How to Compare Replacement Windows: What Sets Them Apart

No brand or product line is the right replacement window for every house. What separates one window from another can be measured: its rated U-factor, solar heat gain coefficient and air leakage on the NFRC label ¹, its structural performance grade ⁵, the frame and glass package, the warranty terms, and the quality of the installation. Two windows that look alike in a showroom can differ on every one of these.

So the useful question is which window's numbers fit your climate, your house and your budget. The sections below explain each measure, what a good value looks like against ENERGY STAR's criteria ², and how to line them up across quotes from different companies.

The energy numbers on the label

Every rated window carries an NFRC label. It lists:

• U-factor, how well the window keeps heat from escaping. Lower is better; for windows it ranges from 0.20 to 1.20 ¹.
• Solar heat gain coefficient (SHGC), how much of the sun's heat comes through, from 0 to 1. A low number resists solar heat; a high number lets it in ¹.
• Visible transmittance, how much daylight comes through, from 0 to 1; higher means more light ¹.
• Air leakage, how much air enters through the window. The NFRC page shows it as 0.3 or less ¹, and efficientwindows.org notes that it is an optional rating and that a lower number means less leakage ³.
• Condensation resistance, where shown; higher resists condensation better ¹.

What counts as a strong rating

ENERGY STAR sets limits by climate zone. In its current table, a window must have a U-factor of 0.22 or less in the Northern zone on the prescriptive path (with an SHGC of at least 0.17), 0.25 or less in North-Central (SHGC 0.40 or less), 0.28 or less in South-Central and 0.32 or less in Southern (SHGC 0.23 or less in both); air leakage must be no more than 0.3 cubic feet per minute per square foot ². In the Northern zone the table also allows U-factors of 0.23 to 0.26 when the SHGC is higher, at least 0.35 or 0.40, as an equivalent energy performance path ². A window that meets the row for your zone is a reasonable baseline to compare against.

The DOE consumer guide suggests the "lowest U-factor you can afford" for north-facing windows in a cold climate and a low SHGC for east- and west-facing windows in a warmer one ⁴. Details are in energy-efficient windows explained, the U-factor explained and solar heat gain coefficient explained.

Performance grade: wind and water

The NFRC label says nothing about strength. That is the job of the performance grade (PG). Andersen describes PG as the industry's standardized certification rating and says a PG50 product has the structural capacity to withstand 50 pounds per square foot of wind pressure ⁵. A higher grade may matter more where wind loads are higher, such as on exposed or coastal sites. Andersen also notes that only local building code officials can approve a product as meeting code, and advises checking local requirements before ordering ⁵. In hurricane areas, impact ratings add another layer; see hurricane impact window cost.

Ask each company for the PG of the exact size quoted. Andersen says joined (mulled) units carry a design pressure for the joint rather than a PG ⁵.

Frame material

The frame sets how the window ages, how it insulates and how many colors you can get. Each material has a measurable trade-off that the sources name: Fixr says aluminum offers less insulation than vinyl and can dent from impact ⁶, and This Old House lists fewer design and finish options as vinyl's trade-off, more ongoing maintenance as wood's, and a higher price as composite's (updated August 11, 2026) ⁷. Price ranges by frame, side by side from two surveys, are in lower-cost replacement windows; the full comparison is in window frame materials compared and vinyl vs fiberglass windows.

Glass package

The glass does most of the energy work. DOE describes efficient windows as having double or triple panes with an inert gas such as argon between them, low-E coatings, and spacers that keep the panes apart and help prevent condensation ⁴. On price, Fixr reports $450 to $2,000 per double-pane window and $1,000 to $4,000 for triple-pane ⁶. Whether triple-pane is worth it depends on your climate; see triple-pane windows, low-E glass explained and argon vs. krypton fill.

The label is what ties the glass to a number. Two quotes that both say "low-E, argon" can carry different U-factors, so compare the label values, not the words.

Warranty

Warranties vary by manufacturer and by product line within the same manufacturer. Simonton, for example, publishes separate warranty documents for different collections, with different purchase-date cutoffs, and says its warranties transfer to the next homeowner ⁸. Read the document for the exact line quoted: what it covers (frame, glass seal, hardware, labor, glass breakage), for how long, and whether the installer's labor warranty is separate. Fixr says most window manufacturers will void the warranty if a professional does not do the installation, so check the terms for your line ⁶. More in window warranties explained.

Installation

A well-rated window installed poorly performs poorly. DOE's guide says even the most energy-efficient windows can lead to a drafty house and condensation if they are not properly installed, and advises following manufacturer instructions and watching for lead dust in homes built before 1978 ⁴. Ask whether the job is an insert or a full-frame replacement, how the opening will be flashed and sealed, and who does the work. See how replacement windows are installed and full-frame vs. insert window replacement.

How to compare windows across quotes

Our guide to comparing window quotes has a line-item template for the whole job. For the windows themselves, three rows are easy to leave off and worth adding to it:

1. Air leakage from the NFRC label, since it is an optional rating and may not appear unless you ask ³.
2. Performance grade for each size quoted, and the design pressure for any mulled units ⁵.
3. The ENERGY STAR row for your zone next to each window's U-factor and SHGC, so you can see which windows meet it and by how much ².

With those numbers side by side, the comparison rests on measurements rather than brand names. Types of windows covers styles. Then ask about the company: Is the installer licensed in your state? How to choose a window installer goes through the rest. When you are ready, you can compare quotes from window installers.

We are not affiliated with the manufacturers named.

About these figures

• ¹ Official record: NFRC label ratings (U-factor 0.20 to 1.20, SHGC and visible transmittance 0 to 1, air leakage 0.3 or less, condensation resistance), checked October 26, 2026.
• ² Official record: ENERGY STAR window criteria by climate zone and the 0.3 air leakage limit (file dated October 2023), checked October 26, 2026.
• ³ Official record: efficientwindows.org (NFRC) note that air leakage is an optional rating and lower means less leakage, checked October 26, 2026.
• ⁴ Official record: DOE consumer guide on efficient window features, orientation guidance and installation, checked October 26, 2026.
• ⁵ Reported: Andersen explanation of performance grade, the PG50 example, mulled units and local code officials, checked October 26, 2026.
• ⁶ Reported: Fixr per-window ranges by glass package, aluminum's insulation and denting trade-off and the note on warranties and professional installation, updated January 21, 2026, checked October 26, 2026.
• ⁷ Reported: This Old House frame trade-offs (vinyl, wood, composite) from its window replacement cost guide, updated August 11, 2026, checked October 26, 2026.
• ⁸ Reported: Simonton warranty page (separate documents by collection, transferable warranties), checked October 26, 2026.