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When specifying a U-Value for glazing whether windows, doors or facades it is extremely important to choose and specify the correct assembly of components for the window system, glass and spandrel panel type etc, that suit the grid size and not just a system that is classified as being Document L approved.
Avoid using high performing glass with a low performing aluminium framing system such as a Document L approved 45mm standard aluminium window system because the method used to evaluate a system of this type is dated and gives only average U values.
To achieve a more accurate evaluation for the U-value of a proposed design, the overall areas of aluminium framing along with the areas of glass and spandrel panels, plus an allowance for the spacer in the double glazed units need to be calculated and collated a total area values. The combined total of these values then needs to be divided by the total area to get the average U value for a specific façade or schedule.
Click image to enlarge.
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An example of this type of calculation is illustrated below and demonstrates the collation of figures for a grid of 2m x 2m based on a façade of 10m x 10m with a glass centre pane U-value of 1.1W/m²K using 50mm wide curtain wall mullions and transoms. (Please use this example for guidance only as different parameters alter the values).
When choosing an aluminium system for a building, attention should be given to the thermal performance of the frame because if you choose a system with only average U-value performance substantial reductions in overall thermal performance can be expected. Where possible specify window, door and modular curtain wall systems that are assembled with a polyamide thermal barrier. These systems perform better than those assembled by a standard pour and cut thermal barrier, as they provide superior insulation against cold bridging.
Using up to date thermal analysis software packages, APA Systems Ltd can help you choose the right type of aluminium system and glass to meet design specifications and regulations. One software package evaluates the aluminium framing system along with producing condensation prediction calculations, while the other evaluates the façade or window schedule and calculates an overall U-value for it. |
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