Thermal performance of windows and doors

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Thermal performance of windows and doors

Thermal performance refers the heat transfer between the building and the surrounding. It is a factor that is concerned how well a structure responds to heat flow between inside and outside of the structure. Thermal performance is measured in terms of heat loss. The heat loss occurs by three modes: convection, conduction and radiation:

  • Conduction: Transfer of heat within a body or between two bodies in direct contact. No material is physically moved during this type of transfer.
  • Convection: Transfer of heat between the surface of a solid and a liquid or a gas. This type of transfer involves movement via circulation.
  • Radiation: Transfer of heat by radiation between two bodies at different temperatures.

Whenever differences in temperature exist between surfaces, heat will migrate from the warmer area to the cooler area. This is true of all surfaces. Insulation products are designed to frustrate the transfer of heat across the material itself.

Thermal insulation

Thermal insulation is the method of preventing heat transfer between objects in thermal contact. The purpose of thermal insulation is to reduce or even eliminate heat exchanges between two bodies of different temperatures. It is used in many areas including building, industry, the automobile, the kitchen and the textile.

The thermal insulation of a building reduces the heat exchange between the interior of the building and the outside environment. It is the basic principle of the passive house . It traps warmth inside in winter and keeps the house cool in summer. Improved insulation reduces energy consumption.

Insulation materials limit the flow of energy (heat) between two bodies that are not at the same temperature. Greater insulation performance is directly attributable to the thermal conductivity of the insulant. That is, the rate at which a fixed amount of energy transfers across a known thickness of the material.


U value, also known as thermal transmittence, is used for measuring heat transfer rates in walls, windows, doors, floors and roofs. U-value, usually expressed in W/m2K (Watts per square metre per degree Kelvin) and describe the heat transfer, in watts, through a square metre of a building element such as a wall, floor or roof.

For example, if a window (2m x 2m=4 m2) had a U-value of 1.5 W/m2K, this window will conduct 1.5 W of energy per m2 for every degree (K) difference in temperature from one side to other side. So, if the temperature is 22 ºC inside and -1ºC outside, this is a 22ºC difference. The potential energy loss is calculated as follows:

= 1.5 W/m2K x 2 m2 x 22 = 66 W

For the same size window with a 1.2 W/m2K U-value, the enery loss is calculated as follows:

= 1.2 W/m2K x 2 m2 x 22 = 52.8 W

Measuring thermal performance of windows and doors

There are 3 types of U-values for windows and doors:

  • Uf – refers to the U Value of the window frame itself without glass.
  • Ug – refers to the U Value of the glass itself.
  • Uw – refers to the combined U Value of the frame and the glass combined. (Sometimes Ud is used for doors instead of Uw)

To calculate the U value of a building part such as a window or door, you need to know the U-value for the window frame (Uf) and U-value for the glass (Ug). The U-value for a whole window or door (Uw) combines the Uf value and Ug value.

The Uf-value will depend on the design of the frame profile, the material used, the number of chambers in the profile, the type of thermal
break used (aluminium windows), and the type and number of glazing gaskets and weather gaskets.

The Ug-value will vary depending on the specification of the sealed unit. Variables include the types and thicknesses of glass used, the type of spacer bar, the size of the gap between the panes and whether the space between the panes of glass is flooded with gas (usually Argon and sometimes Krypton).

Combining the Uf- and Ug-values will give a Uw-value which is a measure of heat loss for the whole window. A lower U-value signifies a well-insulated building whereas a higher value indicates that the building has a poor thermal performance.

Reducing U-Value

When choosing a aluminium window or door, you should consider the U-value. The lower Uw value means the better thermal performance of the window or door. Improving the U-value means reducing the transfer of heat by conduction, convection and radiation.

In order to select the higher thermal performance for the window and/or door, you should consider the following points:

  • Lower Uf value for frame (Slimmer profiles has better Uf values)
  • Lower Ug value for glass (Low-e coating, argon gas between glass panes, triple glasses etc. increase thermal performance)
  • More glass area (Glass Ug value is usually lower than profile Uf value. So when you increse the glass area, you can decrase overall Uw value for windows)

Regulations for windows and doors

Building Regulations state that certain standards must be met when a window or door is replaced. Since April 2002, all replacement glazing has come within the scope of building regulations.

Building Regulations Part L documents state what an acceptable thermal performance is for all building elements, including windows and doors.

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