Skylights Explained: Types, Glazing and How They Work

A skylight is a window set into the roof, and that single fact explains most of what makes it different from a window in a wall. It faces the sky, it sits where rain, snow and sun arrive with the most force, and it usually has a shaft or a ceiling opening beneath it. This guide covers the main types, how they are mounted and opened, what the glazing is made of, how the performance ratings work, and where tubular devices fit in. It is educational only, and each section points to a page that goes deeper.

What counts as a skylight

The vocabulary overlaps, which confuses almost everyone at first. The Efficient Windows Collaborative introduces roof windows "known as skylights" in the same breath, and its glossary defines a roof window as a fixed or operable window similar to a skylight placed in the sloping surface of a roof. It also defines an operable skylight as a roof window that gives both light and ventilation. This site uses the words interchangeably unless a distinction matters.

Building codes draw the line by angle rather than by name. The International Residential Code commentary published by the International Code Council describes skylights and sloped glazing as glazing installed in roofs or walls that are sloped 15 degrees or more from the vertical. The same commentary separates a unit skylight, which is factory assembled and tested as one component, from site-built glazing, where each part must be designed for the loads on its own. A tubular daylighting device is treated as its own category because it is typically assembled in the field from a kit, and we cover it in tubular daylighting devices (sun tunnels) explained.

How a skylight is mounted: flush, curb and integral curb

Two terms come up in every mounting discussion. A curb is a raised frame that lifts the skylight above the surface of the roof. Flashing is sheet metal or another material applied to seal the joint where two different materials or surfaces meet, and on a roof that usually means the edge of the skylight against the roofing.

The Efficient Windows Collaborative lists three arrangements. A flush-mount skylight sits on the roof deck with no curb, and the roofing material overlaps and seals it. A curb-mount skylight sits on a curb raised above the roof plane, and that curb is not supplied with the skylight, so it is built separately and the unit can be fixed or operable. An integral-curb skylight arrives as a complete unit with its own curb, and it may also be fixed or operable.

Which one fits depends on the roof. The same guide suggests starting with the pitch of the roof and deciding whether a deck-mounted, curb-mounted or pan-flashed product would work best. The joint between skylight and roof is where most leak stories begin, so we give it a full page: flashing, curbs and why skylights leak. The broader sequence of installation, leaks and lifespan sits in skylight installation, leaks and lifespan.

Fixed, vented and electric operation

A fixed skylight does not open. Its job is light and view, and the absence of a moving sash keeps the construction simple. An operable skylight opens by hand or by a motorized system, and the Efficient Windows Collaborative notes that some designs include a venting mechanism that lets a little air through without opening the window fully. Because warm air collects near the ceiling, an opening at roof level can exhaust that hot air from the room below.

The same source mentions two practical touches: rotating sashes that let the exterior pane be cleaned from inside, and interior shading such as solar, blackout, Venetian or roller shades to soften or block intense sun. A motorized unit also needs a power source and a control, which brings electrical work into the picture. That work belongs to a qualified professional working to local code, a theme we return to below. The trade-offs are laid out in fixed, vented and electric skylights compared.

What the glazing is made of

Glazing is the glass or plastic panes in a window, door or skylight. Skylight glazing falls into two broad families, and the right choice depends on the shape of the unit, the roof and the climate.

Glass units

Framed roof windows are glazed with glass and offer most of the glazing and solar-control options of ordinary windows, according to the Efficient Windows Collaborative. A modern glass unit is usually insulating glass, meaning two or more panes spaced apart and sealed at the edges. The glossary describes two common upgrades. A low-emittance (low-E) coating is a microscopically thin, nearly invisible metal or metal-oxide layer that mainly reduces heat flow through the glazing. A gas fill, usually argon or krypton, sits between the panes to suppress conduction and convection.

Safety is part of the picture because a skylight hangs over the room. The glossary notes that laminated glass is two or more sheets with a plastic inner layer that holds the glass together if it breaks, and that tempered glass is reheated and then cooled quickly so that it shatters into small pieces. The International Code Council commentary on the residential code limits skylight glazing to laminated, fully tempered, heat-strengthened and wired glass, plus approved rigid plastics, and it does not allow annealed (ordinary, not heat-treated) glass because it breaks into large sharp shards. Tempered and heat-strengthened panes generally need a retention screen below them, with limited exceptions based on size and height, while laminated glass and approved rigid plastics do not. Local adoption of the code varies, so the building department decides what applies.

Plastic glazing

Plastic glazing is most familiar in domed units. The glossary describes acrylic as a thermoplastic with good weather resistance, shatter resistance and optical clarity. The Whole Building Design Guide observes that most skylights are passive, with a clear or diffusing medium that is usually acrylic, often built as a double layer for added insulation. GreenBuildingAdvisor adds that acrylic and polycarbonate cost less than glass but resist scratches less well, and that after long sun exposure plastic may become brittle and discolored. It also reports that polycarbonate has higher impact resistance than acrylic and costs more. The full comparison is in glass versus acrylic skylight glazing.

The numbers that describe performance

Three ratings do most of the work. U-factor measures the rate of non-solar heat loss or gain, and a lower number means better insulation. Solar heat gain coefficient (SHGC) is the fraction of solar radiation admitted, on a scale from 0 to 1, where a lower number means less solar heat comes in. Visible transmittance (VT) is the fraction of visible light that passes through.

The ratings come from the National Fenestration Rating Council (NFRC). ENERGY STAR certified skylights are made by an ENERGY STAR partner, independently tested and verified through NFRC, and meet criteria that vary by climate zone. Those zone-by-zone tables are published as images on the ENERGY STAR page, so check it directly for current figures.

A stricter recognition tier gives a feel for where the top end sits. The ENERGY STAR Most Efficient 2024 criteria for residential skylights and tubular devices were set as follows.

Reading a label is a skill of its own, and reading NFRC and ENERGY STAR labels walks through one. How those numbers translate into heating, cooling and comfort is the subject of skylights, energy and comfort.

Daylight, glare and heat

More glass does not automatically mean better daylight. The Whole Building Design Guide, writing mainly about larger buildings, makes the point that natural light has to be controlled and distributed, and that designers must avoid glare and overheating when placing openings. Direct sun in the eyes causes glare, and excess opening area can raise summer cooling needs or speed up winter heat loss. The guide also describes active skylights, which use a mirror system to track the sun, though it notes that the majority are passive.

For a home, the practical version is that where a skylight goes, which way it faces and what sits beneath it all shape the result. Orientation and room use are covered in skylight placement and room orientation, and the space between roof and ceiling is covered in skylights, attics, light shafts and insulation. A light shaft is the passage that carries daylight from the roof opening down to the ceiling opening, and how well it is insulated and finished affects both comfort and moisture.

Condensation is often mistaken for a leak

The Efficient Windows Collaborative states that "leaky" skylights are often misdiagnosed. Drops of water that look like a leak are more often condensation forming on the cold skylight surface, and a skylight is usually the first place condensation appears when indoor air carries too much moisture. It suggests insulating the skylight well, providing adequate air movement, and using high-insulating glazing with a well-designed frame. A real leak deserves a proper inspection, and we cannot diagnose any particular case. More is in skylight condensation and moisture.

Tubular daylighting devices as an alternative

Where a conventional skylight would need a wide shaft, a tubular device can reach the same room through a much smaller roof opening. The Efficient Windows Collaborative describes a clear plastic dome on the roof joined by a highly reflective tube, 10 to 14 inches in diameter and bendable, to a small diffuser in the ceiling, so it can be placed without reframing the roof or building a light well. A study from Lawrence Berkeley National Laboratory confirms that these devices need a much smaller roof opening than conventional skylights and can carry daylight farther from the building envelope.

Codes, permits and who does the work

Cutting a hole in a roof changes the structure, the weatherproofing and sometimes the electrical system. Permit rules, structural requirements and glazing requirements differ from place to place, and the building department and the product instructions have the final word. Building permits and codes for roof openings explains what usually triggers review. This site does not recommend that readers do roofing, structural or electrical work themselves, and a qualified professional is the right person for those tasks. If you are weighing who that should be, choosing a roofing or window contractor, and what to ask lists the questions worth asking.

The short version

A skylight is defined by how it is mounted, whether it opens, what its glazing is made of and how the NFRC ratings read. Glass and plastic each have a place, and code limits the materials that may be used overhead. Roof openings involve structure, weatherproofing and sometimes wiring, so local code, product instructions and a qualified professional come first.