Secondary glazing film gets talked about in two extremes. Some people call it miracle insulation, others write it off as cling wrap for windows. After fitting it on plenty of old sash windows over the years, the truth sits in the middle. It does a real job in the right spot, and it disappoints when you ask it to do something it was never built for.
What secondary glazing film is, and what it is not
Secondary glazing film is a thin, clear plastic sheet that you tape around the inside of a window frame and then shrink tight with a hairdryer until it goes drum flat and almost invisible. The point of it is the pocket of still air it traps between the film and the glass. Still air is a decent insulator, so that trapped layer slows down the heat moving out through the cold single pane.
What it is not is a second pane of glass. It will not perform like proper double glazing or a rigid secondary glazing panel bolted to the frame. It is a seasonal, low cost layer you put up for winter and take down in spring. Go in expecting that and you will be happy with it. Go in expecting it to turn a draughty Federation window into a sealed modern unit and you will be let down.
You can see our Secondary Glazing Film for the kit itself. One kit covers a set area, so measure your windows before you order so you buy the right amount.
Fitting it properly
Most of the complaints about this film come down to a rushed install, so slow down here.
- Clean the frame where the tape will stick. Dust, old flaking paint and grease all stop the double sided tape from holding. Give the timber a wipe and let it dry fully.
- Run the double sided tape around the inside face of the frame, right out near the edge so you get the biggest possible air gap behind the film. Press it down along its whole length.
- Peel the tape backing off and lift the film onto it. Start at the top, get it roughly square, then smooth it onto the sides and bottom. It will look loose and wrinkled at this stage, which is normal.
- Go over it with a hairdryer on a medium heat, holding it back a little and keeping it moving. The film pulls tight as it warms and the wrinkles disappear. Do not park the hairdryer in one spot or you can overheat the film.
- Trim the excess film back to the tape line with a sharp blade.
Done carefully, a fitted window looks clean from across the room and lets the light through. Done in a hurry on a dirty frame, it sags, peels at the corners and looks exactly like the cling wrap people complain about.
What it actually does for heat and noise
On heat, the film earns its keep. The trapped air layer takes the edge off the cold radiating in from a single pane, and it stops the draught that leaks through and around an old sash from blowing straight into the room. In a cold room with an old window, you will feel the difference sitting near it. It also tends to cut down the condensation that forms on the inside of the glass on frosty mornings, because the room side surface stays a bit warmer.
On noise, be realistic. The film adds a thin air gap that can soften high pitched sounds. It does very little for the low rumble of traffic, because that kind of noise needs mass and a wide air gap to stop it, and thin plastic has neither. If noise is your main problem rather than cold, read our guide on how to quieten a noisy sash window first, because film is near the bottom of the list of things that help there.
Where it falls down
There are a few honest limits worth knowing before you buy.
- You cannot open the window while it is fitted. The film seals the whole opening, so a window you want to use for ventilation through the season is the wrong window for it. Keep it for windows you are happy to leave shut until spring.
- It only covers the glass opening. If the real draught is coming up through the sash cords, around a loose meeting rail or under the bottom rail, the film over the glass will not reach it. Sealing the moving parts of the sash is a different fix.
- It does not love doors. People try the same film across a draughty door and it rarely lasts, because the door moves and flexes and the film tears at the edges.
- It is seasonal. The tape and film are made to go up for winter and come off again. Expect to replace it rather than treat it as permanent, and expect it to leave some tape residue you will need to clean off the frame.
Film versus the other options
Film is the cheapest way to get a bit of warmth back from a cold window, and for a rental or spare room that is often the right call. But for a window you actually use it is usually not the best option.
If the window rattles and the cold is coming through gaps rather than through the glass, draught sealing the sash is a more permanent fix and still lets you open and close the window as normal. Our Draught Sealing Kit for a double hung window seals the four gaps around a sash and stays in place year round. Film and seals also work together: seal the moving gaps, then add film over the glass on the windows you are not using.
For a fuller picture, including heavier glass and other upgrades, our article on improving the energy efficiency of your home walks through what each option is worth.
Is it worth it?
For the money, yes, on the right window. It is a cheap, reversible way to make a cold single pane less cold for a season, and it does cut condensation. Match it to the job: use it on windows you will leave shut over winter, fit it carefully onto a clean frame, and do not ask it to carry noise or to fix a draught coming from the sash itself.
Frequently asked questions
How long does secondary glazing film last?
Treat it as a seasonal product. It is designed to go up for winter and come down in spring. Fitted on a clean frame it will hold for the season, but the tape and film are not meant to stay up for years.
Does secondary glazing film really reduce heat loss?
Yes, modestly. The trapped air layer slows the heat radiating out through a single pane and stops draughts blowing in around the glass. You will feel it near a cold window, but it will not match proper double glazing.
Does the film help with traffic noise?
Very little. It can soften high pitched sounds a touch, but it does almost nothing for the low rumble of traffic, because stopping that kind of noise needs mass and a wide air gap that thin film does not provide.
Can I still open the window with the film on?
No. The film seals the whole opening, so the window stays shut until you take the film off. Use it on windows you are happy to leave closed for the season.
Will it leave marks on my window frame?
The double sided tape usually leaves some residue when you peel the film off. It wipes off the timber with a little effort, so it is worth cleaning the frame down once the film is removed.