Window film for west-facing windows at home should be chosen by glass type first and darkness second. The common mistake is picking the darkest film for the hottest room, when a highly heat-absorbing film on ordinary glass, partly shaded by curtains or an awning, is a combination that can crack the glass. Once the glass type is identified, a suitable light film usually solves the heat problem without leaving the room dark all day.
Four in the afternoon, a west-facing apartment. The wood floor is hot underfoot, the air conditioning runs at full power and it still feels stuffy near the window, and the glare makes the TV unwatchable. In summer 2026, northern Vietnam's power load set a record 29,667 MW, and most homeowners in this position think of the same thing: put on really dark film and be done with it.
Hold on.
Why can film crack glass?
Glass does not crack because it gets too hot. It cracks because it heats unevenly.
When the afternoon sun hits, the middle of the pane heats up and expands, while the edge held in the frame stays cool. The cool part pulls the hot part into tension. According to a European Window Film Association document, the greater the temperature difference within a pane, the greater the stress, and a thermal crack typically leaves the edge of the glass at an angle of about 90 degrees.
Window film, especially absorptive film, makes the pane hold more heat. That is what it is for, stopping heat at the glass so less enters the home, but it is also what widens the temperature difference.
A few factors make this worse, according to that document and Pilkington's ATS-113 technical bulletin, updated in April 2025:
Partial shading on the glass. Shadows from an awning, an upstairs balcony or trees divide the pane into hot and cool zones.
Curtains hung close to the glass. Curtains trap hot air and push heat back into the glass.
Damaged glass edges. A chip on the edge is the starting point of a crack, and such damage is usually hidden in the frame.
Tinted glass and thick glass. These already absorb heat on their own.
Picture a west-facing unit with a balcony casting a shadow across the lower half of the window, and an owner who pulls blackout curtains close against the glass in the afternoon. Nearly every risk factor is present at once. In a case like this, film should absolutely not be chosen on instinct.
What type of glass does your home have?
This is the first question, and many quotes never ask it.
| Glass type | Thermal stress risk | Comments |
|---|---|---|
| Tempered glass (toughened glass) | Very low | EWFA states it withstands a difference of about 250°C and needs no further thermal stress assessment |
| Heat-strengthened glass (semi-tempered) | Low | Only special conditions need review |
| Ordinary annealed glass (float glass) | High | Compatibility must always be assessed |
| Tinted annealed glass, solar-control coated glass | Higher | Pilkington advises against applying film to single annealed glass with a solar-control coating |
| Insulated glass unit | Depends on construction | May affect the seals and the glass manufacturer's warranty |
| Laminated safety glass | Ask first | The documents we read do not cover this in detail |
Tempered glass usually has a small etched mark in one corner. If you see no mark, ask the window installer or the building management. Do not guess from the thickness.
One point worth making for apartment residents: the exterior of a unit may fall under the building's common rules. Strongly mirrored film changes the color of the facade, so ask the management before installing.
The risk is not zero even when done correctly; the EWFA document itself says so. Anyone who promises "guaranteed never to crack" has not read the documents.
How do you choose window film for west-facing windows?
Set your priorities before you look at film samples.
In a study or a living room with a TV, glare is the biggest problem. In a bedroom, it is heat and privacy. In a room with paintings, a wood floor or a leather sofa, fading is the concern, but remember that UV is only one of many causes; visible light and heat fade furnishings too.
A practical principle: the lightest film that still solves the glare. The reason is everyday life: in midsummer you hate the heat, but for the rest of the year, and every morning, you live with a room that has been made dark. A room with 5% film may need the lights on in the middle of the afternoon.
As for privacy, understand it correctly. Reflective film is "one-way" only when it is brighter outside than inside. After dark, with the lights on indoors, it reverses, and people on the street can see in very clearly. You still need curtains. The subtle point is that curtains should hang some distance from the glass, not close against it, so they do not push more heat back into the glass.
Film does not solve everything either. If your home has a metal roof or a west wall that soaks up heat, the glass is only part of the problem. Awnings, outdoor louvers, even trees that stop the sun before it reaches the glass, are always more effective than stopping it at the glass surface. Film is the solution when you cannot change the architecture. If you can add sun louvers, consider that option first.
The architectural film page describes the glazing survey process, and it is worth reading before you bring in an installer.
Is insulated glass worth filming?
Many apartments and new homes use double-pane insulated glass, and this is where you need a clear head. Pilkington states that solar-control film on the room side of an insulated glass unit only reduces glare and barely reduces heat, judging by the small change in the solar heat gain coefficient. The reason is easy to picture: the film holds heat in the inner pane, behind the gas layer, and that heat still radiates into the room.
So if your goal is less heat and your home has insulated glass, do not assume film on the interior side is the answer. You may be paying for a room with less glare, not a cooler one. The bulletin also warns against applying film to a Low-E surface and notes that heat can affect the insulated glass seals. So ask carefully first on this point, especially while the glass is under warranty.
What should you ask the installer before filming?
These six questions help you spot who really knows the trade.
- Do you check the glass type and glass edges before quoting?
- For this glass, is the film code you chose within the film manufacturer's recommendations?
- If the glass cracks after installation, who is responsible, and where is that written down?
- Can you put a test sample on one pane?
- Will the warranty on the insulated glass or the building's curtain wall be affected?
- Does the warranty card record the film code for each area?
The third question is the one many people hesitate to ask. The Pilkington bulletin states clearly that the manufacturer does not provide warranty coverage where aftermarket film is applied, and advises getting recommendations and commitments in writing before installation. If something goes wrong, you are standing between the glass supplier and the film installer. Knowing this in advance lets you get a clear agreement.
VEGO keeps project warranty cards by work item and area, with the film code tied to each zone. The warranty period goes by code, from 5 to 10 years depending on the code in the system catalog. The product catalog shows the available film lines, but choosing a code for home glass requires an on-site survey.
First step
Take three photos: the whole glass area, a corner of the glass showing the frame, and the shadow on the glass at 3 p.m. Send them with the dimensions through the contact page or call 0924.802.802. If you have already filmed the glass and see a crack starting at the edge, stop pulling the curtains close to the glass and see the support center for guidance.
