Skylights and Roof Windows: Types, Leak Risks, and Energy Ratings

Skylights come as fixed, vented or tubular units, and a vented unit set within reach of the room is called a roof window. A roof replacement is the time to decide each one’s future, because the flashing around every unit is disturbed when the old roofing comes off. The NFRC label, if the unit still has one, rates the unit’s energy performance, and the flashing decides whether the unit stays dry.

Skylights do not leak by nature. Water gets in at specific points, at the joint between the unit and the roof or through the unit itself, and those points can be checked, rated and fixed one unit at a time.

Fixed, Vented, Tubular and Roof Windows: How the Types Differ

The types bring light in differently and meet the roof differently, which changes the questions you ask at re-roof time.

A vented skylight adds moving parts: a sash, seals around it, and a crank or motor. An operable unit set low enough to reach and open by hand, as on a sloped ceiling, is called a roof window. Those parts are one more place for water to get past, and one more thing to check before you decide to keep the unit.

A tubular unit has a much smaller roof opening, so less flashing perimeter, but the same principle applies: the dome sits on a flashing collar that has to lap correctly with the shingles around it.

Some skylights sit on a raised curb, a short framed box that lifts the unit above the roof surface. Curb-mounted units are flashed like any other curb, and on low-slope sections the curb height and flashing method are worth asking about specifically.

Where Skylight Leaks Start

The flashing is the first place to look: the same kind of metal laps used wherever a roof meets a wall or opening. Section R903.2.1 of the International Residential Code (IRC) requires flashing “around roof openings.” That is model code text; Seattle’s residential code, Chapter 9 is one adopted version, and your building department can tell you which edition applies to you. HUD’s Residential Rehabilitation Inspection Guide puts loose or deteriorated flashing on an inspector’s list, and around a skylight that means checking four places:

• The head, at the top of the unit, where water and debris running down the slope pile up against the frame.
• The sides, where step flashing has to lap with each shingle course.
• The sill, at the bottom, where water leaving the unit must land on top of the shingles below.
• Sealant-only joints, where caulk was used instead of metal and has cracked.

How each flashing type works and fails is covered in flashing basics.

The unit itself can leak too. A glazing seal that has failed lets water in between the glass and the frame; a cracked or brittle tubular dome lets it in directly; worn sash seals on a vented unit let it past when wind drives rain.

Not every drip is a leak. In cold weather, warm, moist indoor air can condense on the cold glass and frame and run down the shaft, which looks exactly like a leak but shows up on cold, dry days rather than during rain. If the drips follow cold snaps rather than rain, read condensation that looks like a leak before calling a roofer. If they follow the rain, tracing a leak to its source walks through the method.

Reading NFRC and ENERGY STAR Ratings

The National Fenestration Rating Council (NFRC) label is the place to start. DOE’s former Energy Saver page on energy performance ratings for windows, doors and skylights, preserved in an archived copy of June 21, 2026, explained that NFRC runs a voluntary program that “tests, certifies, and labels windows, doors, and skylights based on their energy performance ratings.” The label gives U-factor, solar heat gain coefficient (SHGC) and visible transmittance (VT), and can also show air leakage and condensation resistance.

Read the label this way:

• U-factor measures how readily the unit lets heat pass through. A lower number means less heat lost in winter and gained in summer.
• SHGC measures how much of the sun’s heat gets through. A lower number blocks more solar heat, which matters on a sunny, south- or west-facing slope in a warm climate.
• VT measures how much visible light gets through. A higher number means more daylight.
• Condensation resistance, when shown, is useful if the unit sits over a humid room such as a bathroom.

If you cannot find a label on an older unit, look for a model number on the frame or the inside of the sash. Ask the maker whether it can give you the unit’s ratings from that number.

ENERGY STAR adds a climate test. Its page on residential windows, doors and skylights states that products “must meet U-Factor and, where applicable, Solar Heat Gain Coefficient (SHGC) requirements,” under a specification effective October 23, 2023. The numbers differ by zone; the ENERGY STAR key product criteria page shows that the criteria “vary for each of the climate zones shown in the map above.” Find your zone on that map and compare your unit’s NFRC label with the skylight row for it.

Federal tax treatment has changed. The IRS page on the energy efficient home improvement credit lists exterior windows and skylights among eligible components and states, “You can claim the credit for improvements made through December 31, 2025.” A skylight installed in 2026 does not qualify for that credit. Other tax questions are covered in the tax treatment of a new roof.

Skylights During a Roof Replacement

A re-roof removes the shingles that the skylight’s flashing is woven into, so every unit gets touched whether or not anyone planned for it. That is why each one needs a written decision before tear-off, not on the day.

Reusing old flashing is a poor bet. It is nailed, bent and lapped with the old shingle courses, and reusing it means fitting new roofing around metal that has already been worked once. A flashing kit made for the unit’s model gives the new roof clean laps at the head, sides and sill. Ask whether the skylight maker sells a kit for your model and whether the roofer will use it.

Then decide each unit on its condition:

• Keep and reflash when the glazing is clear, the seals are sound, there are no stains around the shaft, the frame is solid, and the NFRC ratings meet your zone’s criteria or energy performance is not a reason for the work.
• Replace when there is fogging between panes (a failed glazing seal), a cracked or yellowed dome, a rotted frame, or a leaking sash. If energy performance is part of the reason for the work, ratings below your zone’s criteria also argue for replacement. Replacing while the roof is open avoids cutting into a new roof later.
• Remove and roof over when you no longer want the unit. That means framing the opening closed and patching the deck, which may need a permit, so ask the roofer and the building department.

Warranties split at the joint. The skylight maker warrants the unit; the roofer warrants the installation. Ask in writing who warrants the flashing joint if a leak shows up at the head of the unit a year after the job. Where skylight work falls in the sequence of a replacement is covered in the stages of a roof replacement, in order.

Use this per-unit checklist before you sign:

1. Photograph the NFRC label or model number on each unit.
2. Note fogging, cracks, stains or soft spots around each shaft.
3. Ask the roofer, for each unit: new flashing kit, replace, or remove?
4. Ask who supplies the kit or the replacement unit, and whose warranty covers what.
5. Get each decision written into the contract by unit location.

Keep, Reflash or Replace Your Skylights?

If the roof is being replaced, every skylight gets new flashing at minimum; reusing old flashing on a new roof puts tired metal at the joint you are paying to make watertight. Replace any unit with a failed glazing seal, a damaged dome or a leaking sash, and weigh replacing units whose ratings fall below your zone’s ENERGY STAR criteria if energy performance matters to you. Keep and reflash the rest.

If the roof is not being replaced, act on a leak at the flashing with a reflash, and on a fogged or cracked unit with a replacement, then plan the full decision for your next re-roof. Before tear-off is scheduled, photograph each NFRC label or model number so every keep-or-replace call rests on that unit’s own ratings. The other decisions a re-roof brings, from materials to insurance, are sorted in the roofing guide’s decision finder.

This guide is general information for U.S. homeowners, not legal, insurance, tax, or engineering advice. Codes, laws, and policy terms vary by location and change over time; confirm details with your local building department, your insurer, or a licensed professional.

Sources

• 2021 Seattle Residential Code, Chapter 9: Roof Assemblies, City of Seattle, Department of Construction and Inspections
• Residential Rehabilitation Inspection Guide, U.S. Department of Housing and Urban Development, Office of Policy Development and Research
• Energy Performance Ratings for Windows, Doors, and Skylights (archived June 21, 2026), U.S. Department of Energy, Energy Saver (page since removed)
• Residential Windows, Doors and Skylights, ENERGY STAR (U.S. Environmental Protection Agency)
• Residential Windows, Doors and Skylights Key Product Criteria, ENERGY STAR (U.S. Environmental Protection Agency)
• Energy Efficient Home Improvement Credit, Internal Revenue Service