensation Resistance is expressed as a number between 1 and 100. Higher condensation resistance numbers indicate that the window can better resist condensation.

[edit] Energy Star Windows

The NFRC's ratings are employed by the Energy Star Windows program which indicates replacement windows that are eligible for tax rebates and credits. The United States Department of Energy relies on the NFRC/Energy Star measurements when advising homeowners about replacement windows which provide them with an energy efficient window solution and, when applicable, a tax credit.

Criteria for windows which are eligible for tax credits vary according to the homeowner's climate. The Energy Star program classifies Energy Star windows according to the NFRC's U-Factor and Solar Heat Gain Coefficient ratings. Energy Star qualifications do not take into account the ratings for Air Leakage, Visible Transmittance or Condensation Resistance.

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