White roof foil for reducing energy demands of buildings

FatrafolNews

Besides photovoltaics the Fatra, a.s. Company also follows trends in the field of reducing energy demands of buildings. An easy and affordable method is represented by the concept of “cool roofs“. White roofs exhibit an increasing trend especially in the U.S., where attention has been focused on research of the influence of characteristics of roof lining insulation surfaces on the environment for some time.

Cool roofs with white foil

It is actually quite simple logic an analogy of which everyone knows. Put a black T-shirt on on a hot summer day and let the sunshine warm you. Then, do the same with a white T-shirt. In the former case you will certainly be considerably hotter and you will also sweat more, i.e. cool yourself.

Similarly, owners of cars with a black body know that if they touch the body in hot summer, it will probably be very hot and it will be much more demanding to cool the interior than in the case of white cars.

Reflectance of roofs with white room foil

The same simple effect can be observed on flat roofs. By selecting a suitable material of the roof lining insulation with a very light colour you can achieve high reflectance of sunrays from the roof surface as well as a considerable reduction of the amount of heat that passes through the roof to the interior, which gets heated excessively. This means a reduction of energy gains of the building and thus reduction of direct costs of cooling, extension of the service life of air-conditioning systems and also reduction of investment costs associated with the purchase of air-conditioning systems. Increased reflectance of light, i.e. UV radiation also has a positive influence on the service life of roofing materials in general. Indirect consequences are no less interesting.

White roof foil reduces energy consumption

A reduction of energy consumption results in a reduction of the need of electricity production, thus to a reduction of production of CO2, which is the most significant greenhouse gas. On a foreign server I found interesting statistics saying that possible use of roofs with high reflectance in the global scale would correspond to a reduction of CO2 emissions by approx. 24 gigatons in the course of the service line of the roofs. This corresponds to the sum of 600 billion of USD saved on the production of CO2 emissions. By reflecting heat back to the atmosphere “cold roofs” also contribute to a better atmosphere in large towns.

White roof foil reduces the temperature

It is a well-known fact that in large towns that contain a lot of surfaces with a high capacity of absorbing heat and solar radiation the temperature is several °C higher during the summer months as compared to less populated areas. Surfaces that absorb heat during the day irradiate it at night and the air does not cool down sufficiently.

Massive use of large roofs with a surface that is highly reflecting for light and heat can lead to a certain cooling effect in towns. This effect can also be indirectly associated with an improvement of the atmosphere in towns as a reduction of the temperature also means a lower risk and concentration of smog. In technical terms the ability of hydro-insulating materials to reflect solar energy is called "solar reflectance". Reflectance can be measured both in the range of visible light spectrum and in the ranges of infrared and ultraviolet spectrum. Reflectance represents fractions of reflected light from the surface of the evaluated material compared to total light falling on the evaluated surface.

Reflectance of a roof with white roof foil

Reflectance of white surfaces is approx. 0.8 to 0.9. Another measurable parameter is infrared emittance. Both the coefficients can be expressed with one summarizing coefficient called the SRI coefficient. The SRI (Solar Reflectance Index) coefficient is a value that defines the ability to reflect solar heat at a small increase of temperatures. The definition says that the SRI value of a standard black object with the reflectance of 0.05 and emittance of 0.9 is 0 while the SRI value of a standard white object with the reflectance of 0.8 and emittance of 0.9 is 100. The higher the SRI index is, the better the parameters from the point of view of "cold roofs".

In general, the SRI value can even be negative on the one hand, but also higher than 100 on the other hand. The Fatra, a.s. Company has responded to this new trend by having its selected products tested in the BAVARIAN CENTER FOR APPLIED ENERGY RESEARCH in summer 2000. Foil samples based on TPO Fatrafol P916 in the RAL 9003 colour shade and samples of Fatrafol 810 PVC-P foil in the colour shade of RAL 9010 were tested.

Tests of white roof foil

In the case of both the evaluated roof foil samples the tests proved a higher value of the SRI index than 100. The Fatrafol P916 foil achieved the value of 103 and the Fatrafol 810 foil even 108. These excellent values mean for the Fatra, a.s. Company and all the partners that are involved in this trend of building design and development a huge potential based on a simple and cheap solution of reducing energy demands of newly built or existing buildings.

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