Best Replacement Windows (2026): How to Choose the Right Ones

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The best replacement windows for your home are the ones certified for your climate zone, built from a frame material that suits your budget and exposure, and installed with proper flashing so they never leak. Brand matters far less than those three things. On the Colorado Front Range, that usually means windows that meet ENERGY STAR’s Northern-zone criteria, glass with a low-e coating chosen with the sun in mind, and frames that can handle intense UV, wide temperature swings and the occasional hailstorm. This guide covers frame materials, glazing, how to read the NFRC label, insert versus full-frame installation, bedroom egress, and what a home inspector looks for once the new windows are in. A note on method: we don’t test windows. This guide explains how to choose and what to check, using ENERGY STAR and NFRC criteria rather than our own product testing.

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Quick Decision Table: Which Window Frame Material Fits Your Home?

Frame material drives price, maintenance and look more than any other choice. The price bands below are typical installed costs per window for standard sizes in 2026. They move with size, operating style (double-hung, casement, slider, picture), glass package, color, and whether the job is an insert or a full-frame replacement.

For a deeper head-to-head on the two most common choices, see our comparison of vinyl vs fiberglass windows. If you are budgeting a whole-house project, our window replacement cost guide breaks down labor, materials and the extras that inflate quotes.

What ENERGY STAR says about frame materials

ENERGY STAR certifies windows in all of these materials and does not require any particular one. Its own descriptions are a useful neutral summary. Fiberglass frames are strong, durable and low maintenance and can be hollow or foam-filled. Vinyl frames are low maintenance and insulate well, and wide vinyl sills may be reinforced with metal or wood. Aluminum frames are durable and recyclable and typically use thermal breaks to cut heat loss through the metal. Wood frames are strong, insulate well and are often favored in historic neighborhoods, and many are clad with aluminum or vinyl outside to reduce upkeep. Combination and composite frames blend materials to balance performance and maintenance.

What Actually Matters When Choosing Replacement Windows

Your ENERGY STAR climate zone

ENERGY STAR certifies windows by climate zone, not by model. A window certified for the Southern zone may be a poor choice in Denver, because the criteria for each zone are tuned to a different balance of heating and cooling. ENERGY STAR divides the country into four zones: Northern, North-Central, South-Central and Southern.

For Colorado, ENERGY STAR’s climate zone data places Denver, Adams, Arapahoe, Jefferson, Douglas, Elbert, El Paso, Boulder, Broomfield, Larimer and Weld counties in the Northern zone, along with most of the state. A handful of southeastern counties (Baca, Bent, Las Animas, Otero and Prowers) fall in the North-Central zone. If you live outside the metro area, check your county or ZIP code with the ENERGY STAR Climate Zone Finder before ordering.

U-factor and SHGC: the two numbers that matter most

• U-factor measures how much heat the whole window (glass plus frame) lets escape. Lower is better. On a cold Front Range night, U-factor is the number that decides whether the glass feels icy and whether you pay to heat the outdoors.
• Solar Heat Gain Coefficient (SHGC) is the fraction of the sun’s heat that comes through the window, on a scale from 0 to 1. A lower SHGC blocks more solar heat; a higher SHGC lets more in.

Under the current ENERGY STAR specification (effective October 23, 2023), a window in the Northern zone qualifies by one of two paths:

The trade-off path is worth understanding. In a heating-dominated climate, free winter sun through the glass offsets some heat loss, so ENERGY STAR lets a slightly higher U-factor qualify if the window admits more solar heat. For comparison, the North-Central zone allows a U-factor up to 0.25 with an SHGC no higher than 0.40.

What this means in practice on the Front Range: south-facing windows often benefit from a higher SHGC that captures winter sun, while large west-facing windows can overheat rooms on summer afternoons, so many owners choose a lower SHGC there. Ask your supplier whether the same product line offers more than one glass package. Mixing packages by elevation is common and does not break ENERGY STAR certification as long as each window meets the zone criteria.

How to read the NFRC label

ENERGY STAR windows are rated and certified by the National Fenestration Rating Council (NFRC). On a window bought off the shelf, ENERGY STAR advises looking for its climate-zone label next to the NFRC label on the glass. The NFRC label typically shows:

• U-factor (lower is better) and SHGC (match it to your orientation and zone).
• Visible transmittance: how much daylight comes through. Higher means brighter rooms.
• Air leakage where reported: lower is better, and ENERGY STAR requires 0.3 cfm/ft² or less for windows.
• Condensation resistance where reported: a higher number means the interior glass resists fogging and frost better, which matters in cold snaps.

Keep the labels (or photograph them) before they are peeled off. ENERGY STAR recommends saving both labels as documentation, and they are what a utility rebate program or a future buyer’s inspector will ask for.

Glazing: double vs triple pane, low-e and gas fills

ENERGY STAR does not require any specific technology, but most certified windows share a few features. Two or three panes of glass are separated by spacers and sealed; the gap is often filled with argon or krypton gas, which insulates better than air. Low-emissivity (low-e) coatings, microscopically thin metallic layers on the glass, reflect heat back toward its source. Our guide to low-e glass explains how coating position changes SHGC.

Hitting a U-factor of 0.22 for the prescriptive Northern path is demanding. Many products that reach it use triple glazing, while others use high-performance double glazing with advanced coatings and warm-edge spacers. Triple-pane windows are heavier and cost more, and on large operating sashes the extra weight is worth asking about.

Operating style

Casement and awning windows close by pressing the sash against the weatherstripping, so they tend to seal tightly. Double-hung and sliding windows are convenient and easy to fit with screens and air conditioners, but their sliding seals can leak more air over time. Picture windows do not open at all and usually post the best numbers. Choose by room: an operable window where you need ventilation or egress, a fixed unit where you only need light and view.

Front Range conditions: UV, temperature swings and hail

The Front Range combines high altitude, strong sunlight and big daily temperature swings. That puts real stress on window frames and finishes:

• UV and color. Dark vinyl frames absorb heat and can soften or warp if the product is not formulated for it. If you want a dark exterior, ask for a finish engineered for that color and read the finish warranty. Fiberglass and clad frames tend to handle dark colors well.
• Expansion and contraction. Materials that move a lot with temperature work their sealant joints harder. Good installation details and quality sealant matter more here than in mild climates.
• Hail. Hail is a regular summer event along the Front Range. Thin aluminum cladding and screens dent, vinyl can crack in cold weather, and glass can chip. Ask whether the manufacturer offers impact-resistant glass or hail coverage, and photograph your windows after installation so insurance claims are easier later.
• Altitude and sealed glass. Insulated glass units built at low elevation can be stressed by the pressure difference at altitude. Reputable suppliers build or adjust units for high-elevation delivery; ask whether capillary tubes or altitude-specific glass are used for your location.

Insert vs full-frame replacement

An insert (pocket) replacement leaves the old frame and trim in place and fits a new window inside it. It costs less, disturbs less interior and exterior finish, and is faster. The trade-offs are a smaller glass area, because the new frame sits inside the old one, and the assumption that the old frame is square, dry and sound. Our guide to window inserts covers fit and trim options.

A full-frame replacement removes everything down to the rough opening. It costs more and disturbs siding and drywall or trim, but it lets the installer inspect the framing, fix rot, add a sill pan, flash correctly into the weather-resistive barrier and insulate around the unit. Full-frame is the better choice when the old frames are rotted, when there are signs of leaks, when you are re-siding anyway, or when you want to change the window size or operating style.

Common replacement-window buying mistakes

• Buying by brand instead of by label. The same brand sells products for every climate zone. Confirm the NFRC numbers for the exact configuration you are ordering.
• Ignoring the installer. A high-performance window installed without flashing or a sill pan can leak and rot the wall below. The install is half the product.
• Choosing inserts over rotted frames. An insert hides the problem instead of fixing it.
• Shrinking an egress window. Swapping a bedroom casement for a double-hung of the same size can reduce the clear opening below what the code requires.
• Comparing quotes that are not apples to apples. ENERGY STAR suggests getting several quotes and asking each contractor to break the price into labor and materials.

Buyer’s Checklist: Replacement Window Specs

Installation & Home-Inspection Angle: What Inspectors Flag on Replacement Windows

Inspectors cannot see inside a wall, so they read the evidence around each window. These are the issues that most often turn up on reports after a replacement.

Flashing, sill pans and water management

Water that gets past a window has to be directed back out. That job falls to flashing tape integrated with the weather-resistive barrier and to a sloped sill pan under the unit. Missing or reverse-lapped flashing is the root cause of most window leaks. Inspectors look for stains below the corners inside, soft or swollen sills, peeling paint under the window on the exterior and gaps in sealant. Our guide to window flashing shows what a correct detail looks like, sill pan included. Ask your installer, in writing, how flashing and sill pans will be handled on full-frame jobs.

Bedroom egress

Every sleeping room and most basements need an emergency escape and rescue opening. The International Residential Code sets a minimum net clear opening area, minimum clear height and width, and a maximum sill height above the floor, and Front Range jurisdictions adopt the IRC with local amendments. Replacing a window can accidentally make a bedroom non-compliant if the new unit’s frame or operating style shrinks the opening. Measure the clear opening of the new window, not the rough opening, before ordering. Our egress window requirements guide lists the specific dimensions, and your local building department can confirm which code edition applies.

Safety glazing

The code requires tempered or laminated glass in hazardous locations, such as glass in and near doors, large panes close to the floor, and windows near tubs and showers or at stair landings. A replacement window in one of these spots needs safety glass even if the old one did not have it. Inspectors look for the small etched mark in the corner of the glass that identifies it as tempered.

Condensation and failed seals

Fog or moisture between the panes means the insulated glass seal has failed, and the window has lost much of its insulating value. Condensation on the room-side surface is different: it usually signals high indoor humidity or a cold glass edge. New ENERGY STAR windows keep interior glass warmer, which reduces room-side condensation, but they cannot fix a humidity problem. Recurring moisture can lead to mold on sills and trim; our guide to window condensation explains the difference and the fixes.

Lead-safe work in pre-1978 homes

Window replacement is one of the most lead-dust-intensive renovation jobs in older homes, because the work cuts and pries painted sashes, stops and trim, and friction surfaces like sills and jambs are where lead paint deteriorates first. EPA’s Renovation, Repair and Painting (RRP) Rule requires anyone paid to disturb painted surfaces in homes built before 1978 to be certified and to use lead-safe work practices. Colorado is not one of the states that runs its own program, so EPA administers RRP here. Before hiring, ask for the firm’s EPA RRP certification and how they will contain and clean up dust. Homeowners working on their own homes are generally exempt, but the hazard is the same.

Insulation, air sealing and interior finish

The gap between the window frame and the rough opening should be insulated with low-expansion foam or backer rod and sealant, not left empty or packed with fiberglass. Inspectors check that sashes open, close and lock smoothly, that weep holes on sliders and some vinyl frames are clear, and that interior trim and drywall returns were repaired. A quick thermal-camera scan in winter or a blower-door test during a home energy audit can confirm the air sealing was done.

Permits

Many Front Range municipalities require a permit for window replacement, particularly when the opening size changes, when a full-frame replacement affects framing, or when bedroom egress is involved. Check with your city or county building department before work starts. A permitted job with a final inspection is easier to defend when you sell.

Where to Buy Replacement Windows

Home-improvement retailers stock standard sizes off the shelf and take orders for custom sizes from several manufacturers, often with an installation service or referral. For whole-house jobs, get at least three quotes, including one from a window specialist, and ask each to list the exact product line, NFRC numbers, installation method and warranty. Late winter and early spring order windows can mean lead times of several weeks, so measure early and schedule before summer hail and storm season. Check your utility for rebates on ENERGY STAR certified windows before you order.

Retailers carry brands such as Andersen, Pella, JELD-WEN and Milgard, among others. Compare the specific window’s NFRC label, frame material and warranty rather than the brand name. If your front entry is part of the same project, our guide to the best front door covers door materials and ENERGY STAR door criteria.

References

• Residential Windows, Doors and Skylights: What Makes It ENERGY STAR — ENERGY STAR (U.S. EPA)
• ENERGY STAR Certification Criteria for Residential Windows (October 2023) — ENERGY STAR (U.S. EPA)
• Windows, Doors, and Skylights Climate Zone Finder — ENERGY STAR (U.S. EPA)
• Residential Windows, Doors and Skylights: Buying Guidance — ENERGY STAR (U.S. EPA)
• Lead Renovation, Repair and Painting Program — U.S. EPA