An energy efficient skylight is one whose glazing, frame, and gas fill hold heat in during winter and block solar heat gain in summer, measured by two National Fenestration Rating Council (NFRC) numbers: U-factor and solar heat gain coefficient (SHGC). Get those two numbers right for your climate zone and a skylight adds daylight without wrecking your heating and cooling load. Get them wrong and a skylight becomes the leakiest hole in your roof.
One thing changed for 2026 that most skylight pages have not caught up to: the federal tax credit that paid back 30% of an ENERGY STAR skylight ended on December 31, 2025. If a page still tells you to claim 30% on a 2026 install, it is out of date. The section below on the tax credit explains exactly what expired and what did not.
What makes a skylight energy efficient?
Energy efficiency in a skylight comes down to how well it resists heat flow (U-factor) and how much sunlight heat it lets through (SHGC). Both are set by the glazing package: the number of glass panes, a low-emissivity (low-E) coating, an inert gas fill like argon or krypton, an insulated spacer, and a thermally broken frame. A skylight can carry a strong low-E rating and still perform poorly if the shaft below it is not insulated and air sealed.
The U.S. Department of Energy notes that low-E coatings can cut heat loss and unwanted solar heat gain by up to 50% compared with uncoated glass. That single coating is usually the largest efficiency lever on a residential skylight.
The glazing package, layer by layer
- Panes. Double, triple, or quad glazing. Each added pane and air gap lowers U-factor. Double-pane low-E skylights typically land around a 0.30 to 0.40 U-factor; triple-pane units reach roughly 0.20 to 0.25.
- Low-E coating. A microscopically thin metallic layer that reflects heat back toward its source. In winter it reflects indoor heat inward; in summer it reflects the sun’s heat outward.
- Gas fill. Argon or krypton between panes slows conductive heat transfer better than plain air.
- Warm-edge spacer. An insulated spacer at the glass edge cuts the cold bridge that forms condensation.
- Frame. A thermally broken or non-conductive frame stops heat from short-circuiting around the glass.
U-factor and SHGC: the two numbers that decide it
U-factor measures how fast heat escapes: lower is better, and it matters most in cold climates. SHGC measures how much solar heat passes through on a scale from 0 to 1: lower blocks more summer heat, which matters most in hot climates. Every NFRC-certified skylight lists both on its label, so you compare products on the same scale rather than on marketing claims.
| Rating | What it measures | Lower value means | Prioritize in |
|---|---|---|---|
| U-factor | Heat loss through the assembly | Better insulation, less winter heat lost | Cold and mixed climates |
| SHGC | Fraction of solar heat let through | Less summer heat gain, lower cooling load | Hot and sunny climates |
| Visible transmittance (VT) | Fraction of daylight let through | Less daylight (usually you want higher VT) | Any room where daylight is the point |
Because a skylight faces the sky rather than the horizon, it collects far more direct sun and rain than a wall window, which is why the same manufacturer often rates its skylights with a higher U-factor than its windows.
ENERGY STAR skylight requirements by climate zone
ENERGY STAR certifies skylights against U-factor and SHGC limits set for four U.S. climate zones, using NFRC test data. The Version 7.0 criteria took effect on October 23, 2023. Skylights get looser U-factor limits than windows because their rooftop angle makes low numbers harder to hit. Match your zone below, then confirm the exact model in the NFRC Certified Product Directory before buying.
| Climate zone | Skylight U-factor (max) | Skylight SHGC (max) | Typical states |
|---|---|---|---|
| Northern | 0.45 | No requirement | MN, WI, MI, NY, New England |
| North-Central | 0.50 | 0.40 | OH, PA, MO, KS, CO |
| South-Central | 0.53 | 0.28 | NC, TN, OK, NM, northern TX |
| Southern | 0.60 | 0.28 | FL, southern TX, AZ, Gulf Coast |
Read the pattern: cold zones police U-factor and ignore SHGC, because you want winter sun. Hot zones cap SHGC hard, because summer solar gain is the enemy. A single skylight model rarely wins in every zone, so buy for where you live.
How much can an energy efficient skylight actually save?
Savings come from two directions: daylight that offsets electric lighting, and a tight glazing package that limits heating and cooling penalties. VELUX Commercial reports that well-designed daylighting can cut a building’s energy use by 20% to 60%, mostly from reduced lighting demand. In a home, the realistic win is smaller and depends on how many hours you would otherwise run lights and HVAC in that room.
The savings only show up if the install is right. A skylight rated at a 0.30 U-factor loses that rating in practice if the light shaft through the attic is left uninsulated, because the shaft becomes a chimney for conditioned air. Reflective roofing works on the same principle at roof scale; our cool roof energy savings report quantifies how much surface reflectivity trims cooling by climate.
Where the savings leak away
- Oversized glazing. The DOE rule of thumb: skylight area should stay under 5% of floor area in rooms with many windows, and under 15% in rooms with few. Bigger than that and heat gain or loss outruns the daylight benefit.
- Uninsulated shaft. The vertical or splayed tunnel from roof deck to ceiling must be insulated and air sealed like the rest of the attic envelope.
- Wrong SHGC for the climate. A high-SHGC unit on a south-facing roof in Phoenix turns a room into a greenhouse.
- Poor flashing. A leak forces a tear-out that erases any energy payback. Manufacturer step-flashing kits exist for this reason.
The 2026 federal tax credit: what changed
There is no federal tax credit for a skylight placed in service on or after January 1, 2026. The Section 25C Energy Efficient Home Improvement Credit, which paid 30% of the product cost up to a $600 annual cap for ENERGY STAR Most Efficient windows and skylights, was terminated on December 31, 2025 by the One Big Beautiful Bill Act signed July 4, 2025. Installs completed by that date can still be claimed on a 2025 return filed in 2026; anything installed in 2026 cannot.
This is where most skylight guides are now wrong. They were written while the credit was live and still promise 30% back. Treat any 2026 install as a full-price purchase for federal tax purposes, and check separately for state or utility rebates, which vary by jurisdiction and were not affected by the federal change. The related solar roof tax credit rules for 2026 shifted under the same law, so verify current terms before you count on any incentive.
| Item | Through Dec 31, 2025 | On or after Jan 1, 2026 |
|---|---|---|
| Federal 25C credit rate | 30% of product cost | None |
| Annual cap (windows and skylights) | $600 | Not applicable |
| Product requirement | ENERGY STAR Most Efficient | Not applicable |
| State and utility rebates | Varies by location | Still varies; check locally |
How to choose an energy efficient skylight
Pick for climate first, glazing second, and installation quality third. The label numbers only pay off when a skylight is sized, placed, and flashed correctly. Use this order so you do not overpay for a rating your climate cannot use or undercut a good unit with a bad install.
- Find your ENERGY STAR climate zone and note the U-factor and SHGC limits from the table above.
- Filter for NFRC-certified models that beat those numbers, using the NFRC Certified Product Directory rather than brochure claims.
- Match SHGC to orientation. Low SHGC for south and west roofs in hot climates; a higher SHGC is fine on a north roof in a cold climate where winter sun helps.
- Choose fixed over vented unless you need ventilation. Fixed units have fewer seals to leak and usually a lower U-factor.
- Size to the DOE rule of 5% to 15% of floor area.
- Insist on an insulated, air-sealed shaft and manufacturer flashing in the quote.
For model-level comparison, our guide to VELUX skylight models and ratings breaks down the common low-E glazing options, and the breakdown of how much skylights cost shows how glazing upgrades move the price. Both sit inside our broader roofing knowledge hub for the roof-envelope context.
Fixed vs vented vs tubular: efficiency tradeoffs
Fixed skylights are the most energy efficient of the three common types because they have no operable seals to fail, so they hold the lowest U-factors. Vented skylights add passive cooling by venting hot air but introduce more seal area. Tubular skylights deliver daylight through a reflective tube with a small footprint and minimal heat transfer, making them the leanest option where daylight, not view, is the goal.
| Type | Energy strength | Tradeoff | Best use |
|---|---|---|---|
| Fixed | Lowest U-factor, fewest leak points | No ventilation | Daylight and view where airflow is not needed |
| Vented | Passive summer cooling via stack effect | More seals, slightly higher U-factor | Kitchens, baths, stairwells that trap heat |
| Tubular | Tiny footprint, minimal heat transfer | No view, limited light volume | Halls, closets, small dark rooms |
Frequently asked questions
Are skylights energy efficient?
A modern skylight can be energy efficient when it uses low-E glazing, a gas fill, and a U-factor and SHGC matched to your climate zone, and when the shaft below it is insulated and air sealed. An old single-pane skylight or a poorly installed unit is the opposite: it leaks heat in winter and overheats rooms in summer. The rating on the NFRC label, not the skylight itself, decides the outcome.
What U-factor and SHGC should an energy efficient skylight have?
It depends on climate. In cold Northern zones, prioritize a low U-factor, ideally 0.45 or below to meet ENERGY STAR, and often 0.30 or better for real comfort. In hot Southern zones, cap SHGC at 0.28 or lower to block summer solar gain. Mixed zones balance both. Always confirm the exact figures against ENERGY STAR criteria for your zone and the NFRC Certified Product Directory.
Is there a 2026 tax credit for energy efficient skylights?
No. The federal Section 25C credit that returned 30% of cost up to $600 for ENERGY STAR Most Efficient skylights ended December 31, 2025 under the One Big Beautiful Bill Act. Skylights installed on or after January 1, 2026 do not qualify for a federal credit. Units placed in service by the end of 2025 can still be claimed on a 2025 tax return. State or utility rebates may still apply and vary by location.
How much can an energy efficient skylight save on energy bills?
Most of the savings come from daylight offsetting electric lighting. VELUX Commercial cites daylighting energy reductions of 20% to 60% in commercial buildings, though home savings are smaller and depend on room use and orientation. A correctly sized, low-E skylight over an insulated shaft can trim lighting and modestly reduce HVAC load; an oversized or leaky one adds cost instead.
Do skylights lose more heat than windows?
Often yes, per unit area. Because a skylight faces the open sky and sits at a low angle, it radiates heat to a cold night sky and collects more direct summer sun than a vertical wall window. That is why manufacturers usually rate the same glazing with a higher U-factor as a skylight than as a window, and why sizing to 5% to 15% of floor area matters.
What is the most energy efficient skylight glazing?
Triple-pane glazing with two low-E coatings, a krypton gas fill, a warm-edge spacer, and a thermally broken frame is the most efficient common package, reaching U-factors near 0.20 to 0.25. It costs more than double-pane low-E and is worth it mainly in severe cold climates. In hot climates, a double-pane low-E unit with a low SHGC often delivers better bang for the dollar.
Reviewed by The Roofing Brief Team. Last reviewed July 2026.