How do I fit window film for heat or privacy?

Window film works well and is fitted badly more often than not, because the temptation is to spend five minutes cleaning the glass and an hour fighting bubbles.

Safety

What you'll need

• Film of the correct type and size
• Spray bottle with water and a few drops of washing-up liquid
• Squeegee, a sharp knife with fresh blades and a lint-free cloth
• Scraper for cleaning the glass, and a tape measure

Step by step

1. Identify the actual problem first.Heat and glare, being overlooked, glass safety, or heat loss. Each has a different film, and buying the wrong one is the commonest mistake — frosted film does nothing for heat and solar film does not obscure much.
2. Check compatibility with your glazing.Dark solar film on double glazing can cause thermal stress cracking, and may void a glazing warranty. Check the manufacturer's chart against your glass type and pane size before buying.
3. Understand what each type does.Solar film reflects heat and glare and darkens the room. Frosted or privacy film obscures but passes light. Safety film holds broken glass together. Low-e film reduces heat loss slightly in winter.
4. Be realistic about one-way film.Reflective privacy film works only when the outside is brighter than the inside. At night with the lights on it works in reverse and you are more visible, not less.
5. Measure and cut oversized.About 20mm extra all round. You trim it in place once it is positioned — cutting to the exact size first almost never fits.
6. Clean the glass far more thoroughly than feels necessary.Scrape it, wash it with soapy water, squeegee, then check with a raking light. This is where the job is won or lost — every speck left becomes a permanent bubble.
7. Clean the frame edges too.Dirt in the corners and along the beading transfers onto the film as you position it, and it is the part people skip.
8. Work on a mild day out of direct sun.Hot glass dries the solution before you can position the film. Cold glass makes it stiff. An overcast mild day is ideal.
9. Wet everything generously.Water with a few drops of washing-up liquid, sprayed on the glass, on the film's adhesive side as you peel the backing, and on your hands. The film should float, which lets you slide it into position.
10. Peel the backing with two people or with tape.Two pieces of tape on opposite sides of a corner pulled apart separates the layers. Spray continuously as you peel so the adhesive never touches anything dry.
11. Position, then squeegee from the centre outwards.Slide it into place while it floats, then work the solution out from the middle towards the edges in overlapping strokes. Starting at an edge traps solution in the middle.
12. Trim with a fresh blade against a straight edge.Leave about a millimetre gap from the frame so the edge is not lifted by the seal. A blunt blade drags and tears the film.
13. Do a final squeegee through a cloth.A lint-free cloth over the squeegee protects the film's surface while you work out the last of the solution, particularly at the edges.
14. Expect some haze and small bubbles for a few weeks.Residual moisture evaporates through the film. Small water bubbles clear within two to four weeks; dirt bubbles never do.
15. Leave it alone while it cures.Do not clean it or pick at bubbles for about a month. After that, clean with a soft cloth and mild detergent, never an abrasive or ammonia-based cleaner.

Tips

• Spend three times as long cleaning the glass as you expect to. Every bubble that never goes is a speck you left.
• Wet everything — the glass, the film and your hands. The film should float so you can slide it into position.
• Cut it 20mm oversized and trim in place. Pre-cut film almost never fits an actual window.

Common mistakes

• Applying dark solar film without checking glazing compatibility — It can cause thermal stress cracking in double glazing and may void the glazing warranty. Check the manufacturer's chart against your glass type and pane size.
• Rushing the glass cleaning — Every remaining speck of dirt becomes a permanent bubble under the film. Water bubbles clear in a few weeks; dirt bubbles never do.

If it doesn't work

Questions people ask