Determining the Heat Transfer Coefficient of Aluminium Windows: Hotbox or Software Simulation?

By Bogdan Grebenyuk, Product Manager at Thermevo
on May 28, 2018
The heat transfer coefficient of windows is usually given from a U value calculation, but how is this determined?
Although there are a number of ISO standards covering the thermal insulation properties of windows, one of the most-used current British standards is BS EN ISO 12567-1:2010, or Building Regulations Part L in England. Within the BS standard, window products are tested in a so-called hotbox which is a method that is also used for many other structural elements, such as walls and cladding.

The basic structure of a hotbox

Hotboxes are often large industrial machines which are finely tuned with different temperatures on either side of a division. In the case of a window, this is installed in the middle of the hotbox and how much heat is transferred from the warm side to the cold side can therefore be carefully measured.

According to Ronald Phillip Tye, this method is conducted in a, "conditioned laboratory space."
The hotbox is commonly set up with a 20 degree differential on either side of the pane. Simultaneous measurements of the temperature difference across the window's surface are then taken. From the multiple measurements recorded, a single U value calculation is determined so that different products can be compared on a like-for-like basis. For building designers concerned with the physical properties of structural products, hotboxes have long been trusted as a method for determining U values for exterior doors and windows accurately.

A U value calculation can also be garnered from the air temperature on either side of the window.

According to Bertil Jonsson of the SP Techincal Research of Sweden, "this requires the equipment to be calibrated, so that the coefficients of surface thermal insulance (Rsi, Rse) can be determined."
In this set up, the sum of Rsi and Rse equals the surface thermal insulation on the inside and the outside of the item under test, respectively. Whichever hotbox technique is applied, the process can be time consuming and technically challenging in many cases. It is universally acknowledged to provide very accurate results, but this must be considered against the downsides, which include the fact that it is often costly and that it can delay building projects which have specified specialist or bespoke window products.

Along with the U value of a section of glazing, designers need to consider the frame, too. U ratings that relate to the thermal insulation properties of a frame, for example, are called Uf values. Likewise, Ug refers to the U value of the glazing itself whilst Uw, which is probably of most benefit to the majority of architects and civil engineers, relates to the window product as a whole. When it comes to architecturally pleasing frames, such as aluminium ones, the U value calculation is of great importance because some metal frames perform poorly thermally. Nevertheless, high-end aluminium frames, fitted with a strong EPDM seal to the glazing and either a PA insulation bar or an ABS insulation bar within the frame itself will frequently produce exceptionally low heat transfer coefficients.

The alternative to hotbox: using software simulations to calculate the U value

Thankfully, all of the U values – Uf, Ug and Uw – can be worked out without the expense of trialling them with the hotbox method so that different insulation bars and seals can be tried out in differing combinations. Many of today's building designers prefer modelling software to predict the thermal transference properties of their window and door products to see how they will. Indeed, modelling software has been used commercially since first developed in 2002 by Allen and Aynsley.
Flixo approach: Finite Element Method for U(f) calculation

This approach often gives them a further advantage over testing real products in hotboxes , in that software can allow them to determine a number of heat transfer coefficients for aluminium windows and other sorts of products in various combinations. Thus, the thermal properties of an entire building can be modelled in advance, if wanted, allowing clients to gain a great understanding of the potential energy costs they will face once their windows and doors have been specified.

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