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Windows: How to choose between double and triple glazing?

The choice of windows is a crucial element when building or renovating a house. It will considerably influence the thermal comfort of a house and therefore its energy consumption. So how to choose between double and triple glazing?
Focus on glazing composition

First of all, let’s define the composition of the glazing: a double-glazed window consists of two panes of glass, separated by a spacer which may be filled of air or gas. Spacer composed of gas improves the insulating performance. The principle of triple glazing is the same: three panes of glass separated by two spacers.

As for single glazing, it is now banned from new buildings and renovations because it is particularly poor in terms of thermal and acoustic performance.

Identifying the performance of a window 

Window is such an essential element in the consideration of the energy performance of a house, that it’s one of the pillar elements of passive construction, among, insulation, airtightness and mechanical ventilation.

To identify the thermal performance of a window, it is necessary to refer to the U-value indicated by the manufacturers. Three U-values, all expressed in W/(m²K), must be considered:

  • Uw value for the performance of the whole window
  • Uf value for frame performance
  • Ug value for glazing performance

To ensure a high-performance window, the Uw value (glazing + frame) is the most important factor to consider. The lower the value is, the more insulating the window is.

Which glazing should I use in Portugal?

In Europe, the Passivhaus Institute recommends a Uw value of 0.80 W/(m²K), with triple glazing, for passive buildings located in areas with cold and temperate climates (North of France, Belgium, Germany, the Netherlands,…), and quadruple glazing for countries with cold climates (Sweden, Norway, Finland,…). In countries with warmer climates, such as Portugal, South of Spain and South of Italy, Passivhaus recommends double glazing with a Uw value of up to 1.20 W/m²K.

In addition to U-values and climate, it is important to consider a range of factors that have a direct impact on the thermal performance of a window : 

  • Its orientation
  • The solar factor of the glazing
  • The shape and size of the window
  • The proportion of the frame
  • The frame material

Finally, installation is a very important step to ensure optimal energy performance in the end. It is essential to make sure that the size of the frame is perfectly adapted to the wall framing, to ensure air and water tightness to avoid any infiltration and finally to not forget about thermal bridges.

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