High-Performance Windows: Looking Through the Options: Page 6 of 6

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High-performance windows
High-performance windows are multipurpose—including providing an inside view of the outside world, while contributing to a home’s overall energy efficiency.
riple-glazed, low-e window from Unilux
This triple-glazed, low-e window from German company Unilux offers excellent insulation performance, yet transmits enough sunlight to be effective for passive solar heating.
Well-placed windows
Well-placed windows can provide ample daylighting and ventilation to interior spaces.
Old single-pane window
Old single-pane windows are a constant household energy drain and compromise a home’s overall thermal comfort.
Alpen HPP windows
Alpen HPP can manufacture windows that have up to five glazing layers (three suspended films between two layers of glass), resulting in lightweight high-performance units that weigh only a little more than standard double-pane windows.
Gas-filled window
Some gas-filled windows, such as this one from Alpen HPP, are shipped with bladders to equalize pressure fluctuations in transit. They are removed during installation.
wood with aluminum cladding
Frame options include wood with aluminum cladding.
vinyl with polyurethane foam fill
Frame options include vinyl with polyurethane foam fill.
uninsulated vinyl
Frame options include uninsulated vinyl.
NFRC window label
NFRC window labels include U-factor, solar gain, transmittance, and optionally, air leakage and condensation resistance.
High-performance windows
riple-glazed, low-e window from Unilux
Well-placed windows
Old single-pane window
Alpen HPP windows
Gas-filled window
wood with aluminum cladding
vinyl with polyurethane foam fill
uninsulated vinyl
NFRC window label

Model energy performance. Installing too many windows in a house can introduce too much solar gain, causing overheating, which may force the air conditioner to work harder (particularly if those windows are east- or west-facing). Too many windows can also create excessive heat loss, depending on the U-factor of the windows. To determine how the windows will affect the home’s energy performance, nothing beats energy modeling. There are excellent software tools for modeling performance, including REM/Design ($347) and EnergyPlus (free)—though these programs take some training or experience to use effectively. Using these tools during a home’s design can influence the placement and size of the windows—leading to lower energy bills and also greater thermal comfort.

Look to the NFRC labels. When shopping for windows, make sure you’re comparing apples to apples. Use NFRC labels to compare different products’ U-factors, visible transmittance, and solar heat gain. Some NFRC labels also include numbers for air leakage and condensation resistance. The Efficient Windows Collaborative (efficientwindows.org) has an excellent online tool for help with window selection.

Define energy performance targets. This depends on both the climate and the location of the windows in the house. Given the many glazing options to choose from, it increasingly makes sense to “tune” windows by orientation—in other words, specify a certain type of glazing depending on the window’s location. With this approach, the primary variables are U-factor and SHGC.

If you want passive solar heating, especially in northern climates, specifying high-SHGC windows for the south makes sense, even if it means a sacrifice in the U-factor. For windows located on east and west walls, glazing with lower SHGCs will help limit unwanted solar gain. For north-facing windows, the SHGC value doesn’t matter much; a low U-factor window is more important. For practical reasons, it usually makes sense to specify one type of glazing for south-facing windows, and another type for all of the other windows. Some designers and builders like to design the south windows to be slightly different dimensions to ensure that they aren’t mixed up with windows slated for other walls (see the “Tuning Windows for Orientation & Climate” table).

Egress considerations. Certain windows must be designed for getting out if there’s a fire. These windows have to be a specified minimum size, and they should be readily operable—usually without separate storm windows or window attachments that require opening. In general, egress is easier with casement than double-hung windows. A building official will be able to advise on egress requirements in your area.

Warranties. Warranties make an important difference. Many have 20-year warranties or 20/10 warranties (20 years for the glass and 10 years for the non-glass components). Labor to replace failed windows is often covered for a much shorter period of time. Some manufacturers offer “limited lifetime” warranties, though understanding the fine print on “lifetime” tends to be an art form.

Access

Alex Wilson is the founder and executive editor at BuildingGreen, which publishes Environmental Building News. He also is president of the nonprofit Resilient Design Institute, which works to advance measures to improve the resilience of our buildings and communities.

Efficient Windows Collaborative • efficientwindows.org

National Fenestration Rating Council (NFRC) • nfrc.org

Manufacturers:

Alpen High Performance Products • alpenhpp.com

Cardinal Glass Industries • cardinalcorp.com

Guardian Industries • guardian.com

Hurd Windows and Doors • hurd.com

Marvin Windows and Doors • marvin.com

Pilkington Glass • pilkington.com

Quanex Building Products (Super Spacer) • quanex.com

Sage Electrochromics • sageglass.com • Dynamic glass

View • viewglass.com • Dynamic glass

Comments (1)

Jim and Elaine Stack's picture

another very ECO choice is to modify the windows already in place. Instead of ripping out the old and adding to the landfill you can add Inflectors on the inside of the window opening. They have a reflective side for summer and dark side to absorb IR heat in winter. They are also air tight and take makes a big difference.
Since they are held in place with magnetic strips you can flip them over for heat rejection or gain in a minute. You can even take them off and enjoy a fresh breeze when it's temperate out.
I put them on all my windows and it makes a big difference. Our little 4 Kw system runs our 100% electric home and vehicle with energy to spare. .

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