Calculating the thermal transmittance of windows: the new radiosity method

//Calculating the thermal transmittance of windows: the new radiosity method

Calculating the thermal transmittance of windows: the new radiosity method

metodo della radiosity - 2We spoke about the radiosity method – in the ISO 10077-2:2017 standard – during the meetings organised in the last months of 2017. It is not mentioned often, but we would like to talk about it again briefly because this new method could change the rules of the doors and windows market, especially aluminum ones.

What is radiosity?

Radiosity is a global illumination algorithm used during 3D computer rendering. It was discovered in the 1950’s to study heat transfer, and was then refined in the 1980’s and 1990’s to better simulate illumination phenomena in computer graphics, obtaining much more realistic results compared to traditional methods (see the 2nd picture).

How does it work?

Radiosity method - EkuThe method is based on thermal radiation, since it consists in the calculation of the amount of energy transferred between surfaces. To simplify the calculation, one assumes that all surfaces are perfectly diffusive.

The surfaces of the scene to be rendered are each divided into one or more smaller surfaces (patches) and the algorithm considers one patch at a time. At each pass, the light that one patch receives from another is calculated: a part of the light is considered absorbed, while the rest is reflected in the scene. The more passes made, the greater the precision and realism of the final result.

Benefits for aluminum doors and windows

The radiosity method allows to calculate heat transfer inside the non-ventilated cavities of the profiles, defining the quantity of light absorbed and emitted by the various surfaces. This finite element calculation method provides more accurate and more realistic thermal transmittance values.

At the Fenstertage in Rosenheim, a study on the effects of introducing this method into the ISO 10077-2:2017 standard was presented. The first analyses show basically insignificant results as regards wood and PVC systems, but improvements for aluminum systems.

metodo della radiosity - 4Fixture junction configuration being equal, the radiosity method allows to assign aluminum doors and windows an advantage (delta) ΔUranging between 0.1 and 0.3 W/m2K, according to the assessments made by the ift Institute of Rosenheim.

The overall advantage for thermal transmittance Uw drops to a ΔUw equal to 0.1 that may seem negligible, but in practice may prove very useful when the overall thermal transmittance is close to the threshold values. For example: if one is close to Uw equal to 1.4 or to 1.0 W/m2K, that 0.1 W/m2K may prove fundamental in complying with the law and in not losing the market.

The radiosity method widens the horizon in terms of Research & Development and innovation. It could open new doors in the development of profile geometries, in the configuration of the profile’s thermal core and in the use of new materials.

The Uf results obtained with the 2012 standard remain valid, along with those obtained with the 2017 edition via the updating of finite element calculation programs (Flixo, Bisco, etc.). On the Flixo website, for instance, there is an interesting calculation example using the radiosity method (in English). According to the Swiss firm, in an aluminum fixture the Uf value can improve by up to 15%.
At this point, all that is left is to update the ‘queen’ of doors and windows standards, EN 14351-1.


For more information and details, contact our technical department
2018-06-29T00:05:22+00:00giugno 29th, 2018|News|0 Comments

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