6 Metrics to Assess Whole-Window Performance

A house can’t be completed without windows. Windows are responsible for facilitating the entry of natural light into the living space, allowing the residents to appreciate the gorgeous views of their yard, garden or neighborhood and, they keep the home sufficiently ventilated. As such, homes with ample windows seldom feel stuffy or look ill-lit. Today, one of the leading qualities of good, functional windows is their energy efficiency. In order to determine whether or not a replacement window is energy-efficient enough, it is important to focus on whole-window performance.

In this article, Direct Windows breaks down the metrics used to assess whole-window performance and discusses the benefits of energy-efficient windows.

Metrics of Whole-Window Performance

The National Fenestration Rating Council has developed an energy rating system that determines the performance of the entire window, skylight or door. On the market, the NFRC label is the only reliable method to assess the energy efficiency properties of windows and compare products. It appears on all products that meet the standards of the organization. At this time, this label on windows units showcases pertinent ratings for the following factors:

1. U-Factor

U-factor refers to the measurement of thermal resistance. The lower the U-factor of a window, the greater its resistance to heat flow and the better its insulating properties. While the U-factor of windows can take any value, it generally ranges from 0.20 to 1.20. High-performance double-pane windows can have U-factors of 0.30 or lower, while some models of triple-pane windows can even reach U-factors as low as 0.15. Low U-factors are most essential in climates that require a lot of heating, although they are also incredibly useful in climates that need frequent air conditioning.

2. Solar Heat Gain Coefficient

The solar heat gain coefficient is the fraction of solar radiation admitted through a window replacement, either transmitted directly or absorbed, and then released as heat inside the home. In general, it is expressed as a number between 0 and 1. The lower the SHGC of a window, the less solar heat it transmits and the greater its shading ability. This rating method pertains to the whole window, including the effects of the frame.

So, why is SHGC so important? This is because while it can provide free heat during the winter months, it can also cause overheating during the summer. To correctly balance the solar heat gain with an appropriate SHGC, several factors must be considered, such as the climate, orientation, shading conditions and more.

3. Visible Transmittance

Visible transmittance refers to the amount of visible light that passes through a glazing material. It is distinct from the solar heat gain coefficient since most present-day windows include selective coatings that can allow different amounts of visible, ultraviolet and infrared light. While the visible transmittance theoretically varies between 0 and 1, most double- and triple-pane windows possess values ranging from 0.30 to 0.70. The higher the rating of visible transmittance, the more light is transmitted through the window. If you want to maximize the daylighting in your home, choose replacement windows that have high visible transmittance.

4. Air Leakage

The level of heat loss and gain that occur through cracks in the window assembly is indicated by the air leakage rating. In general, the lower the rating, the less air will pass through cracks. To acquire an ENERGY STAR certification, the rating must be 0.30 or less. Keep in mind, however, that the air leakage rating of a window replacement can be displayed on the NFRCor another program, like the AAMA’s Gold Label or Keystone Certifications, Inc.’s NAFS Structural Certification Label. This means that the value may not always show up on the NFRC label.

5. Condensation Resistance

How well a window unit resists condensation on its interior surface is measured by the condensation resistance. Often expressed as a number between 1 and 100, the rating is based on the interior surface temperatures at 30, 50, and 70 percent indoor relative humidity. This is for an outside air temperature of 0° Fahrenheit under an estimated wind speed of 15 mph. The higher the condensation resistance rating, the better the window is able to resist the formation of condensation. It is, however, an optional rating on the NFRC label.

6. Sound

A side benefit to houses bespattered with tight, insulated windows is that they are much quieter than houses with leaking windows. In order to reduce sound transmission, it is important to minimize air infiltration first. The use of double glazing will be beneficial as well.

Benefits of Energy-Efficient Windows

1. Energy and Cost Savings

In regions that encounter intense heating seasons, energy-inefficient windows can be a significant cause of unwanted heat loss, condensation issues and discomfort. Now, it is possible to achieve minimal heat loss, less air leakage, reduced condensation and improved comfort at home, thanks to recent advances in technology. On the flip side, in climates that often require cooling, energy-inefficient windows can prompt heat gain. Fortunately, low-E coatings made it possible to reduce solar heat gain and therefore increase comfort levels while providing daylight and clear, unobstructed views.

According to window installation experts, if you install replacement windows that allow long-term air leakage reduction, the heating or cooling load of your HVAC system will not be as substantial and your energy savings will dramatically shoot up over time.

2. Improved Comfort

High-performance windows equipped with new glazing technologies not only reduce energy consumption and costs but make residential spaces more comfortable as well. In cold, freezing winter months, outdoor temperatures drive the temperatures of interior glass surfaces down below room air temperature. If household occupants are exposed to the effects of a cold surface, they can experience notable radiant heat loss, almost instantaneously making them feel less restful. During warmer weather, strong, direct sunlight can cause discomfort. Fortunately, an energy-efficient window can reduce heat gain while providing sufficient light.

3. Reduced Condensation

Since high-performance, energy-efficient windows create warmer interior glass surfaces, they actively reduce the chances of condensation and frost. This is due to the presence of insulating frames and warm edge technology, which refers to the edge construction of double or multiple glazed units that conducts less heat or cold than traditional windows. If the frame and glazing system are poor performers for a specific climate, both elements can contribute to the occurrence of condensation.

To control condensation, the level of humidity indoors should be reduced. Use vents, dehumidifiers or air-to-air heat exchangers along with energy-efficient window installation to make sure your home’s humidity levels are regulated year-round.

4. Increased Natural Light

Letting sunlight into the living space is an essential function of windows. Natural light is a powerful tool in defining certain interior spaces, improving the health and well-being and boosting the productivity of occupants. However, optimal daylight requires careful placement of windows and interior design that emphasizes visual comfort and balanced light levels and minimizes the fading of furnishings.

An energy-efficient window can diffuse the direct sunlight and minimize glare from the sun in order to maintain the home cool and comfortable. 

5. Reduced Fading

Your selection of windows influences the intensity of transmitted radiation. The most detrimental radiation in sunlight is ultraviolet rays, which encourage the breaking of chemical bonds, leading to fading and degradation. The good news is that using low-E glass can reduce the ultraviolet light transmitted by up to 75 percent. Moreover, applied films or window units that incorporate plastic layers will reduce fading on a lot of modern interior furnishings.

Let STL Windows Direct Replace Your Windows and Doors

Breathe new life into your living space with new, energy-efficient Direct Windows from STL Windows Direct. Our experienced team is committed to providing the comfort and efficiency every client needs through industry-leading products and outstanding customer service. Call us at (314) 690-5800 or fill out our contact form to request a free, no-obligation estimate today. We work with homeowners in Maryland Heights, MO and surrounding areas.