Where a rooflight actually loses heat
Most of the heat leaves through the glass in the middle, but the perimeter is where the losses concentrate per metre, and it is the perimeter that differs between frameless and framed designs.
Think of any rooflight as three zones. The centre of the pane is where the low-E coating and argon or krypton fill do their work, and it is the best performing part of the unit. Around it runs a band roughly 60 to 100mm wide where the spacer bar holds the panes apart and the edge seal keeps the gas in. Then there is whatever supports the glass: an aluminium or timber frame on a framed unit, or a bonded seating on the kerb for a frameless one. Heat looks for the easiest route out, and in the edge band and the support it finds materials that conduct far better than a still gas layer.
So the useful question is not whether a unit has a frame. It is how well the whole edge assembly, from spacer to upstand, resists that short, fast route. Our guide to rooflight heat loss in winter breaks the losses down across the whole opening.
What changes when the frame disappears
Taking the frame away removes one cold path and exposes another: the edge of the sealed unit now does the job the frame used to do, so the spacer and the seating carry more of the thermal load.
On a typical frameless flat rooflight the outer pane is larger than the inner one. It oversails the kerb, and the inner pane drops inside the upstand so the glass sits down into the opening. The unit is bonded to the kerb with structural silicone, and there is no aluminium section to warm or chill. That is a genuine advantage: an aluminium frame without a thermal break is a very efficient conductor, and even a thermally broken frame is a denser, colder path than insulated glass.
What the frameless design gives up is a separate component to absorb the edge losses. The spacer, the secondary seal and the stepped edge of the outer pane now sit directly between the warm room and the winter air. If the spacer is plain aluminium, the edge band becomes the coldest line on the unit, and in a steamy kitchen that line can show as a strip of condensation just inside the upstand. With a warm edge spacer made from a composite or stainless steel profile, that line runs several degrees warmer.
How the two designs compare
Judged on the same glass, the differences come down to edge detailing rather than the frame itself.
The table sets out where each design tends to be stronger and weaker. It is a comparison of approaches, drawn from how the products are built, not a ranking of any make.
| Point | Frameless | Thermally broken frame |
|---|---|---|
| Metal at the edge | None, or hidden | Aluminium with a polyamide break |
| Main cold path | Spacer and seating | Frame and spacer |
| Glass share of opening | Higher | Lower |
| Relies most on | Upstand insulation | Quality of the break |
The glass share is the point people overlook. Because a frameless unit has no frame border, more of the opening is high performing glass centre and less is edge. For the same structural opening that tilts the balance in its favour. A framed unit fights back by insulating its frame well; our guide to thermally broken rooflight frames explains how the polyamide strips work and why cheaper frames still run cold.
Why size and shape matter more than the label
Edge losses scale with the perimeter and centre losses with the area, so small frameless units feel the edge effect more than large ones.
A 600mm square rooflight has 2.4 metres of edge wrapped around 0.36 square metres of glass, which works out at about 6.7 metres of edge for every square metre. A 1500mm square unit has only 2.7 metres of edge per square metre. A long, narrow 500 by 2000mm strip over a Mill Road side return sits at 5 metres of edge per square metre, well above a square unit of the same area. The edge accounts for a much larger slice of the total heat loss in the small and narrow units, whichever design you choose.
That is why two rooflights with identical glass can publish different U-values, and why you should compare the declared whole-unit figure for the size you are actually buying. Rooflights are now tested in the horizontal position, which gives higher figures than the old vertical method, and the Part L limiting value of 2.2 W/m2K applies to the whole unit, edge included. A frameless product that meets that figure in horizontal testing has already accounted for its edge. For how those numbers are measured, see rooflight U-values explained.
The kerb: the part nobody sees
A frameless rooflight is only as warm as the upstand it is bonded to, and the upstand is often built on site rather than supplied with the glass.
On a Cambridge flat roof extension, the kerb is typically a timber upstand at least 150mm above the finished roof, wrapped in the roof membrane. If the timber is left uninsulated, the flat roof insulation stops at the base of the kerb and the upstand becomes a cold collar around the opening. Heat leaks through the timber, the inside face of the kerb runs cold, and moisture condenses on the plasterboard lining just below the glass. That is a detailing fault, not a glass fault, and it affects framed units on site-built kerbs just as much.
The cure is to carry insulation up the kerb, or to use a factory insulated upstand, so that the thermal line runs unbroken from the roof deck to the glass. When we plan a frameless unit we draw the kerb as part of the rooflight, not as carpentry around it. Poor kerbs are also a common reason a skylight feels cold underneath even when the glass itself is good.
Choosing without trading heat for looks
You can have the clean frameless sightline and good thermal performance; it just takes a closer look at the specification than a brochure photo gives.
- Ask for the whole-unit U-value in the horizontal position, for your size.
- Check that the spacer is a warm edge type, not plain aluminium.
- Confirm how the upstand will be insulated and who builds it.
- For a kitchen or bathroom, weigh triple glazing, which warms the edge band as well as the centre.
The glass decisions then follow the orientation. A frameless unit over a south facing kitchen in Trumpington gathers a lot of summer sun, so a solar control coating may matter more to your comfort than a small edge difference. A north facing unit over a loft stair in a Romsey terrace sees little winter gain, so its U-value and edge detail carry more weight. We work those choices out on the light plan before quoting, and the full range of options sits on our energy efficient rooflights page. If you would like us to look at your roof, book a light survey.
Questions people also ask
Do frameless rooflights get more condensation?
Only when the edge is poorly detailed. A plain aluminium spacer or an uninsulated kerb can leave a cold strip at the perimeter where moisture forms. With a warm edge spacer and an insulated upstand, a frameless skylight is no more prone to it than a framed one. Our rooflight condensation guide covers ventilation too.
Is triple glazing worth it on a frameless unit?
It helps the centre and the edge, and it cuts the downdraught under large units. The extra weight bears on the bonded seating, so the kerb must be sized for it, and there is a small loss of light. It is most useful on north slopes and over rooms with high humidity.
Can I replace a framed rooflight with a frameless one?
Usually, provided the kerb is rebuilt or adapted to the size and fall the frameless unit needs. A replacement rooflight in an existing home still has to meet Part L, and we handle the Building Control notification where the work is notifiable.
Do frameless rooflights let in more light?
For the same opening, yes, because there is no frame border shading the edge. The gain is most noticeable on small units, where a frame takes up a larger share of the view.
