How window film works

What is arriving at your window

Sunlight reaching a pane is not one thing. It is roughly three.

Visible light is the part you see by. It is what makes a room feel bright and what lets you see out, and it is also the part responsible for glare when it arrives at an awkward angle.

Infrared is the part you feel as heat. You cannot see it at all, which is why a room can be uncomfortably hot through glass that does not look particularly bright.

Ultraviolet is the part that does most of the damage to your furnishings. You cannot see or feel it, and it is the easiest of the three for film to deal with.

A plain pane of glass lets a great deal of all three through. The point of film is to change those three numbers independently rather than simply dimming everything at once.

What the film itself is

It is not a spray, a coating applied by brush, or a tint baked into the glass. It is a manufactured film, usually built from layers of polyester, that arrives on a roll and is bonded to the inside face of your existing glass with a pressure sensitive adhesive.

Between those polyester layers sit whatever the film is designed to do. A solar control film carries a very thin metallised or ceramic layer that reflects or absorbs part of the incoming energy. An obscuring film carries a diffusing layer that scatters light instead. A safety film is mostly thickness and adhesive, built to hold a pane together rather than to change what passes through it. The outer face carries a scratch resistant coat, because the film is now the surface you will be cleaning.

Two practical consequences follow from it going on the inside. It is protected from the weather and from the street, which is why it behaves the way it does for a long time. And it can be removed later without touching the glass itself, which is often the detail that settles a conversation with a landlord or a managing agent.

What it does well

Heat. Solar control film reduces how much of the sun's energy comes through the pane. The room still gets its daylight. What changes is how much heat arrives with it, which is why a west facing room can go from unusable in the late afternoon to simply warm.

Glare. Reducing visible light at the glass reduces glare at the desk. This one always involves a trade, which is covered properly in the article on heat and glare, because the light you remove to kill glare is the same light you wanted for the room.

Ultraviolet. This is where film is at its strongest. It filters out the large majority of the ultraviolet arriving at that pane, which removes the single biggest contributor to fading.

Privacy. Either by reflecting at the surface so the glass returns the brighter side, or by diffusing so that shapes break up in both directions. These behave very differently after dark.

Holding glass together. Safety film does not stop glass breaking. What it does is hold the broken pieces in the frame rather than releasing them into the room, which is a different and more useful promise.

What it does not do

It is worth being equally clear about the limits, because a film sold on the wrong expectation disappoints no matter how well it is fitted.

Film is not double glazing. It will do very little about the cold coming off your glass in July, and nothing at all about a draught around a frame. If your complaint is winter rather than summer, film is probably not your answer and we will tell you so.

It does not change the glass. A pane that is already shaded, already tinted or already struggling has those properties before the film goes on and after.

It will not remove glare entirely. Where the sun sits low and looks straight down a room, no film on its own fixes that, and you are into blinds or external shading.

And it does not stop fading. It slows it considerably by removing the ultraviolet, but visible light and heat fade things too, and anything sitting in direct sun will still change over time. Anyone telling you otherwise is overselling.

Why "how dark" is the wrong question

The instinct is to assume that a film which blocks more heat must make the room darker, and once upon a time that was broadly true. Modern spectrally selective films are built specifically to separate those two, taking out a good deal of the infrared while letting most of the visible light through.

Which means you can often have a markedly cooler room that still looks like a room with a window in it. Whether you can depends on your glass, your aspect and what else you are trying to solve at the same time, and that is exactly the kind of thing worth working out on site rather than from a chart.

The families of film

Most of what we specify falls into four groups, and plenty of jobs use more than one across different elevations.

Solar control , for heat and glare, in a range that runs from near invisible to distinctly reflective.

Obscuring and decorative , for privacy and for treating glass as a surface, including bands, patterns and frosted panels.

Safety , for holding broken glass in its frame.

Ultraviolet focused , where fading is the whole complaint and you want to change as little else as possible.

How a film gets chosen

Briefly, because it has an article of its own: the aspect the glass faces, the hours that matter to you, the glass itself, and what you want the room and the building to feel like. The film follows from those. It is never picked off a list first and justified afterwards.

What we specify is named in your quotation, along with the glass it suits and what it will and will not do. If film is the wrong answer for your problem, we would rather tell you at the measure than have you find out in February.