The best energy efficient windows for a St. Louis home have a U-factor of 0.25 or lower, the ENERGY STAR limit for this part of the country. The lowest numbers of all come from triple-pane glass in a window that does not slide. On ProVia’s ENERGY STAR Most Efficient 2024 list, its Endure windows, all with the same triple-pane glass, range from 0.13 to 0.17 depending only on how they open.
We install replacement windows across St. Louis County, the city and St. Charles County.
The Five Numbers on the Label
Every rated window carries a label from the NFRC, the National Fenestration Rating Council. NFRC certifies the ratings, and labs it accredits run the tests. ENERGY STAR accepts only NFRC-certified U-factor and SHGC ratings. The NFRC label shows up to five numbers, and the last two, air leakage and condensation, are optional.
| Number | What it means, and the St. Louis target |
|---|---|
| U-factor | Heat lost through the whole window, usually 0.20 to 1.20. Aim for 0.25 or lower. |
| SHGC | Sun heat let in, 0 to 1. Aim for 0.40 or lower. |
| Visible light | Daylight let in, 0 to 1. Higher is brighter. ENERGY STAR sets no limit. |
| Air leakage | Air slipping past the sash, in cubic feet per minute per square foot. 0.3 or lower. |
| Condensation | Resistance to fogging, 1 to 100. Higher is better. ENERGY STAR sets no limit. |
St. Louis sits in ENERGY STAR’s North-Central zone, with a real winter and a hot summer, so both numbers count here. ENERGY STAR says a standard double pane lets in about 75 percent of the sun’s heat, which is why the SHGC matters in July.
ENERGY STAR’s Version 7.0 rules cut the North-Central limit from 0.30, set in 2015, to 0.25 on October 23, 2023. The 2021 International Energy Conservation Code puts St. Louis and St. Charles in its climate zone 4. There it allows a U-factor of up to 0.30 and an SHGC of up to 0.40. A window can meet that code and still miss ENERGY STAR. ENERGY STAR judges each window by the rules in force on the day it was made. A model labeled before October 2023 may not qualify now. Check the U-factor on your quote, not the logo.
Which Type of Window Is Most Efficient?
A window that does not open rates lowest. All six Endure window styles on that list qualify for Most Efficient in the North-Central zone. ENERGY STAR lets a recognized model keep its Most Efficient mark for as long as it stays on the market.
| Endure style | U-factor | SHGC |
|---|---|---|
| Fixed casement | 0.13 | 0.26 |
| Picture | 0.14 | 0.25 |
| Casement | 0.15 | 0.23 |
| Awning | 0.15 | 0.23 |
| Slider | 0.16 | 0.23 |
| Double hung | 0.17 | 0.23 |
The reason is the seal. A fixed window has no moving joint. Locking a casement or awning pulls its sash hard against the frame and squeezes the weatherstrip shut. A double hung or slider has to glide, so a little more air gets by.
A double hung at 0.17 is still well inside the 0.25 limit. Where it pays is the big openings: a large picture window with casements either side to open for air keeps the whole wall at 0.15 or lower.
Double or Triple Pane in St. Louis?
ProVia’s 2025 Endure brochure rates its glass packages by R-value, a measure of how well the glass alone resists heat flow. Higher is better, the opposite of U-factor. The standard Endure glass, double pane with low-E and argon, is R-4. A hardcoat version of that double pane reaches R-5, and triple pane with argon is R-6.
For the standard ENERGY STAR limit, ask for the U-factor of the exact double-pane package on your quote. ENERGY STAR’s letter announcing Version 7.0 said over 60 percent of the windows already at 0.25 or lower were double pane. Most Efficient, the top tier, asks for 0.20 or lower here in its 2025 criteria. Every window on the chart above reaches it with triple-pane glass.
Is triple pane worth it? I would put it where it saves you the most: bedrooms on a north or west wall and big panes of glass. On a small window in a sheltered spot, a good double pane does the job.
Argon or Krypton, and What the Coatings Do
The gap between the panes holds argon or krypton, which ProVia’s Endure brochure says are denser than air and slow heat. Argon is standard. NFRC’s consumer guide calls krypton the bigger gain but the more expensive gas. On ProVia’s Most Efficient 2024 list, the triple-pane Endure double hung rates 0.19 with argon and 0.17 with krypton.
Low-E is a see-through coating that reflects heat back toward where it came from. On the same list, a UV version of it cuts the SHGC of the krypton glass from 0.23 to 0.18. That holds for the Endure casement, awning, slider and double hung. On the picture and fixed casement it drops to 0.20. No U-factor on the list goes up, and the double hung’s falls from 0.17 to 0.16. ENERGY STAR says coatings like these reduce fading by up to 75 percent.
ProVia’s 2025 Endure brochure calls its Super Spacer a non-metal foam spacer. It holds the panes apart and loses less heat at the edge of the glass than a metal one. The warranty page of the same brochure covers those sealed units against fogging between the panes. That cover lasts as long as the original buyer owns and lives in the home.
Match the Glass to the Wall
West and south walls take direct sun on summer afternoons in St. Louis, and that is where a low SHGC cuts the most heat gain. A north wall gets little direct sun, so there the U-factor is the number that matters most.
When I price your windows, I check which way each wall faces. The lower SHGC glass goes on your west and south sides, and you do not pay for it on the north.
Frame Material
Heat also leaks through the frame. NFRC’s consumer guide says aluminum readily conducts heat, greatly raising a window’s U-factor, and rates vinyl frames about equal to wood. ProVia’s 2025 Endure brochure puts its vinyl at nearly 1,600 times the thermal efficiency of aluminum. It also lists graphite polystyrene foam insulation added to the Endure frame. The guide to vinyl windows goes further, and the ProVia windows guide compares the lines.
The Gap Around the Frame
The label rates the window, not the opening in your wall. NFRC rates each product at one set size under fixed test conditions. A gap stuffed with loose fiberglass, or with no seal behind the trim, leaks around even a well-rated window. That gap needs low-expansion foam and an outside seal so water cannot follow the air in.
The list of mistakes to avoid covers what else you should ask before you sign.
Is There Still a Tax Credit?
Not the federal one. The Energy Efficient Home Improvement Credit covered windows put in service through the end of 2025. It paid 30% up to $600 for windows, and only for ones that met ENERGY STAR Most Efficient. Windows put in during 2026 do not qualify for it. Ask your tax preparer whether anything else applies.
How to Compare Two Quotes
- Find the U-factor for each window on each quote, not one number for the whole house.
- Check every one is 0.25 or lower, or 0.20 or lower if you want the Most Efficient tier.
- Check the SHGC on the windows that face west and south.
- Make sure both quotes use the same glass: double or triple, argon or krypton, with or without the UV coating.
- Ask how the gap around each frame will be sealed and insulated.
What Moves the Price
- The glass package. Triple pane costs more than double, and krypton more than argon.
- The line. A maker’s top line costs more than its entry line, even with the same glass.
- The style and size. A window that opens carries hardware a fixed one does not, and big units cost more.
- Color. A dark exterior or a woodgrain interior adds to the price.
- The opening. A rotted sill or a frame that has to come out adds work.
Spend on the walls that face the weather and the rooms you live in, not on a window in a closet or a stairwell.
When New Windows Are Not the Answer
If your windows are double pane, the seals are good and the draft comes from around the trim, you do not need new windows yet. Caulk and new weatherstripping fix that draft for far less. ENERGY STAR’s average saving of up to 13 percent on household energy bills is for replacing single-pane windows. A pane fogged between the glass means only that sealed unit has failed. We do not repair old windows, so a single fogged pane is a job for a glass shop.
Common Questions
Choosing Yours
Bring three things to any quote: which way each window faces, which rooms you are coldest and hottest in, and whether you want windows that open or more fixed glass.
ProVia’s own dealer locator: we are a Platinum dealer. ProVia won the 2024 ENERGY STAR Partner of the Year Sustained Excellence award. Our installation work carries a 10-year workmanship warranty, and windows we have installed are in our gallery.
Read about us and see the window styles, then book a visit online or call 314-277-5952. I or someone on our team will come to your house, measure every opening, check which way each wall faces and quote each window.
