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Commercial

VELUX FXG Acrylic Double Dome Skylight

The acrylic dome skylight you can order off the shelf: two thermoformed acrylic domes in a thermally broken aluminum curb frame, with a ten-year no-leak warranty and a clear/clear U-factor of about 0.56. It gives a flat roof the most daylight per dollar of any glazing, but clear acrylic runs hot, sits in fire class CC2, and costs four to ten times a polycarbonate dome the same size.

3.9
$509 to $647 for the 30½ × 30½ in unit
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Pros

  • Acrylic transmits 92 percent of visible light and holds that clarity without a UV cap layer, so the dome does not yellow the way uncoated polycarbonate does
  • Double dome halves the heat loss of a single dome: U-0.56 against U-1.02 on Kingspan's published acrylic figures
  • Thermally broken, encapsulated inner frame with a wicking gasket system that drains interior condensation to the outside
  • Curb mount fits roof pitches from 0 to 60 degrees with no top or bottom orientation, and a ten-year no-leak warranty with VELUX flashing
  • Stocked at Home Depot and skylight dealers in about a dozen curb sizes; tinted domes ship in about four weeks

Cons

  • Clear-over-clear SHGC of about 0.89 is well above the 0.60 the 2021 IECC allows even for daylit zones; choose the white or bronze dome for a conditioned building
  • Acrylic is fire class CC2, so code caps plastic skylights at 25 percent of floor area (33 percent for CC1 polycarbonate) and 100 sq ft per unit
  • Roughly ten times the impact resistance of glass but a fraction of polycarbonate's; hail country buys polycarbonate
  • Not Energy Star qualified, and no NFRC-certified U-factor is published for the FXG itself
  • $500 to $650 for a 30-inch unit against $49 to $138 for a polycarbonate dome the same size

Specifications

GlazingTwo thermoformed acrylic domes (clear/clear standard; clear/white and bronze/clear to order)
FrameAluminum, neutral gray, encapsulated and 100% thermally broken inner frame
MountingCurb mount; roof pitch 0 to 60 degrees; outside curb = rough opening + 3 in
Light transmissionAbout 92% visible light for clear/clear acrylic double dome (Kingspan estimate)
U-factorAbout 0.56 double dome, 1.02 single (Kingspan estimate; no NFRC rating for FXG)
SHGCAbout 0.89 clear/clear, 0.63 white/clear, 0.43 bronze/clear (Kingspan estimate)
FireCC2
ImpactRoughly 10× glass; polycarbonate roughly 250× glass
CondensationGasket wicking system drains interior condensation to the exterior
Energy StarNot qualified
Warranty10 years, no leaks, when installed with a VELUX flashing kit
PriceFXG 305305 (30½ × 30½ in): $647 Home Depot, $509 to $525 skylight dealers; FXG 225465 (22½ × 46½ in): $436; FXG 345345 (37½ in curb): $533

Our Verdict

Buy the FXG when the job is daylight over a flat or low-slope commercial roof and you want the clearest, longest-lasting lens in a curb frame that is built to not leak. Order it in white or bronze if the space is air-conditioned. Buy polycarbonate instead if the roof sees hail or the fire code is tight.

3.9

Overview

Most of the plastic domes on flat commercial roofs are polycarbonate now. The big retail name in dome skylights, Sun-Tek, does not make an acrylic dome at all; its whole line is glass or UV-capped polycarbonate, marketed as "30 times more impact resistant than acrylic." The acrylic dome that a facilities manager or roofer can still walk in and buy is the VELUX FXG, a double acrylic dome in a curb-mount aluminum frame, sold through Home Depot and the skylight dealers. It replaced the older CG2 model number; the FXG 305305 is the old CG2 3333.

This review covers the FXG as the buyable product, and borrows published performance figures from Kingspan Light + Air's Bristol Acrylic line (the former Bristolite, acquired by Kingspan in 2016), because Kingspan is the one acrylic dome maker that publishes a full optical and thermal table. We have not installed one. The facts come from the retailer and dealer listings, the Kingspan product data sheet, an ICC-ES evaluation report on acrylic domes, and the building codes, all listed at the end.

What you get

Two acrylic domes, an outer and an inner, spaced for an air gap and set in a neutral-gray aluminum frame. VELUX encapsulates the inner frame and calls it 100 percent thermally broken, which matters because a bare aluminum frame sweats in winter. The gasket design is meant to block air and water from outside while wicking interior condensation out to the roof, which is the failure that makes cheap domes drip on a January morning.

It is a curb-mount unit. You build a wood curb on the roof, at least 4 inches above the roof plane by code, flash it, and set the skylight over it; the outside curb dimension is the rough opening plus 3 inches. It works on pitches from flat to 60 degrees and has no designated top or bottom. The standard dome is clear over clear; clear over white and bronze over clear are available on about a four-week lead time, and you will probably want one of them (see the heat section).

VELUX backs it with a ten-year no-leak warranty, provided it is installed with a VELUX flashing kit. Kingspan's commercial acrylic domes carry the same ten years against defects and uncontrolled water leakage.

Light

Colorless acrylic passes about 92 percent of visible light regardless of thickness, and it does it without a UV-protective cap layer, because the polymer itself resists UV. Kingspan's estimates for a clear/clear acrylic double dome put visible transmittance at 92 percent; single clear at 95 percent. Compare that with VELUX's own NFRC-rated glass skylights, which run 38 to 64 percent visible transmittance because the low-e coatings that make them efficient also cut light. If the job is daylighting a warehouse, workshop, or corridor, the acrylic dome brings in roughly half again as much light per square foot as an insulated glass unit.

The common claim that acrylic yellows and polycarbonate does not is backwards on the raw resins. Polycarbonate yellows without a UV cap; acrylic does not need one. What ages an acrylic dome is crazing, the fine surface cracks that develop after years of thermal cycling and cleaning chemicals, plus fogging between the two skins if the seal fails. Skylight vendors put an acrylic dome's service life at 15 to 20 years; that figure comes from installer blogs, not from manufacturer weathering data, so treat it as trade experience rather than a spec. When the frame and flashing are sound, the dome alone can be replaced; Home Depot stocks replacement domes as a category.

Heat: the part to get right

Here is where a clear acrylic dome needs thought. Kingspan's figures for a clear/clear acrylic double dome are a U-factor of about 0.56 and a solar heat gain coefficient of about 0.89. The 2021 IECC, in its most generous allowance for skylights over daylight zones with automatic daylight controls, permits a U-factor up to 0.90 in climate zones 0 to 3 and 0.75 in zones 4 to 8, and an SHGC up to 0.60 in zones 0 to 6. The double dome clears the U-factor ceiling; a single acrylic dome at U-1.02 does not. The clear dome fails the SHGC allowance badly. A clear acrylic dome is, in effect, a heat lamp over a conditioned space.

The tinted domes fix most of that: white over clear estimates at SHGC 0.63 and 49 percent light, bronze over clear at 0.43 and 25 percent light. For an unconditioned warehouse, clear is fine and the free heat may even be welcome. For anything air-conditioned, specify white or bronze, or the energy inspector will.

For reference, VELUX's glass skylights carry NFRC whole-unit U-factors of 0.37 to 0.49 and SHGCs of 0.18 to 0.28. Glass is the efficient choice; acrylic is the bright one. Our acrylic vs glass guide walks through that trade in general.

Fire class, impact, and code

Acrylic domes are fire class CC2; polycarbonate domes are CC1. IBC Section 2610 uses that distinction: plastic skylights may total one third of the floor area for CC1 material but only 25 percent for CC2, each limit doubled in a sprinklered building. Each unit is capped at 100 square feet inside the curb, units must sit at least 4 feet apart, the curb must rise at least 4 inches, and no plastic skylight goes within 6 feet of an exterior wall that needs fire protection. Every unit carries a label with its CC class and plastic thickness, and an ANSI Z35.1 fall-hazard warning, because a dome is not a walking surface. OSHA requires a screen, cage, guardrail, or fall arrest for anyone working near an unprotected skylight 4 feet or more above the floor below; Kingspan sells external cages and internal screens and recommends an annual inspection for cracks.

On impact, the trade rule of thumb is that acrylic is about ten times tougher than glass and polycarbonate about 250 times. Neither number is a test class; no FM or UL hail rating is published for these acrylic domes. Large hail can crack an acrylic dome. If the building sits in hail country, this is the wrong material, and our acrylic vs polycarbonate guide explains why.

Installation notes

The ICC-ES report on acrylic dome skylights spells out what makes them not leak: a curb of at least nominal 2×6 lumber tall enough to put the dome 4 inches above the roof, 1/4-inch corrosion-resistant lag screws in every mounting hole at no more than 12 inches on center biting 1½ inches into framing, a half-inch bead of butyl or silicone on the curb top before the unit is set, and every gap between frame and curb fully shimmed. Skipped shims are the classic cause of a dome that leaks in the second winter. VELUX's warranty condition, the flashing kit, is the same lesson from the other side.

Price

At the time of writing, the FXG 305305 (30½ × 30½ inch rough opening) lists at $647 at Home Depot and $509 to $525 at skylight dealers with free shipping. The 22½ × 46½ inch FXG 225465 was $436 at a dealer, and the 37½-inch-curb FXG 345345 was $533. A 14-inch acrylic-dome VELUX sun tunnel with rigid tube was $540.

Set against Sun-Tek's polycarbonate curb-mount domes at the same store on the same day, $50 for a 14-inch, $69 for 14 × 22, $138 for 22 × 46, the acrylic double dome costs four to ten times as much. Part of that is the second dome and the thermally broken frame, and part is that acrylic domes are now a specialty product. You are paying for clarity and a frame built to a no-leak warranty, not for toughness.

Kingspan's Bristol Acrylic line has no published price; it is a specified commercial product quoted through reps, with curb sizes from 14 × 14 up to 92 × 92 inches, single or double glazing, clear, white, or bronze domes, and thermally broken frame options. If a job needs more than a dozen skylights or a size Home Depot does not carry, that is the line to spec.

Who should buy it

Anyone daylighting a flat or low-slope roof who wants the most light and the longest clarity from a dome, in a frame with a ten-year leak warranty, and who is not in a hail belt. Order white or bronze over a conditioned space. If the code official is counting plastic skylight area or the roof takes hail, buy the polycarbonate dome instead and read our acrylic window buying guide for where acrylic does belong.

Sources