Heat transfer through Glazing
Heat transfer through glazing is a complex issue.
While the vast majority of manufacturers and installers are ethical in their performance reporting, some misleading claims have entered the market. When considering window film, WFAANZ advises you keep the following in mind...
- Temperature reduction
WFAANZ considers unsupported claims of internal temperature reduction without calculation or appropriate reference to be misleading.
The purpose of most window films is to reduce solar energy entering the window. Any reduction in solar energy in a building following a film installation can be calculated simply and with reasonable accuracy using local weather files. Glass + film performance can be calculated by the manufacturer, and local weather files provide levels of solar energy hitting the window.
Calculating ambient temperature reduction, however, requires complex calculation and extensive building information. Size, shape, air conditioning system details, building materials, occupant numbers, occupancy patterns throughout the day and geographic location must be considered.
Interaction of all aspects of the building contribute to ambient temperature at any given time. The reduced solar energy load through the windows is not enough information to calculate ambient room temperature.
- Infra red heat or infra red (IR) energy reduction
WFAANZ considers the promotion of infra red rejection values in isolation is misleading to consumers.
Energy (or light) from the sun is divided into three wavelength categories - ultraviolet, visible and infra red. All sunlight brings heat, so to properly calculate how much solar heat a window film can reduce, the sun’s whole spectrum must be considered.
Infra red from the sun brings about half the heat, the other half comes mainly in the visible light wavelengths and a small amount in UV. To report only a film’s infra red reduction is therefore only telling part of the story. A particular window film may have an infra red reduction of say 90%, but the actual net heat rejection may be only 50%.
The true indication of heat reduction through glazing is the film’s Total Solar Energy Rejection (TSER). This is calculated across all the sun’s wavelengths and even includes the portion of heat that is absorbed by the glass and reradiated inwards. Anyone promoting an infra red performance value must also declare the film’s TSER.
- Angled heat rejection
WFAANZ does not consider the mentioning of angled heat rejection is necessarily misleading to a consumer - it can be a point of interest - but consumers should be aware that the phenomenon is present for all glass and window films.
When window films are measured with laboratory equipment for their solar performance, the heat source is directed straight on or perpendicular to the sample. This is the reporting standard.
A consumer may also want to know the angled heat rejection.
What this means is that at say 2pm when the outside temperature is hottest, the sun is high in the sky and the sunlight striking the glass is at an angle, not perpendicular. The solar energy reflection of the glass + film in this case is increased, so the total solar energy rejection of the glass + film increases.
This is not a unique property of window film. Clear glass alone, with a very low reflection normally, can be highly reflective when the light source is at a sharp angle.