- Heat and glare can still be a problem with CLEAR double glazing
- Many dark films can't be used on double glazing
- Avoid double glazing of hot aspects
- Low-E film is an alternative
Double Glazing Explained
Double glazing acts as an insulator and not as a shading device.
As an insulator, heat passes through the window much more slowly than usual. Although this is mainly a benefit in winter the insulating property is also an advantage on very hot days as the heat from the outside enters more slowly. This contrasts directly with sun control Window Film which works as a shade rather than an insulator.
Double glazing, also known as an Insulated Glazing Unit or IGU, is made of two sheets of glass sealed around the perimeter with space in between filled with an inert gas. The inert gas reduces convection inside the IGU to help slow the transfer of heat.
Double glazing is quite effective as an insulator, more so than Low-E film. It is also good at reducing noise. When it comes to summer performance, window film wins hands down, unless the double glazing is made of special glass treated with a coating similar to window film. Although available locally, most high performance double glazed units are imported and quite expensive.
Most double glazed units are made of clear glass which allows the winter (and summer) sun to shine in, yet reducing the heat transfer caused by temperature differences on either side of the window. This helps them to work as a heat trap which can be great in winter but not so good in summer.
Double Glazing and Heat Stress
Unfortunately many of the films that are used with single glazing are not suitable for double glazing. This is because the heat trapped between the two pieces of glass can cause excessive expansion and result in breakage. This is known as heat stress. In particular, putting dark films on double glazed windows should be avoided. This is especially a problem if the glass is laminated and less of a problem if it is toughened.
In most cases a site visit is required. Solar Gard® have produced a glass to film guide to help with selecting a suitable film. 3M use computer stress analysis. The collection of data allows computer modelling to predict the expected stress and the suitability of particular films. This is available for 3M films only. The one 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.
Dealing with heat and glare on double glazed windows
Since clear glass is mostly used, many people find that they still need something to deal with solar heat and glare.In many cases a reflective film can be used. I usually recommend a product with solar absorption near or below 40%. The Solar Gard Sterling™ family of products such as ST20A, ST40A, orST50A are good options, as is Silver 20. For glare reduction Silver 20 or possibly Slate 10 can be considered.
For heat reduction another option is Solar Gard LX70. This is a very light film with modest heat reduction, low absorption, low reflection but a high price.
Related articles:
- Insulation Film
- Glass Stress
- Laminated Glass offsite at Wikipedia. Note comment about blocking 99%UV is not true for most laminated glass available locally.
- Toughened Glass offsite at Wikipedia.
- Fading