Can you retrofit solar film to a rooflight?

Solar film is a thin polyester layer, usually with a metallic or ceramic coating and an adhesive back, that you squeegee onto glass you already own. The good ones reflect and absorb a chunk of the sun’s energy before it becomes heat in the room, and they can knock a fair amount off glare at the same time. On paper that is exactly what a hot rooflight needs.

The catch is position. On a sealed double-glazed unit the only face you can reach is the inner surface of the inner pane, the one nearest your head. By the time the sun’s energy reaches that surface it has already crossed the outer pane, the air or gas cavity and the inner pane, warming all of them on the way. A factory solar-control coating sits on the inner face of the outer pane, out at the front of the unit, where it can turn energy away and shed heat back to the sky before it ever gets inside. Retrofit film is working at the wrong end of the sandwich, which is why the same nominal performance reads better on a datasheet than it delivers in the room.

There is a second number worth watching. Film cuts visible daylight along with heat, and a strong solar film can take a bright rooflight down to something noticeably dimmer. Ask for both figures, the total solar energy rejected and the visible light transmittance, and judge them together rather than chasing heat rejection alone.

This is the part installers skate over. Applying an absorbing film to a sealed unit makes the glass run hotter, because energy the film absorbs turns straight into heat in the pane. If that heat is uneven across the sheet, the warm middle wants to expand while the cooler edges held in the frame do not, and the stress that builds up can split the glass. This is thermal stress cracking, and rooflights are unusually exposed to it because they face straight up at the sky and take the full load of the summer sun.

Whether a given pane survives depends on the glass. Toughened glass tolerates the stress well and is generally the safe candidate for film. Annealed float glass, which is what a lot of older units use, is far more likely to crack, and you often cannot tell which you have by looking. Laminated panes and any glass with an existing low-emissivity or solar-control coating change the sums again, sometimes enough that a film manufacturer will not warrant the job at all.

Two further points matter before anyone reaches for the squeegee:

• The sealed-unit guarantee. Most double-glazing units carry a manufacturer’s guarantee against the seal failing and the cavity misting up. Applying film raises the temperature the unit runs at, and many makers treat film as grounds to void that guarantee. Check the wording before you commit, because a filmed unit that fogs a year later is no longer covered, and the answer for a fogged sealed unit is a replacement.
• Overhead application. A rooflight is above your head, often over a stairwell or a deep kitchen, and film has to go on clean, bubble-free and dust-free in one attempt. That is hard enough on a vertical window at chest height. Done over a lantern from a ladder or a tower, it is harder still, and a trapped bit of grit or a lifted edge shows against the sky for the life of the film.

If your rooflight is a moulded plastic dome, a polycarbonate multiwall sheet or an acrylic flat panel rather than glass, treat film as a non-starter. These materials expand and contract far more than glass with temperature, so a rigid film bonded to the surface tends to bubble, wrinkle and peel within a season or two. Acrylic in particular can craze under the combination of adhesive solvents and heat, leaving a fine web of cracks that will not come out.

Plastic rooflights also let more of the spectrum through in the first place, and they yellow with age, so the honest route to a cooler, clearer room is usually to replace the whole unit rather than to coat a surface that will fight the film. We cover the trade-offs on our plastic and polycarbonate skylight installation page.

Round here this decision has a particular edge to it. Southend faces south across the Thames Estuary, so a great many rear extensions and loft rooms take sun on the glass from mid-morning to late afternoon across the whole cooling season, with reflected light off the water adding to the load on a bright day. A rooflight over one of those rooms is a solar collector, and film is the thing people reach for once the room has already become unbearable.

Weigh it in G-value terms. Clear double glazing sits around 0.6, meaning roughly 60 per cent of the sun’s energy comes through as heat. Solar-control glazing built for the job comes in nearer 0.3, and it does that from the front face of the unit where the physics works in your favour. Retrofit film can move the number, but it starts from behind and it degrades under exactly the ultraviolet it is there to block, so its performance is highest the day it goes on and slips from there.

If the existing rooflight is due to come out anyway, or the room is bad enough that you were going to spend money on it, putting that money into the glass specification rather than a film gives you the better G-value, keeps the manufacturer’s guarantee intact and lasts the life of the unit. That is the whole basis of our energy-efficient skylight installation work: specify for the orientation first, then choose the glass, then fit the unit. Film has its place as a low-cost trial on a toughened glass rooflight that is otherwise sound, but it treats the symptom rather than the specification.

Start by working out three things: whether your rooflight is glass or plastic, and if glass, whether it is toughened, and which way the roof faces. A plastic rooflight rules film out. A north-facing glass one rarely needs it. A south or west-facing glass one over a room that cooks in July is the case where the choice between film and a glass swap actually matters, and the swap usually wins.

If the unit is old, already misting, or the room is a serious overheating problem, tell us the orientation and the rough opening size and we will come back with a glass specification and a price for replacing it properly. Ring 01702 898232 or request a quote, and have a look at our skylight installation costs before you decide. We will look at the whole roof rather than just the pane you were about to cover.