Do Energy Efficient Skylights Block UV?

A U-value tells you how much heat escapes through the unit in winter. A g-value tells you how much solar heat comes in during summer. Neither number says anything about ultraviolet light on its own, because UV is a different part of the spectrum from the heat and visible light those figures describe. A rooflight can carry an excellent U-value and still let through more UV than a cheaper unit next to it, if the glass make-up is different.

UV matters in a room for one plain reason: it fades things. Sunlight bleaches the colour out of fabric, carpet and upholstery over time, dulls the finish on a wood floor, and yellows paper and some paints faster than a room lit only from the side. A rooflight sits directly over whatever is below it, so the sofa, the bed or the reading chair placed under the glass takes more of that exposure than furniture against a wall ever would.

The part doing most of the work is the interlayer inside a laminated pane, not the coating on the glass or the gas between the panes. Laminated glass is built from two sheets of glass bonded either side of a plastic interlayer, usually PVB, and that interlayer is what stops the UV rather than the glass itself. It is a standard part of glazing science rather than a claim specific to any one product, and it is the same interlayer that holds the glass together if it is ever struck, which is why laminated glass is specified for safety in the first place.

Many rooflights already carry a laminated inner pane, because Building Regulations ask for a safety pane on the inside face wherever someone could fall against or through the glass, particularly over a stairwell, a landing or anywhere low to the floor. Where that safety requirement applies, strong UV protection comes along with it as a side effect, at no extra decision needed. Where it does not apply, for example a small roof window well above head height, the fitter may offer toughened glass on its own, which does not carry the same interlayer and lets through more UV.

Low-E coatings and solar control coatings are aimed at heat, not fading, though they take a little UV out along the way. The invisible layers guide covers what a low-E coating does to keep heat in during winter, and solar control glass for skylights covers the summer side of the same coin. Both change how much heat and visible light pass through the glass, and both trim UV a little as a by-product, but neither is specified as a fading control on its own. If fading is the concern in a specific room, the laminated interlayer is the part to ask about, not the coating.

This is also why two rooflights with the same U-value can protect a room differently. One might use a laminated inner pane for safety and pick up strong UV blocking as a result, while the other, fitted higher up where no safety pane is required, relies only on toughened glass and a coating chosen for heat performance. The numbers on the energy label look similar; the fading risk underneath does not.

A rooflight faces the sky directly, so it collects sun for more hours of the day than a vertical window on any one wall, whatever the aspect of the roof. A south-facing slope over a kitchen extension or a loft bedroom in one of Peterborough's new-town townships will see stronger, more direct sun for longer than a north-facing one, and a run of roof windows over a landing in a Fletton-brick terrace can throw sun across a wide strip of floor rather than a single patch. None of that is a defect. It is simply more hours of light reaching whatever sits beneath the glass, which is exactly why the glass build-up is worth getting right at the survey stage rather than after the floor has already faded.

Furniture placement plays its part too. A reading chair, a bed or a rug positioned directly under a rooflight, rather than to one side of it, takes the fullest exposure. Moving the piece a little, or choosing curtains, blinds or a low-E blind fabric for the room, reduces exposure further, though the glass itself is where the largest single reduction in UV comes from.

If fading matters in the room, because there is a pale sofa, a wood floor, a rug or artwork sitting under the glazing, ask specifically about the inner pane rather than trusting the U-value alone to cover it. A laminated inner pane is the single change that does the most for UV, and it is often already part of the specification for safety reasons before fading is even raised. Layer a solar control or low-E coating on top for the heat side of the job, and the two together cover warmth, glare and fading without one being sacrificed for the other.

• Ask whether the inner pane is laminated, and for which reason: safety, UV, or both.
• Check the g-value and light transmittance figures separately, since UV is not shown by either number.
• Consider a blind or curtain as a second layer for a room with a south-facing slope and furniture directly beneath the glass.

We work through all of this at survey as part of the energy efficient rooflight service, alongside the sightline sketch that sets out the view from where you actually sit, so the glass chosen answers the heat, the glare and the fading question together rather than one at a time. Our guide to g-value and our guide to light transmittance both explain the figures that do appear on a glass specification, and will solar control glass dim the room below answers the glare question directly. For an existing rooflight with a plain, unlaminated pane, replacing single-glazed rooflights with modern glazing covers what changes when the whole unit is upgraded. What affects the cost of a rooflight sets out where glass specification sits in the wider budget for a job.

Does tinted glass block more UV than clear glass?

Not reliably. Tint changes how much visible light and colour comes through, but UV blocking depends on whether the pane is laminated and what coating is applied, not on how dark the glass looks. A clear laminated pane can block more UV than a tinted pane that is not laminated.

Will a rooflight fade my sofa faster than a normal window?

It can, if the sofa sits directly beneath the glass and the inner pane is not laminated, because a rooflight collects more hours of direct sun on a given spot of floor or furniture than a vertical wall window usually does. Moving the piece to one side, or specifying a laminated inner pane, reduces that exposure.

Do I need to ask for UV protection separately from energy efficiency?

Yes, in the sense that the two are specified differently. Energy efficiency is set by U-value and g-value; UV blocking comes mainly from the laminated interlayer in the inner pane. A rooflight can be specified for both at once, but they are not the same request.

Does UV protection wear out over time?

The interlayer inside a laminated pane is sealed between two sheets of glass and is not exposed to weathering the way an external coating can be, so its UV blocking does not fade the way a film applied to the inside of a window might. A sound edge seal on the sealed unit is what keeps the whole assembly performing as specified over the years.