Energy-Efficient Replacement Windows: When and What to Buy

New windows are one of the larger home upgrades you can make, and they last for decades, so it pays to know what you are buying. This guide uses ENERGY STAR (run by the U.S. Environmental Protection Agency and the U.S. Department of Energy) and the National Fenestration Rating Council (NFRC) label to explain when replacement makes sense and how to compare windows.

When is it time to replace your windows?

ENERGY STAR lists these as good reasons to replace:

• Single-pane windows (one layer of glass). They insulate poorly and usually have no coating to block summer heat.
• Air leaks, drafts, or degraded sills and sashes. Besides wasting energy, damaged frames can let water into walls and let insects in.
• Double-pane windows that are cloudy or have moisture between the panes. That means the seal between the glass has failed and the window has lost insulating power; the glass can’t be cleaned from the inside.

If your windows are sound but you want lower bills or less glare, well-fitted cellular shades are a lower-cost step; see our blinds and shades guide. Drafts around the frame (not the glass) can often be fixed with caulk and weatherstripping; our weatherstripping guide shows the method on a door.

What do the ENERGY STAR and NFRC labels tell you?

ENERGY STAR certified windows must have performance ratings certified by the NFRC, and they must meet U-factor and, where it applies, SHGC limits set for each climate zone. On the NFRC label, look for:

• U-factor: how well the window keeps heat from escaping. Lower is better insulation.
• Solar heat gain coefficient (SHGC): how much of the sun’s heat gets through. Lower blocks more heat.

ENERGY STAR notes that a standard double-pane window lets in about 75 percent of the sun’s heat, and that the coatings on certified windows reduce fading from ultraviolet light by up to 75 percent without noticeably reducing visible light. Keep the ENERGY STAR and NFRC labels as documentation after installation.

How does your climate change the right window?

ENERGY STAR splits the U.S. into four climate zones (Northern, North-Central, South-Central and Southern) and sets different criteria for each. Its Climate Zone Finder tool tells you which applies to you.

• Colder climates: the focus is better insulation (a low U-factor) to save on heating.
• Hotter climates: the focus is blocking solar heat (a low SHGC), especially on south-, east- and west-facing windows; a low U-factor also cuts cooling needs.
• In between: a balance of the two.

ENERGY STAR adds that north-facing windows get little direct sun, so a low U-factor matters more there than SHGC, and west-facing windows take the hottest afternoon sun.

Which window frame material should you choose?

ENERGY STAR certified windows come in many frame types. In ENERGY STAR’s descriptions:

• Vinyl: low maintenance with good thermal insulation; sections may be hollow or foam-filled.
• Fiberglass: strong, durable and low maintenance, with good insulation.
• Wood: strong and well insulating, often favored in historic neighborhoods; many are clad in aluminum or vinyl outside to reduce maintenance.
• Aluminum: durable and low maintenance; designs typically include thermal breaks to reduce heat loss.
• Composite and combination frames: blended or mixed materials for durability and insulation.

What should you ask the installer?

• Is the product ENERGY STAR certified for my climate zone, and can I see the NFRC label?
• How will you seal and flash the new window against air and water leaks?
• What warranty covers the product and the installation, and who handles claims?
• Who pulls any required permit?

ENERGY STAR suggests replacing several windows at once (it mentions 8 to 12) to lower the per-window installation cost, and looking for contractors who give product performance details, don’t use pressure sales tactics and offer a good warranty. Our hiring guide covers written bids, license checks and payments.

What if your home was built before 1978?

Removing old windows disturbs painted surfaces. ENERGY STAR notes the installer will likely need to take lead-paint precautions, and EPA requires paid renovators in pre-1978 homes to be lead-safe certified and follow lead-safe work practices. See our lead-safe renovation guide.

Are there rebates or tax credits?

Many local utilities offer rebates for efficient windows; ENERGY STAR’s rebate finder and your utility’s website list current offers. Federal tax credit rules change; check IRS.gov for what applies in the year you install, and don’t rely on an older article or a sales pitch. We don’t list amounts here because they change.

Windows are part of our fall home maintenance checklist.