In the UK, one of the most important factors that building designers need to know about windows is Part L of the Building Regulations. This was updated in 2013 and Part L now requires windows to have a minimum level of thermal performance for all buildings. This performance must be given as either a Window Energy Rating, which is something that is laid down by the British Fenestration Rating Council, or as a U-value rating. This rating is sometimes called a Uf value, a Ug value or a Um value. For example, where the Uf value relates to the thermal transmittance of an aluminium door or window frame, the Ug value relates to the glazing element itself. Of greatest importance to all building professionals is the fact that whichever U-value is used, the requirement for it to get gradually lower has continued year on year. Since 2002, for instance, the thermal properties of doors and windows with an approved L1B status, the documentation which covers the energy efficiency existing dwellings, has required a drop of some fifty percent from 3.3 Watts per m2K to 1.6 Watts per m2K. In addition to such exacting standards, the aluminium façade industry now faces even more change with the introduction of a new standard method of calculation for these values. What does this mean?
The Calculation of Thermal Transmittance
Traditionally, the calculation of thermal resistance in windows has been conducted by measuring the thermal resistances of each element of the window, its R-values, and dividing these by their thermal conductivity, or lambda values. When all of the R-values of a product's materials are added together including things like air gaps, PA insulating bars and ABS insulating bars, it is then possible to work out the so-called reciprocal value, or 1 divided by the sum of the R-values. This reciprocal equates to the aforementioned U-value rating.
In some cases, window products are tested in so-called hot boxes in laboratories to truly determine their thermal retention properties, especially where things like PA insulating bars are added.
However, nowadays software modelling is increasingly commonplace and is likely to be more so, given the European standard changes. The algorithms of the simulation software are defined in the Standard EN ISO 10077-2. The key part is the model for the energy transport inside cavities by radiation and convection, or conduction respectively.
Nothing is Changing – Yet
"Change is the law of life and those who look only to the past or present are certain to miss the future."
John F. Kennedy. The calculation standard EN ISO 10077-2 should be revised this year, the formal vote will take place at the end of November. The main point in this revision is the introduction of an additional calculation method, which models the radiation heat transfer inside cavities physically correct ("radiosity method").
In a transition period calculations done by both methods are accepted. The concentration on one calculation method, as well the validity of old calculations should be fixed in the standard EN 14351-1.
What Will the Future Hold?
It is to be expected that the radiosity method for calculating the thermal properties of doors and windows will get mandatory sometime in the future.
Innovative firms will profit from the new method, because it delivers more accurate values, mainly for polyamide bars that are fitted more than 2 mm away from each other. Moreover, lower U-values are commonly derived from this method of measuring insulated aluminium frames, so the likely changes are bound to suit producers of highly engineered aluminium window products who are seeking ever more improvements to the thermal efficiency of the building designs they contribute to. So if you want to be ready for the future, you should check that your software supports the radiosity method.