- All window films absorb heat
- Heat absorption increases the glass temperature and induces stress
- Some glass / film combinations should be avoided
The reason that glass stress is a problem is that glass doesn't like tensile forces. If a window is in the sun then most of the window is being warmed. However the edges are inside the frame and are cool. This means the centre of the window is expanding and stretching the edges which is a tensile force. Therefore films must be selected carefully.
Window / film combinations that aren't normally at risk
- Toughened or tempered glass (this should be marked on the glass)
- Most residential sized windows
- Most reflective films
- Films with low solar absorption
Factors that increase thermal glass stress
- Rough edges - this can be poorly cut glass or just a chip
- Laminated glass
- Low-E Glass
- Double glazing
- Large sheets of glass
- Unusual shaped windows
- Thick glass
- Partial shading from trees, building structures or window frames
- Hardened sealant
- Internal shading that is dark or non-ventilated
- Internal structures that trap heat at the top of the window such as a dropped ceiling
- Air-conditioning that blows on the glass
- Dark non-reflective film, paint or curtains
- Wired glass breaks easily and can only have external reflective film, although we would recommend new glass.
The risk of breakage increases if these factors combine
- In most cases a site visit is required.
- SolarGard have produced a film to glass guide to help with selecting a suitable film.
- Both 3M and Solar Gard can provide a stress analysis report. The collection of data allows computer modelling to predict the expected stress and the suitability of particular films. The significant item of data that must be supplied by the customer is a history of breakage. It is critical on commercial jobs that we find out if there has been a history of window breakage.
If stress is an issue we would recommended products with solar absorption near or below 40%
Related Products:
Double Glazing and Heat Stress
Please read our article on double glazing for information on the extra stress factors involved.
Related articles:
- Reflective Film
- Double Glazing
- Laminated Glass offsite at Wikipedia. Note comment about blocking 99%UV is not true for most laminated glass available locally.
- Toughened Glass offsite at K&K Glass
Solar Gard recommendations of Film to Glass compatibility