the science behind energy efficiency in windows

PART I: THE ANATOMY OF AN ENERGY-EFFICIENT WINDOW

Energy-efficient windows -- you might have heard of them many times over. But what enables them to promise energy savings is still a mystery to many. Here, we will take a look at the technical and scientific concepts that make newer windows on the market as efficient and noteworthy as they are.

THE COMPONENTS OF A WINDOW

A window is made up of parts and components developed with quality and applied with technological advancements that make them able to resist heat, keep the cold out, and ultimately, make the whole window energy-efficient.

A variety of quality, durable, and low-maintenance framing materials can enhance insulation and reduce heat transfer. Among the framing materials available on the market are the following:

Fiberglass - Strong and durable, fiberglass windows are low maintenance and they also offer good insulation.

Aluminum - Durable, recyclable, and low-maintenance, aluminum frames typically come with thermal breaks that reduce conductive heat loss.

Many windows today come with Low-E (low-emissivity) glass that have special coatings that can reflect infrared light. They keep heat outside during summer and inside during winter. The coatings also reflect damaging UV rays, helping protect your furniture and other belongings from premature deterioration.

Furthermore, windows with more than one pane of glass provide better insulation compared to single-pane windows. Aside from enhanced insulation, multiple-pane windows also have increased impact resistance.

Vinyl - Noted for good thermal insulation and low maintenance requirements.

Wood - Wood frames are generally strong and have good insulation properties. They are also aesthetically attractive.

Combination - Frames of this type use different materials separately (throughout the sash and frame). For instance, the exterior half can be vinyl while the interior is wood. Thus, the features in terms of durability and insulation depends on the materials combined.

Composite - Frames of this type are made of various materials blended together during manufacture. They are highly durable and require less maintenance. They also make for well-insulated windows.


Windows that have gases such as krypton and argon in between the panes offer better energy efficiency than those that have regular air fills.

Spacers keep a window’s glass panes correctly distanced apart. With the right materials, they can add to the window’s energy efficiency. Non-metallic and metal/non-metal hybrid spacers are ideal for added insulation and heat resistance.


PART II: MEASURING PERFORMANCE

New technologies have helped increase windows’ benefits in terms of energy savings and comfort, providing more practical options for consumers. Following are the primary performance factors that you should check for and take into consideration when looking for energy-efficient replacement windows.

INSULATING VALUE

Windows can gain non-solar heat that results from the temperature difference between the outdoors and indoors. The greater the temperature difference, the greater the rate of heat flow, which can affect how much a cooling and heating system should work to keep the home comfortable.

The U-factor is the measurement of the rate of non-solar heat flow that gets through a window. Comparing U-factor figures allows consumers to compare the insulating ability of a product, with lower U-factors indicating lower heat flow.

There are three major types of energy flow that occur through a window:

(1) non-solar heat gain and loss through convection and conduction;

(2) solar heat gain in the form of solar radiation;

(3) airflow in the form of ventilation and infiltration


Single-pane windows depend on the thin films of still air on the inside and moving air on the exterior glazing surfaces. However, the glazing itself does not provide resistance to heat flow. This is why additional panes reduce the U-factor, indicating increased insulating value.

Low-E coating is another feature that can affect the U-factor. It is relatively a new technology applied to windows, which is a microscopically thin, practically invisible metal or metallic oxide coating. It limits radiative heat flow between glass panes -- basically, it reflects the heat back into the home during cold weather and back to the outdoors on warm days.

Gas fills, on the other hand, when combined with low-E coatings, can also increase the insulating value of a window. Furthermore, the more efficient a window’s frame and spacers are, the lower the U-factor is.

SOLAR CONTROL

Even if solar transmission through windows can provide free heating during the cold days, it can also give unnecessary heat during the warm days. In fact, solar-induced cooling costs are greater than heating costs in many parts of the United States. Solar heat gain through windows and skylights can account for at least 30% of the cooling needs in an average American home.


Solar heat gain is measured by SHGC or the Solar Heat Gain Coefficient. Additional glazing layers provide more barriers against solar radiation, reducing a window’s SHGC (the lower the SHGC, the better). Tinted glazing, for instance, gives a lower SHGC. Low-E coatings, when used with tinted glazing, can reduce solar heat gain through preventing the absorption of heat. Exterior and interior window treatments are also recommended for better SHGC values, although exterior shades are considered more beneficial.

The table below shows the average SHGC values of commonly used and available window glass and coating options.


VENTILATION AND AIR TIGHTNESS

Airflow through and around a window is meant to occur in the form of ventilation, and indirectly through infiltration. Basically, if you want air to freely pass through, all you need to do is open the window -- this is controlled ventilation.

On the other hand, uncontrolled air leakage is known as infiltration. This occurs when the window is not airtight, with air and heat escaping through joints and cracks around it. Such leakage can account for about 10% of the energy waste in a home.

The air leakage rating is the standardized measurement of the infiltration rate of a window. The lower this value, the greater air tightness.

A window’s U-factor is affected by the glazing, frame, spacers, and gas fills.

Solar heat gain is primarily affected by the glazing and coatings applied, as well as the interior or exterior shading or window treatments used.

Infiltration and ventilation are affected by the window’s operation type and the quality of the seals and caulk, as well as installation.

PART III: MAKING SENSE OUT OF THE LABELS

Today, one will rarely see a window product that does not carry energy ratings or labels, similar to those that you might see on household appliances. These labels are meant to help guide consumers as they shop for energy-efficient products. With windows, the labels we often see are developed by the National Fenestration Rating Council (NFRC), a non-profit organization that aims to provide homeowners, builders, engineers, and architects, with important information and updates regarding the window industry and related technological advancements. Meanwhile, Energy Star, an Environmental Protection Agency (EPA) voluntary program, also gives its approval to energy-efficient products.

THE IMPORTANCE OF ENERGY RATINGS AND LABELS

Windows, skylights, and doors are known to account for at least 25% of the cooling and heating requirements of an average home. Through the ratings provided by NFRC and Energy Star, designers, builders, and consumers, are provided with a basis for comparing the multitude of products available on the market. As many manufacturers offer energy-efficient products, the ratings let involved parties know which ones suit their requirements and needs.

PART III: MAKING SENSE OUT OF THE LABELS

HOW TO USE THEM

Energy labels show various performance attributes, which enable builders, designers, and homeowners to directly compare products, helping them select items according to their specific energy performance needs. Given that the Energy Star and NFRC ratings are used nationwide, they can be used by just easily comparing the values.

DETERMINING THE VALUES

The ratings are determined using advanced computer tools developed in the United States and Canada. Standardized product performance testing is also conducted to ensure the accuracy of the ratings provided. Lawrence Berkeley National Laboratory’s WINDOW 4.1 program is among the fundamental programs used in the rating system. Together with the FRAME program, U-factor, Solar Heat Gain Coefficients (SHGC), Air Leakage (AL), and other attributes are also rated by the system.

UNDERSTANDING THE NFRC LABEL

1. NFRC Insignia This gives consumers the assurance that the product has been independently evaluated and rated.

2. U-factorThe measure of heat transmission and transfer based on temperature difference; the lower the value, the smaller the amount of heat transmitted.

3. Solar Heat Gain Coefficient (SHGC)The measure of the solar heat that flows through the window; the lower the value, the smaller the amount of solar heat gained.

4. Visible Light Transmittance (VT)The measure of the fraction of visible light that passes through a window; the higher the value, the greater natural light is let in.

5. AIR LEAKAGE (AL) The measure of the rate of infiltration; the lower the value, the smaller the amount of air and energy leaked.

6. INDEPENDENT CERTIFICATION AND INSPECTION AGENCY NAMEIndicates the name of the certification and inspection agency that the manufacturer has selected.

7. MANUFACTURER’S NAMEThe company name of the window manufacturer.

8. PRODUCT DESCRIPTION Typically includes the product’s model number as well as its other specifications such as type of glass and gas fill.

9. RESIDENTIAL AND COMMERCIAL APPLICATION VALUESAs energy efficiency ratings can vary depending on the size of the windows as well as the building in which it will be installed, the NFRC rates windows for both residential and commercial applications.

Up and Above Contractors, LLC

2 Lower Hill Rd Newton, NJ 07860209 Main Street Andover, NJ, 07821

United States

(973) 300-0636973-531-4225

UpAndAboveContractors.com

the science behind energy efficiency in windows

Documents