5.5 Window systems

5.5.1 U value

The U value of a building component expresses the amount of energy transmitted from the warm side to the cold side. The lower the U value, the less energy is transmitted. It is often the aim to reduce the U value of building components in order to reduce the heat loss, and thereby the heating demand, of the building.

The U value is expressed in W/m2K. In glazing constructions, heat is transferred from the inside through the insulating glass unit to the outside by radiation, convection (warm air rises, cold air falls) and conduction. The U value for windows is denominated Uw and is a combination of the frame Uf value, the glazing Ug value and the cold bridge effect between glazing and frame, ψ. To reduce the convection loss inside the glazing cavity, the cavity can be filled with gas, e.g. argon or krypton. To reduce the radiation heat transfer, low emissivity coatings can be applied to the glass panes facing the cavity. Low emissivity coatings are thin layers of metal, invisible to the eye but with emissivity values down to almost 0. A standard glass pane has an emissivity of 0.84. By adding internal or external shading devices to the window, the U value can also be lowered by reducing the radiation to the sky and by improving the thermal resistance. The optimum cavity thickness is about 15 mm for argon and about 10 mm for krypton. VELUX roof windows are usually made with argon.

"Dynamic window systems with VELUX ACTIVE Climate Control improve both the winter and summer energy balance of window systems."

U value for sloped windows (roof windows)

As roof windows are installed in sloped constructions, the Uw value will be higher than for windows installed vertically. The convection in the gas between the glass panes is minimum for a vertical glazing, increases when the glazing starts sloping, and is at maximum with horizontal glazing. Convection also depends on the type of gas and cavity thickness. In general, the cavity is independent slope when the cavity thickness is around 10 mm or less. This has an effect on the energy performance of a building, since the heat loss through the roof window is increased due to the larger Uw value. On the other hand, the solar gain and daylight are also increased. Roof windows are also exposed to a larger part of the sky than facade windows and are normally installed without any constructive shadows, thus increasing the amount of daylight and solar gain, as seen in section 1.5.3.

Traditionally, the U value is the single parameter used for evaluating the energy performance of windows. It is common practice to declare Uw for roof windows at 90°, i.e. as facade windows.

Even though heat transmittance increases with increased slope, passive solar gains increase even more. So the vertical value leads to fairer indication of the performance than the sloped value. VELUX is striving to have the U value of windows replaced by energy balance (see section 5.5.3).

5.5.2 g value

The g value (total solar energy transmittance) is quantified by the amount of solar energy entering through the glazing. The g value is defined as the ratio between the solar gain transmitted through the glazing and the incident solar gain on the glazing. g value will be in the range of 0–1 (or 0 – 100%).

Dynamic window systems

The g value of a combination of window and accessories (for example solar shading) is dynamic and can be changed according to indoor and outdoor conditions. The shading can be controlled by the user or automatically with VELUX ACTIVE Climate Control.

Coatings

By using coated glass, part of the solar gain is blocked by reducing the g value. Depending on the type of coating, different parts of the spectrum can be blocked. For solar protective coatings the goal is usually to block as much as possible of the near-infrared radiation and allow as much of the visible radiation as possible to penetrate the coating. For clear coatings, the goal is usually to allow as much of the total solar radiation as possible to penetrate the coating. Even clear uncoated glass will reduce some wavelengths more than others. Coated glass will always affect colour perception indoors.

5.5.3 Energy balance

The term energy balance is used to describe the energy characteristics of a window. The intention is to communicate the balance between solar gain and heat loss. Energy balance is calculated as the sum of usable solar gain through the window during the heating season minus any heat loss. Energy balance is a more accurate way of describing the energy characteristics of a window than the U value alone, as energy balance includes both Uw value and g value to provide a more complete picture.

Methods

In general, the energy balance of a window is evaluated by determining the amount of useful solar gain during a year and subtracting from that the total heat loss through the window. However, since solar gain during the heating season contributes positively to heating demand, it may have a negative effect during a possible cooling season.

"Roof windows have in general a better energy balance than facade windows during the heating season"

The higher the energy balance, the better. Energy balance is quantified in kWh per m² of window.

The amount of solar gain has to be determined for both the heating and the cooling season. For the heating season, the useful solar gain is determined by a utilisation factor multiplied by the amount of incident solar gain on the window. It is very dependent on the building type and location.

If a building is well insulated, the utilisation factor is low (about 70%), while for a poorly insulated building it is high (about 90%).

The amount of solar gain reaching the window is dependent on the slope of the window and its orientation. The total heat loss from a window is dependent on Uw value and air permeability. The heat loss through a window is found for both the heating and the cooling season, and determined by the number of heat degree hours for a year where there is heat loss in the heating and the cooling season. It is dependent on the building type (insulation level) and climatic conditions. The energy balance of windows for the heating season can be expressed as:

Energy Balance = Isolar x gw – D x (Uw, slope + Lair permeability) [kWh/m2]

Lair permeability expresses the heat loss through the window due to air permeability.

In some European countries (UK, DK) a simplified definition of energy balance for facade windows during the heating season has existed for some years. It is important to note that the energy balance for roof windows during the heating season is generally better than the energy balance for facade windows, which is why it is important that they are distinguished from each other. 

"The use of energy balance ensures that the best available window product can be chosen. The higher the energy balance, the better the window performs"

The simplified method for energy balance considers only existing buildings with a specific distribution of windows per orientation. This method is shown in (Kragh et al., 2008). In the 2010 Danish Building Regulations (Danish Enterprise And Construction Authority, 2010), energy balance for windows is recognised as a legislative requirement for window replacements.

The VELUX Group is convinced that energy balance is a more correct and robust metric for the performance of windows than Uw value and it is working for the acceptance of a standardised method for determining Energy Balance (ISO, 2009).

​Figure 5.5.1 Energy balance for roof windows for each orientation during the heating season based on the method proposed for the Danish 2010 Building Regulations (Danish Enterprise and Construction Authority, 2010).

"For existing buildings the tendency is that the g value is at least as important as the U value for the energy performance"

​Figure 5.5.2 Energy balance for roof and facade windows with different pane types for the heating season, based on the current draft for the Danish 2010 Building Regulations (danish Enterprise and Construction Authority, 2010).