High-Performance Windows

Before you make an investment in new windows, consider these low cost alternatives:

  • Keep cool when it is hot outside. Install overhangs or awnings to shade south-facing windows in the summer. For west-facing windows that are difficult to shade, consider applying a reflective film. And close the curtains on west-facing windows in the late afternoon.
  • Stop drafts. To block air leaks year-round, apply caulk and weather stripping. Insulating shades or curtains can also keep out cold drafts at night.
  • Put your landscaping to work. Plant deciduous trees or vines to shade your windows in summer. In winter, they drop their leaves, letting in the sun's warmth.
  • Let in daylight. Save energy by keeping your lights off during the day. Sheer curtains can provide daytime privacy without blocking daylight. Use light-colored walls, ceilings and furnishings to help reflect daylight into a room.

Understanding Window Choices

Making sense of window & skylight labels
The NFRC label found on most windows and skylights provides three key types of information: energy performance data; the glass characteristics; and a description of the frame material.

Energy Performance

NFRC label

U-factor. A measure of the rate of heat loss by the window (including glass and frame). A lower U-factor does a better job of keeping heat inside when it's cold outside. In San Mateo County, select windows with a U-factor of about 0.40 or less.

Solar heat gain coefficient (SHGC). A measure of how much of the sun's heat will pass through the glass. The lower the SHGC, the less heat will enter your home. In mild climates, a low SHGC isn't that important. In fact, you may want more of the sun's warmth to enter your home during the cooler months. In San Mateo County, select windows with an SHGC of about 0.55 (or higher if you want to let in more of the sun's heat). For San Mateo County homes that are air conditioned, consider windows with an SHGC between 0.40 and 0.55.

Visible transmittance (VT). A measure of the percent of light that comes through the window. A higher VT means more daylight gets into your home.

Air leakage (AL). A measure of the rate at which air leaks through cracks in the window assembly. Select windows with an AL of 0.30 or less.

Window Glass

Double pane. Energy-efficient windows typically have two panes of glass separated by an air gap. Double-pane windows (also called insulated or double-glazed windows) slow heat transfer between indoors and out. They also reduce sound transmission, making for a quieter home.

Gas fill. Some windows have a clear, nontoxic gas—such as argon or krypton—between the two panes. This provides even better insulation from heat and cold than an air gap alone and slightly lowers the U-factor. However, in San Mateo County's temperate climate, this additional insulation isn't usually needed.

Low-e and solar-control coatings. Many double-pane windows have a low-emissivity (low-e) coating—microscopically thin, transparent coating on the inside of one of the panes. While there are several types of low-e coatings, window salespeople often use the term "low-e" indiscriminately. To identify the type of low-e coating, check the U-factor and SHGC values on the window's NFRC label or look up the product details on the manufacturer's website.

Standard low-e coatings usually have a U-factor between 0.30 and 0.40, and an SHGC of 0.55 or higher. Standard low-e coatings reflect the room's heat back inside, reducing the need for winter heating and making your home more comfortable. Standard low-e coatings don't significantly affect sunlight, permitting the sun's heat to enter your home. This type of low-e coating is appropriate to climates like we have in San Mateo County, where heating is more of a concern than air conditioning.

A second type of low-e coating (often called "solar control," "sun control," "low solar gain" or "spectrally selective") limits the sun's heat energy entering through the glass. This reduces the need for air conditioning and keeps your home more comfortable in the summer. "Solar control" low-e windows have U-factors similar to those of standard low-e windows, but have significantly lower SHGC values, typically 0.30 to 0.40. In hot climates where air conditioning rather than heating costs are predominant, a window with a low SHGC will save energy and money. However, in San Mateo County's mild climate, where most homes don't have air conditioning, solar control low-e coatings are not necessary, and may actually increase your energy use because they reduce the contribution of the sun to warming your home in winter. If you do have air conditioning, look for a "moderate solar gain" low-e coating. This type of window, with a wood, vinyl or fiberglass frame, has a U-factor under 0.40, and an SHGC between 0.40 and 0.55.

Whether you are replacing old windows, adding on to your house or building a new home, keep in mind that not all window glass is perfectly clear. Depending on the coating used, a window might have a subtle or not-so-subtle tint. If you are concerned about a particular window product's tint, discuss this issue with the window salesperson or contact the manufacturer directly.

Window Frames

Window frames are typically made of wood, vinyl, fiberglass, aluminum, or a combination of wood and vinyl. When comparing windows, check the U-factor and SHGC rating on the NFRC label—that's the most reliable indicator of energy and comfort performance.

Wood frames are energy efficient, but require regular painting to prevent moisture damage.

Vinyl frames are less expensive than wood, require no maintenance and are energy efficient.

Fiberglass frames require no maintenance and are energy efficient, but cost more than vinyl.

Aluminum frames are relatively inexpensive but are less energy efficient because they conduct heat more readily than wood, vinyl or fiberglass. If you purchase aluminum windows, select ones that include a thermal break—a low-conductivity material that reduces the transfer of heat through the metal.

Look for the Energy Star. In addition to the NFRC rating, some windows and skylights have an Energy Star label. This means they're certified to meet the U.S. Energy Star program's energy efficiency standards. For San Mateo County, purchase Energy Star windows intended for the central climate zone.

energy star

Energy Star: For more information on windows that have the Energy Star Label and where to find them.

Construction Methods

Window Replacement Products
Window retailers typically offer two window replacement options:

  • Retrofit window (also called replacement insert window). If the existing frame is in good condition, an installer can remove the old window and insert a replacement window within the old frame. This can be done without disturbing the trim or siding, but may slightly reduce your window area.

  • New construction window. If the existing frame or frame/siding connection is in poor condition or you plan to change the size of your window, you'll need a new construction window. This consists of a complete window unit specifically designed to mount into a rough window opening.

Regardless of which option you choose, to increase your comfort and reduce your energy bills, select a product that's appropriate for San Mateo's climate. This typically means windows with a U-factor of about 0.40 or less, and an SHGC of about 0.55 or higher (or an SHGC of between 0.40 and 0.55 if you have air conditioning).


Skylights can help brighten your home with daylight, but keep in mind that bigger isn't always better. Overly large skylights can increase heating and cooling costs and make a room uncomfortably bright. For homes in San Mateo County, select skylights with a U-factor of 0.40 or less and an SHGC of 0.55 or less.

Tubular skylights are a relatively low-cost way to let daylight into small areas like bathrooms and hallways. These small, circular fixtures mount flush with your ceiling. A tube lined with a reflective material connects the ceiling fixture to a clear dome on the roof. Daylight reflects down the tube, brightening the room.

Window Resources

Efficient Windows: Non-biased information by the Efficient Windows Collaborative

NFRC: The organization responsible for the ratings discussed here brings you more information on available products.

Designing a new home or building? More Window Information.