Window Shopping

Press Enter to show all options, press Tab go to next option

by Planning Technician Carol Borck

At Town Hall's Building and Planning counter, we frequently accept building permit applications for "window replacements." Changing out windows is a practical and cost-effective way to increase your home's comfort and security, spruce up its exterior appearance, and reduce your energy bills. Whether you are remodeling or completely rebuilding your home, choosing the right windows involves being aware of your options and understanding which features will best suit your home and climate.

NFRC labelThere are volumes of information on new window technologies, and claims of "low-e" this and "Energy Star" that can be confusing to understand. New windows generally come with a National Fenestration Rating Council (NFRC) label that identifies their energy performance. These labels make it easier to compare one window to another so you can determine which will be appropriate for your individual needs. The two primary energy ratings to be concerned with are the U-Factor and the Solar Heat Gain.

U-Factor: The U-factor is also known as thermal transmission; it tells us how well the product prevents heat from escaping. U-factor ratings generally range between 0.20 and 1.20, with the lower the rating, the lower the amount of heat loss. In San Mateo County, windows with a U-factor of 0.40 or less are recommended.

Solar Heat Gain: The Solar Heat Gain Coefficient (SHGC) measures how well the product blocks out the heat of the sun. This rating is indicated as a number between 0 and 1, with the lower the SHGC, the more effective the window is at preventing unwanted heat gain. Windows with an SHGC rating of 0.55 are recommended for San Mateo County. However, if you have an air conditioned home, you might consider a rating between 0.40 and 0.55.

Additional Ratings: Other ratings that can appear on an NFRC label include visible transmittance, air leakage, and condensation resistance. Visible transmittance (VT) indicates how much light is allowed to pass through the window and is specified by a number between 0 and 1. The higher the VT rating, the more daylight will be allowed into your home. Air leakage (AL) is a measure of how much air leaks through the window assembly and usually ranges between 0.1 and 0.3. The lower the AL, the better the window is at preventing outside air from leaking into your home. Condensation resistance (CR) rates the window's ability to resist the formation of condensation. It is specified by a number between 1 and 100, with the higher rating being better able to resist condensation.

Window Tech

Single-pane glass windows are very poor insulators; they allow for considerable heat loss and gain. Typically, energy efficient windows have two glass panes separated by an air gap. Each pane and the air space contribute to heat flow resistance. In some window assemblies, the air gap is filled with a less conductive gas, such as argon or krypton, to further reduce the transfer of heat between the inside and outside. However, because San Mateo County has a relatively mild climate, this additional insulation is not usually necessary.

Everyone wants low-e windows, but just what are they? Low-e is short for "low-emmissivity." Low-e windows have a thin, transparent coating on the surface of one of the panes that allows light in while blocking out heat. While there are different types of low-e glazings, standard coatings generally have a U-factor between 0.30 and 0.40 and an SHGC of 0.55 or greater. These coatings are designed to reduce winter heat loss by reflecting a room's heat back inside and can reduce energy loss by 30% to 50%.


Window frames are available in several types of materials, both natural, such as wood, and fabricated, such as vinyl or fiberglass. Frames composed of different materials, such as wood clad with vinyl or aluminum are becoming increasingly popular. When choosing windows, be sure to also consider frame options available. Factors that should affect your decision include energy efficiency, cost, maintenance, durability, and color selection.

Wood frames are very traditional and remain perhaps the most commonly used framing material. Wood is visually appealing and offers relatively good energy efficiency. However, high quality wood windows can be expensive and do require regular painting to prevent moisture damage. Wood frames that are not properly maintained can suffer from swell, warping, rot, and sticking. If you can find a manufacturer that offers FSC certified woods, this would likely be the most environmentally friendly window frame option even though wood frames typically have formaldehyde-based finishes.

Aluminum frames, while relatively inexpensive and low-maintenance, are less energy efficient because they are about 1,000 times more heat-conductive than wood or vinyl. If you select aluminum-framed windows, ensure that they have thermal breaks, which are less conductive materials incorporated into the frame to reduce heat transfer. 

Vinyl frame
Clad wood frame
Vinyl frames, made from polyvinyl chloride (PVC), are a popular choice due to their low maintenance and energy efficiency. They are a less expensive option than wood and come in a wide range of styles and colors. However, because the production and incineration of PVC creates highly toxic compounds, such as dioxin, use of vinyl is often discouraged in green building projects.

Fiberglass frames are very strong and durable, provide excellent insulation, and require no maintenance. Fiberglass manufacturing does contribute to the emission of hazardous air pollutants, primarily from the resins used.

Clad wood frames are wood frames that are covered with an outer skin of vinyl, fiberglass, or aluminum, and hence offer the exterior durability of the chosen cladding while providing the warm look of wood on the interior. These frames provide excellent energy performance, are very strong, durable, can be painted, are low maintenance, and some manufacturers can even offer custom colors.

Planning a New House or a Remodel?

If you are planning a new house or redesigning your current home, simple passive solar design can help you get the most out of your energy efficient windows. First, it is suggested that your building be oriented with its longest face within at least 30 degrees of true south. Second, it is important to design your home with a higher percentage of the building's windows on the south side. A design incorporating these two points will allow the building to capture more of the sun's energy in the winter by absorbing it into the materials of the house while also providing year-round natural daylight. The addition of overhangs or awnings over these south-facing windows will provide shading aginst direct beams of sunlight from the high summer sun and help reduce unwanted heat gain.

Window design strategies for each side of your house are summarized in the following:

East-facing: While the rising sun can bring warmth to your home on cold mornings, it also can cause blinding glare. In addition to thinking about placement of east-facing windows, you can also use interior blinds to manage light intrusion. Planting deciduous trees in this area can also help limit direct sunlight and heat gain during the summer.

West-facing: Because of the low angle of the hot late afternoon and evening sun in the summertime, it is recommended that you minimize the number of west-facing windows on your house. Provide shading by planting deciduous trees on the west side.

South-facing: Windows on the south side of your home will provide year-round daylight and help warm your house in the winter. As stated above, maximize the use of windows on the south side of your home and consider adding overhangs or awnings to shade out some of the summer sunlight.

North-facing: As with south-facing windows, windows on the northern side of your home will provide daylight all year, but with little glare. North-facing windows generally do not require exterior shading.

Do Your Homework

  • As always, surf the web - "energy efficient" windows are everywhere, find out what different manufacturers are offering and compare product performance using the NFRC labels.
  • Determine your needs:
    • What performance ratings will you require?
    • Do you want different performance in different areas of your house?
    • What type of frame do you desire (low maintenance? color? durability?)
    • What is your budget?
  • Review manufacturer's product specs, check warranties
  • If you are remodeling/redesigning or building new, work closely with your architect to get the most out of your building layout through passive solar design

Disclaimer: The products pictured in this article do not constitute an endorsement, approval, or recommendation of the product by the Town of Portola Valley. These product images are provided only for informational purposes and as visual examples.

If you have a green building topic you would like explored further in an article on this web page, please feel free to provide me with your suggestions at or via telephone.