On request to explain some of the many different factors
and terminology, I have divided the discussion into eight sections to keep it
When we look at the specifications for the various films, offered on the retro market, they can be very confusing to the layman and/or deliberately misleading, regarding percentage infrared, visible light and ultra violet transmission.
2. RSA CLIMATE
South African climate is very different because we have eight months of warm to very hot weather where cooling is important, and four months of cool to cold weather. (Heating season).
3. BUILDING ORIENTATION
In South Africa, we are in the Southern Hemisphere and windows of importance are facing east, west and north in comparison with east, west and south for the USA and Europe. Our north facing windows will receive more radiation and therefore needs infrared protection. Dual or triple insulation of windows in RSA is definitely not necessary and will be an “overkill”. It is important to reduce heat, entering through windows and overburdening the air conditioning, but also to reduce heat loss from buildings during the cold season.
4. SOLAR ENERGY SYSTEM
The Solar Energy system is the major player in this whole story but there are many people who do not know how it functions. Energy, created by the sun, comes to Earth in various wavelengths that vary from short to long. This is called the Electro Magnetic Spectrum (E M Spec.). The E M band has the following:
rays – short wavelength
waves – longest wavelength.
solar energy that results from the Solar Spectrum, can be controlled by window
film. It is divided in three distinct bands:
Roughly 53% of the sun’s energy reaches Earth as Infrared waves.Approximately 44% is visible light and3% is Ultraviolet.
Infrared is not the total solar energy (radiant heat) that transmits through a window, as visible light and ultraviolet light contributes the remaining 4%. When you look at the specification for a window film, the data on infrared rejection will be important to you. With modern spectrally selective film, that regulates specific wavelengths of the E M spectrum, it is possible to cut out Infrared in part or up to the total of the IR band and then state that you have removed 80 to 90% of radiant heat. But this will not be the total radiant heat percentage, as Visible light and UV light
47% and therefore the “80 to 90%” only represents 40 to 45% of heat reduction.
This can confuse people. As there is no standard (International or local) for IR measurement of solar films, different suppliers will interpret performance differently and specifications will vary widely. To simplify selection, it is better to look at the SOLAR HEAT GAIN Coefficient (SHGC) which is a standardised performance measurement, recognised by for example NFRC and other building control standards institutions.
5. SOLAR HEAT GAIN COEFFICIENT
It measures how well a product blocks heat from the sun entering a building. SHGC is expressed as a number between 0 and 1. The lower the SHGC, the better a product is at blocking unwanted heat gain. The blocking of solar heat gain is important during the summer cooling season. An object with high SHGC, will transmit more solar heat through a window than one with lower SHGC.
Please note that SHGC will be affected by factors such as climate, surface orientation and external obstructions.
The ability of a product to absorb certain types of energy (specifically Infrared) and radiate that energy through itself and out of a room. A product with high emissivity will transfer a large percentage of IR energy from a warm room, outside to the cold air. Lower emissivity and conductivity will allow a lower rate of heat loss. Plain window glass has high emissivity, and will radiate heat at a high rate from the warmer to the cooler side. (Out in winter and into the room in summer.)
7. RECOMMENDATIONS and
We do not have performance standards as such in South Africa and have to accept whatever is passed our way from overseas. We also have to accept performance warrantees, given by suppliers, but without proper technical backup, it is difficult to know whether the product performance (heat control) activity will last as long as the suppliers product warrantee. Product appearance may well comply, but the chemicals in the film, that do the heat control function, may change through evaporation, reaction, oxidation, etc. These changes may not be visible or measurable.
8. GENERAL COMMENTS
When a person has heating or cooling problems in a domestic building, this can be solved easily by retrofitting of Infrared window film that will control heat transmission into the house in summer and loss of heat during the colder months. Low-E Glass or film, although expensive, will also function well. However, obtain clarity re active function of warrantee.
In the business world of large buildings, it is more complicated to solve heating or cooling problems. When making retrofit investment decisions, you should have a full understanding of the impact of any action taken, to avoid false investments. For example, applying dark tinting on glass to reduce heat, may result in poor visibility. More lighting will have to be installed, and that will increase heat generation inside the building, increase the electricity bill and the air conditioner load.
It is advisable to have a building energy audit done by a reputable specialist company, who will take all aspects into consideration before making a recommendation.