How Do You Measure The Performance Of A Window?

How Is A Windows Performance Measured?

When looking at windows, doors or skylights, whether you are replacing old ones in your current home or buying ones for your new home, there are some factors you will want to take into consideration.  These are called fenestration factors.

The National Fenestration Rating Council (NRFC) has developed a rating system for you to compare the performance of different fenestration products. NFRC is a third party, non-profit organization that sponsors certified rating and labeling programs to help consumers compare the energy and performance features of windows, doors, and skylights.  NFRC’s programs do not distinguish between “good” and “bad” products, set minimum performance standards, or mandate performance levels.  This is where ENERGY STAR comes in. ENERGY STAR enables consumers to easily identify NFRC-certified products with superior energy performance.

Let’s look at the NRFC label and see what it tells us.

There are five (5) characteristics to consider when looking at a fenestration product.

1. U-Factor - This measures how well the product prevents heat from escaping your building envelope. The rate of heat loss generally falls between 0.20 and 1.25. The lower the U-Value, the greater the product’s resistance to heat flow, and the better its insulating value.  The U-Factor is particularly important during the winter heating season.
2. Solar Heat Gain Coefficient (SHGC) – The SHGC measures how well the heat caused by sunlight is blocked.  It is expressed as a number between 0 and 1.  The lower the number, the less solar heat is transmitted into your home.  A typical range is 0.25 to 0.80.  Blocking solar heat gain is particularly important during the summer cooling season.
3. Visible Transmittance (VT) - This is expressed in a number between 0 and1, and is a measure of how much light comes through your fenestration product.  The higher the VT, the higher the potential there is for daylighting.  Generally, products range from 0.20 to 0.80.
4. Air Leakage (AL) -Any air which filters through cracks in the product’s assembly results in heat loss and gain.  AL is measured in cubic feet of air passing through one square foot of window area per minute (cfm/ft^2.).  The lower the AL value, the less air is leaked.  Most industry standards and building codes require a minimum AL of 0.3 cfm/ft².  The AL rating is optional and manufacturers can choose not to include it in their labels.
5. Condensation Resistance (CR) -This measures the ability of the product to resist the formation of condensation on its interior surface. The higher the CR rating, the better the product is at resisting the formation of condensation.  The CR rating is optional and manufacturers can choose not to include it in their labels.  The CR does not predict condensation but rather measures the potential for its formation.

So, when you look for windows, doors or skylights, take these characteristics into consideration.  The better NFRC ratings will result in increased energy savings and great comfort for you and your family.  Be sure to talk with your local window contractor to see which window products are best for you.

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Tags: Air Leakage, Condensation Resistance, NFRC ratings, NRFC label, Replacement Windows, Solar Heat Gain Coefficient, U-Factor, Visible Transmittance, Windows