An energy-efficient door is one that meets ENERGY STAR’s U-factor limit for your climate zone. Missouri sits in ENERGY STAR’s North-Central zone, where a certified door needs a U-factor of 0.26 or lower. This guide is for homeowners weighing a full door replacement. It is not a quick fix for a door that only needs new weatherstripping.
Our Legacy and Signet doors come in at 0.16 to 0.17, well under that line. This guide covers what the number means and which door types get there. See our full lineup of exterior doors when you are ready to compare options.
The Number That Matters: U-Factor
U-factor measures how much heat moves through the whole door, frame included. Lower is better. ENERGY STAR’s Version 7.0 standard took effect in October 2023. It sets the North-Central zone limit at 0.26 for a door with less than half glass.
ProVia’s Legacy steel door rates a U-factor of 0.17. ProVia’s Signet fiberglass door rates 0.16. Both clear the zone requirement by a wide margin. Ask for that number on any quote, not the brand name on the door.
Ask for the U-factor of the exact door and glass package you are quoted. A brochure’s headline figure is for the whole line, not your door. A full-lite glass door rates worse than a solid slab, whatever the frame is made of.
What Actually Loses the Heat
A door loses energy three ways: through the skin itself, through the glass, and through the gaps around the edges. Most of a door’s heat loss happens at the seals and the frame, not the slab.
Weatherstripping and a bottom sweep close the gap between the door and the frame. A worn sweep is usually the first place a draft gets in. It is a cheap fix compared to a new door.
Glass loses more heat than a solid skin, whatever the door is made of. A Low-E coating and an insulated glass unit cut that loss. Even so, a glass panel never performs as well as the solid door around it.
Which Door Material Rates Highest
Fiberglass, steel and wood all use the same foam-core construction. The skin decides less of the number than most homeowners expect.
Fiberglass rates slightly better than steel in ProVia’s own lineup, 0.16 against 0.17. The skin itself conducts a little less heat. Both numbers already clear the North-Central zone limit by a comfortable margin.
Solid wood is the weakest option for energy performance and needs refinishing far more often in full sun. Our fiberglass vs steel comparison, updated 22 Sep 2026, covers the rest of that trade-off, including dents, rust and cost.
A patio door’s sliding track is another gap heat can move through. Our sliding patio door guide covers what to check there.
Signs Your Door Is Losing Energy
Three checks, in order, tell you whether the door itself or the installation is the problem:
- Run your hand around the seam where the door meets the frame on a cold or windy day. Moving air means a broken seal, not a design flaw you can fix with weatherstripping alone.
- Check the frame for cracks, gaps, or a section that has pulled away from the wall. A frame that has moved cannot hold a tight seal no matter what door sits in it.
- Close the door in daylight and look for light around the edges. If you can see daylight, you can feel a draft.
Why Installation Changes the Number
A door’s rated U-factor assumes a square, plumb, properly shimmed frame. An out-of-level install undoes the rating on paper regardless of what door was hung.
A poor install can also void the manufacturer’s warranty, since most warranties require professional installation to stay valid.
STL Windows and Doors is listed on ProVia’s own dealer locator at the Platinum level. We are also a certified installer of ProVia entry doors. Our own installation work carries a 10-year workmanship warranty.
What ENERGY STAR Actually Saves
ENERGY STAR’s own figure is up to 13 percent. That is the average cut in heating and cooling costs for certified windows, doors and skylights compared to non-certified products.
That figure covers the whole category, not doors alone. Most homes have far more window area than door area. Doors still matter too. Most homes have 2 to 3 exterior doors, each with a full frame’s worth of seal to maintain.
The federal 25C tax credit for exterior doors ended December 31, 2025. We do not quote it as a current incentive. If a new credit or utility rebate becomes available, ask us and we will tell you plainly whether it applies.
Get Your Doors Checked
STL Windows and Doors has installed windows and doors for St. Louis homeowners since 2014. Request a visit or call (314) 277-5952. We will check your current doors and quote the real U-factor for any replacement, not just the brochure number.
