What is the U-value of Aluminium?

What is the best aluminum window

The PVC window and door market presents a great variety of manufacturers and products. There is no doubt that the breadth of supply is a good thing. However, it is sometimes difficult to discern which of the many products on offer will be the most suitable for us. Here are some questions to ask when choosing windows.

Energy efficiency is undoubtedly one of the key properties to consider when buying windows. To find out the thermal insulation capacity of a window, check the value of its thermal transmittance. Its symbol is the letters Uw and the unit W/(m2K). The Uw coefficient expresses the heat losses suffered by our house, i.e. how much heat “escapes” through the window. Therefore, the lower the value of the thermal transmittance, the greater the thermal insulation provided by the window. For example, a window with a Uw coefficient equal to 1.3 W/(m2K) is more efficient than a window with a thermal transmittance equal to 1.8 W/(m2K).

Aluminum windows thermal insulation

Thermal conductivity is the ability of a material to transfer heat through it. Thermal transmittance is the measure of heat flowing through a building system. It is the inverse of “thermal resistance”, which relates the conductivity and the thickness of the layers that make up the system.

A thermal bridge occurs when a particular element (the frame of a window, for example) has a higher transmittance U-value than its neighbors (the walls). This situation means that heat, which always seeks the easiest escape route, tends to escape through these elements. In other words, thermal bridges are highways along which heat can escape at a higher speed.

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These images correspond to a comparison between aluminum carpentry with and without thermal breaks. In the first one, heat finds it very difficult to pass through the RPT. Two faces can be seen, one hot (inside) and the other cold (outside). However, in the next one, the heat distribution remains uniform, with the internal face of the carpentry at 17ºC.

Aluminum thermal transmittance

To check the new factor involved in the calculation of the global U of windows and skylights we go to section HE1. We look for table 3.1.1.a -HE1. This table shows the new limit values of thermal transmittance of openings. They are much lower than the values of the 2013 version.

The louver box is the new factor involved in the calculation. Therefore, in addition to reducing the overall U limit value, it must include the thermal transmittance of the louver box (W/m²K) for its calculation. This can be checked in the supporting document DA DB HE/1. Exactly on page 12 and 13.

One of the parameters that determine its thermal behavior is the overall thermal transmittance of the window. This value depends on the thermal transmittance of the frame and also on that of the glass. If the window is also composed of opaque panels, their thermal transmittance will also influence the overall value of the window. In this post we explain how to calculate it.


In reality, it is not fair to give an absolute answer to this question. PVC and aluminum are two very different materials, with different values capable of solving different needs. So, first of all, we always recommend that you get advice from a good professional in the sale and installation of windows who can help you determine the characteristics of your home and choose the option that will allow you to enjoy the best quality of life.

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Without getting too technical, transmittance represents the rate at which the window lets heat and cold in and out. Therefore, the lower the transmittance, the greater the comfort.

As we have anticipated, the selling price does not seem to us to be the most relevant aspect in this comparison. A product is not expensive because its price is high, but because it does not offer the equivalent value that justifies the investment.

Moreover, the selling price does not reflect either the costs or the future savings. A window with a higher price may be the cheaper option if its maintenance cost is very low and if it is helping us to save on our heating or cooling bill every month.

What is the U-value of Aluminium?
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