Skylight

What is a Skylight

A skylight is a light-permitting structure or window, usually made of transparent or translucent glazing, that forms all or part of the roof of a building. Functionally it serves three purposes: daylighting (replacing electric light during the day), ventilation (when operable), and a visual connection to the sky. In code and trade language the broad family is called fenestration in the roof plane, and the IBC groups it with sloped glazing, defined as glazing installed more than 15 degrees from vertical. A skylight differs from an ordinary rooflight or pavement light in that it is engineered as a weathertight, load-rated assembly with a defined energy rating and safety-glazing build-up.

Structurally a unit skylight consists of four parts: (1) the frame, typically thermally broken aluminium, wood, or a wood-and-aluminium composite, which carries the glazing and transfers wind, snow and live loads to the roof; (2) the glazing infill, most often a sealed insulating glass unit (IGU) or, on domes, a plastic glazing sheet; (3) the mounting interface, either a raised curb or a deck-mounting flashing kit; and (4) the weatherproofing, comprising the gasket seals, engineered flashing and internal condensation gutter that keep water out and weep condensate back to the exterior.

Skylights are among the oldest architectural devices. The open oculus of the Pantheon in Rome, completed around 126 AD, admitted daylight through an unglazed roof opening. Glazed, weathertight skylights became practical only when industrial glass manufacturing matured in the 19th century: the Parisian shopping galleries, the great museum roofs, and Joseph Paxton's Crystal Palace of 1851 established the iron-and-glass roof as a building type. The modern factory-glazed unit skylight, the operable roof window, and the tubular daylighting device are all 20th-century developments, with tubular devices reaching mass residential use in the 1990s.

The case for skylights is primarily energy and well-being. Toplighting is roughly three to ten times more efficient than sidelighting at delivering useful daylight per unit of glazing, because the aperture sees the whole sky dome rather than a slice of the horizon, and this advantage holds even under overcast conditions. Where daylighting is well designed and paired with photoelectric dimming controls, electric lighting energy in suitable spaces can fall by up to 80 percent. Against this benefit, the engineer must weigh the heat the same aperture admits in summer and loses in winter, which is why every skylight selection is a balance between daylight, solar heat gain, and conductive heat loss.

Four engineering metrics dominate skylight quality: thermal performance (U-factor), solar control (SHGC), the safety-glazing build-up that determines what happens if the glass breaks overhead, and watertightness over the roof's service life. A skylight that daylights beautifully but leaks, drips condensation, overheats the room, or sheds glass when broken is an engineering failure regardless of its appearance.

Skylight Types and Classification

Skylights classify first by operation (fixed versus operable) and then by daylighting method (direct glazed aperture versus tubular conduit). The IBC and FGIA further group large architectural assemblies as sloped glazing when they exceed roughly 1.4 square metres (15 square feet) of glass or fall outside the unit-skylight definition. Choosing the wrong family is the most common early error: specifying a fixed unit where ventilation is needed, or a small tubular device where a furnished room actually needs a sizeable view aperture. The table below summarises the five mainstream families.

Fixed unit skylights are self-contained, factory-glazed assemblies with no moving parts. The structural perimeter frame supports a glazing infill and the unit is sealed for life, making it the most weathertight and lowest-maintenance choice. It suits any space that needs light but not air: hallways, stairwells, attics, and spaces above storage. Because there is no operating mechanism, the fixed unit also carries the lowest air-leakage rating in its class.

Vented unit skylights and roof windows add a hinged or pivoting sash for ventilation and passive cooling. When the unit sits within arm's reach of the occupant it is conventionally called a roof window; when out of reach it is a vented skylight, opened by a manual rod, an electric chain motor, or a solar-powered motor with a rain sensor that closes the sash automatically. An open roof window creates a chimney (stack) effect, drawing warm stale air out at the high point and pulling cooler air in through lower windows, which is a low-energy way to ventilate a top floor.

Tubular daylighting devices (TDDs), also called sun tunnels or light pipes, are non-operable units that capture daylight at a small roof dome and pipe it through a highly reflective rigid tube to a ceiling diffuser. They are the only practical way to daylight a small, deep, or hard-to-reach interior room such as a closet, corridor, or windowless bathroom, and they avoid the structural and waterproofing disruption of a full skylight shaft. Tube diameters typically run 250 mm, 350 mm and 530 mm (10 in, 14 in and 21 in); with premium reflective tubing the tube can run many metres while still delivering useful light.

Dome and plastic-glazed units use a formed acrylic or polycarbonate dome rather than flat glass, giving a self-shedding shape, light diffusion, and high impact resistance at lower cost and weight than glass. They dominate commercial low-slope roofs, warehouses and factories, and many double as code-rated smoke and heat vents. Sloped glazing systems are large architectural assemblies built from curtain-wall or storefront framing into ridge skylights, barrel vaults, pyramids and full atrium roofs. These are engineered project by project under ASCE/SEI 7 loads and ASTM E1300 glass sizing rather than bought as catalogue units.

Glazing Build-Ups and Materials

The glazing is where daylight, thermal performance and overhead safety are decided. Three material families serve skylights: insulating glass units, monolithic or domed plastics, and the reflective tubing of TDDs. Each has an optimal application, and the overhead position imposes a safety constraint absent from vertical windows: whatever is used, it must not fall on people when it fails. The table below compares the mainstream glazing options.

Insulating glass units are the residential and architectural standard. A typical high-performance build-up is a sealed unit with a fully tempered or heat-strengthened lite outboard, a 12 to 16 mm cavity filled with argon, a low-emissivity (Low-E) soft coat on an inner surface, and a laminated lite inboard. The Low-E coating reflects long-wave heat to cut U-factor and can be tuned to lower SHGC; the argon fill further reduces conductive loss. Triple-glazed units add a third lite and a second cavity for cold climates, reaching U-factors that single-glazed plastic cannot approach, at the cost of weight and reduced visible transmittance.

The inboard laminated lite is the safety keystone. Laminated glass bonds two glass plies to a polyvinyl butyral (PVB) interlayer; when broken, the shards stay adhered to the interlayer instead of falling. IBC Section 2405 and FGIA practice require laminated glass as the inboard (interior) lite of overhead glazing over occupied space, and where a 30 mil (0.76 mm) or thicker interlayer is used the code permits omitting the protective screen otherwise mandated below the glazing. Monolithic fully tempered glass alone is not accepted overhead because, although it dices into blunt cubes, those cubes still fall. Annealed glass is prohibited overhead unless laminated.

Acrylic (PMMA) domes are light-transmitting, easily thermoformed thermoplastics offering high clarity and good weatherability; impact-modified grades improve toughness. Polycarbonate (PC) is a light-transmitting, impact-resistant thermoplastic used as a shatter-resistant glass substitute, with impact strength far above acrylic, making it the choice for hail-prone, vandal-prone and security applications. Both plastics block ultraviolet through formulation or coatings, but acrylic holds clarity longer outdoors while polycarbonate yellows faster without a protective cap layer. Plastic domes fall under IBC Section 2610 rather than the glass provisions.

Tubular reflective tubing is a category of its own. The optical efficiency of a TDD depends almost entirely on the tube's specular reflectance, because the light bounces many times along the run. Premium tubing such as Solatube's Spectralight Infinity holds specular reflectance above 99 percent, reaching up to about 99.7 percent across the visible spectrum (380 to 760 nm), losing only roughly 0.3 percent of light per reflection and remaining spectrally neutral so it does not tint the daylight. That efficiency is what allows a narrow tube to deliver useful illumination after a long roof-to-ceiling run with bends.

Mounting, Flashing and Standards

More skylight failures trace to water and structure than to glass. Mounting method, flashing detail and roof slope decide whether a unit stays dry over its life, while a stack of structural standards decides whether the glass survives the design wind, snow and live loads. This chapter covers both. The first decision is curb-mounted versus deck-mounted.

Curb-mounted skylights sit on a raised wood or metal frame, the curb, built up roughly 100 to 150 mm (4 to 6 in) above the roof deck and flashed by the roofer; the unit then drops onto the curb. Lifting the glass above the roof plane tolerates ponding water and makes the unit suitable for low-slope and commercial roofs. Curb mounting is also more forgiving of roof movement and re-roofing, since the curb stays in place when the membrane is replaced. Deck-mounted skylights install directly onto the roof deck with an integral or step-flashing kit, sitting low for a sleeker profile and better water shedding on pitched roofs; this is the residential default on shingle and tile roofs.

Self-flashing units carry a one-piece integral flange that mounts directly to the roof without a separate curb, simplifying installation on suitable slopes. Whichever mounting is chosen, an engineered flashing kit matched to the roof covering is essential: head, sill, step and saddle flashing route water around and away from the opening. Most makers require a minimum roof pitch (commonly around 3:12, or a curb to lift the glass) so that water sheds rather than ponds; sloped glazing itself is defined as glazing more than 15 degrees from vertical. An internal condensation gutter, a trough cast into the frame, collects any condensate that forms on the cold glass and weeps it back to the exterior so it does not drip into the room.

The structural standards form a load path. ASCE/SEI 7 establishes the minimum design loads: wind uplift and pressure, ground and roof snow, and roof live load, all a function of location, exposure and roof geometry. Those loads feed ASTM E1300, the standard practice for determining the load resistance of glass in buildings, which uses a failure-prediction model based on weathered glass to confirm whether a proposed glass type (annealed, heat-strengthened, fully tempered or laminated) and thickness meets the load. The table below summarises the governing documents.

AAMA/FGIA GDSG-1, the Glass Design Guide for Sloped Glazing and Skylights, ties the load and safety requirements together for non-residential work, describing minimum sloped-glazing requirements and the conditions under which protective screens may be omitted. Unit skylights are also performance-tested as assemblies under AAMA/WDMA/CSA 101/I.S.2/A440 for air infiltration, water penetration resistance and structural load. Energy labelling follows the NFRC procedures (NFRC 100 for U-factor, 200 for SHGC and VT, 500 for condensation resistance), which is why a credible skylight spec quotes the NFRC whole-product rating, not a center-of-glass marketing figure. Tubular daylighting devices are commonly evaluated under ICC-ES acceptance criteria covering structural, water and fire performance of the dome, tube and diffuser as a system.

Key Specification Parameters

A skylight data sheet may list 15 to 30 parameters, but only a handful drive the selection decision. The five that matter most are the thermal and optical trio (U-factor, SHGC, visible transmittance), air leakage, condensation resistance, the structural and water ratings, and the safety-glazing build-up. Each is explained below; all energy figures should be the NFRC whole-product values that include frame and spacer, not center-of-glass.

U-factor is the rate of non-solar heat flow through the whole assembly, expressed in W per square metre per kelvin (SI) or Btu per hour per square foot per degree F (US). Lower is better. Skylights inherently carry higher U-factors than vertical windows of the same glazing, because the unit faces the cold night sky and the mounting is exposed. A single-glazed plastic dome may sit near U 1.0 (US, around 5.7 SI); a double Low-E argon glass unit reaches roughly U 0.45 to 0.50 (US, around 2.6 to 2.8 SI); a triple-glazed unit pushes below U 0.30 (US, around 1.7 SI). Low U-factor matters most in cold climates, both for heat retention and to keep the inner glass warm enough to limit condensation.

Solar Heat Gain Coefficient (SHGC) is the fraction of incident solar radiation admitted as heat, from 0 to 1; lower means less summer heat. Because a skylight faces the high summer sun directly, SHGC is often the more important number in cooling-dominated climates: a value near 0.25 sharply limits cooling load, while a clear high-SHGC unit can overheat a room. Visible Transmittance (VT) is the fraction of visible light let through, also 0 to 1. The engineering tension is that low-SHGC coatings also tend to cut VT, so the figure of merit for a daylighting skylight is a high light-to-solar-gain ratio (VT divided by SHGC): admit the light, reject the heat.

Air leakage (AL) measures uncontrolled air infiltration through the unit, in litres per second per square metre or cubic feet per minute per square foot, tested under NFRC and the 101/I.S.2/A440 procedures. Fixed units leak least; operable units depend on the quality of the sash gasket and latch. Condensation Resistance (CR), rated 1 to 100 under NFRC 500, predicts how well the unit resists interior condensation; higher is better and a thermally broken frame plus warm-edge spacer raises it.

For energy compliance, ENERGY STAR Version 7.0 gives skylights their own, more lenient criteria than vertical windows. The approximate skylight thresholds are summarised below; always confirm the current numbers and your climate zone against the NFRC label and the ENERGY STAR specification.

Beyond energy, the data sheet must state the structural rating (design pressure or load resistance per ASTM E1300 and ASCE/SEI 7), the water penetration resistance (the test pressure to which no water passes), the glazing build-up (which lite is tempered, which is laminated, and the interlayer thickness, since this is the overhead-safety qualifier), the frame material and thermal break, the operation and motor type for vented units, and the warranty, which on quality units separates the glass seal (often 10 to 20 years) from the unit and the operating mechanism.

Selection Decision Factors

To turn the preceding chapters into a specific model, follow the decision sequence below. Most selection mistakes come not from a single wrong choice but from deciding the visible product before settling the function and the constraints. These eight steps can serve as a fixed RFQ template.

1. Function and type: First decide whether the space needs light only (fixed unit or TDD), light plus ventilation (vented unit or roof window), or a large architectural aperture (sloped glazing). A deep interior room with no roof access for a shaft points to a tubular device; a top-floor bathroom points to a vented roof window with a rain sensor.
2. Daylight sizing: Size glazed skylight area at roughly 4 to 8 percent of floor area for general daylighting, trimming toward the low end in hot or cold climates to control heat flow. For TDDs, size the tube diameter (250, 350 or 530 mm) to the room area and the run length, allowing for bends that reduce output.
3. Energy targets: Set U-factor and SHGC from the climate zone. Cold climate: prioritise low U-factor (0.30 US or below) and high condensation resistance. Hot climate: prioritise low SHGC (near 0.25) and a high VT-to-SHGC ratio. Specify NFRC whole-product values and target ENERGY STAR qualification.
4. Glazing and overhead safety: Require an inboard laminated lite with a PVB interlayer over any occupied space, per IBC 2405. Decide tempered versus heat-strengthened outboard, single versus double versus triple IGU, and whether a 30 mil or thicker interlayer is used to waive the protective screen. For domes choose acrylic (clarity) or polycarbonate (impact).
5. Mounting and slope: Match curb-mounted (low-slope, commercial, re-roof-friendly) or deck-mounted (pitched, low-profile, residential) to the roof. Confirm the roof pitch meets the maker's minimum (commonly around 3:12) or specify a curb. Order the engineered flashing kit matched to the exact roof covering.
6. Structural and water rating: Confirm the unit's design pressure and water penetration rating cover the site wind, snow and live loads from ASCE/SEI 7, and that the glass is sized to ASTM E1300. For sloped glazing, this is a project-specific engineering calculation, not a catalogue lookup.
7. Operation and controls: For vented units choose manual rod, electric chain motor, or solar-powered motor; add a rain sensor for auto-close, and consider integral or accessory blinds and shades for glare and heat control. Verify the control protocol if integrating with home or building automation.
8. Total cost of ownership (TCO): Purchase price plus flashing kit plus installation (a skylight is a roof penetration, so labour and waterproofing dominate) plus shades plus long-term risk of leak repair and seal failure. A cheaper unit with a weaker flashing kit or a 5-year seal warranty can cost far more over a roof's life than a quality unit with a 20-year seal warranty.

One last commonly overlooked dimension is manufacturer serviceability: availability of the matching flashing kit for your roof type, replacement glass and gasket parts, motor and rain-sensor spares for powered units, and local installer training. A skylight is a 20-to-40-year roof element, so spare-part availability and the seal warranty matter more than the headline price. For residential and light-commercial units, VELUX and Fakro are the volume leaders; for tubular devices, Solatube and VELUX SUN TUNNEL; for commercial domes and sloped glazing, Wasco, Kingspan Light + Air, Major Industries and Kalwall; for architectural glass skylight systems, Kawneer, YKK AP and Schueco. Verify each model's NFRC label and any fire or smoke-vent rating against your specification before committing.