Window design in low-energy buildings

space heating demand decreases and both dynamically controlled shadings or glazing with solar-control coating could be used.

Dynamic solar shadings can be very costly to install and maintain, and might not always be the house owners’ choice when aesthetics are considered. Furthermore, when these shadings are not operated as intended, this can result in reduced occupant comfort. On the other hand, glazing with solar-control coating has a light transmittance which at maximum is twice the solar transmittance, which means that larger window areas are needed to allow the same use of daylight as with clear glazing or when dynamic solar shading is not active. Moreover, one should keep in mind that some types of solar-coating can give a slight tint to the glass, which might be undesirable. The decision on whether to use external solar shading or glazing with solar coating should be left to the building owner. In the end, it does not matter which option is used, as long as prevention of overheating is integrated in the design process from the beginning.

5.1.3 Climate-dependent daylight target vs. CBDM

When considering various room geometries that can achieve the daylight target without overheating, the use of a climate-dependent target (DFtarget) shows that glazing-to-floor ratios of approximately 17-25% are needed to achieve the specified daylight target for light transmittances of 0.7–0.5 in both north- and south-oriented rooms under the assumption that ideal window positions from perspective of daylight are used. The use of the climate-dependent target takes into account location, but does not take into account realistic sun and sky conditions because it is based on evaluation of the daylight target under a CIE overcast sky. Using the climate-dependent target, greater flexibility in the choice of window size and geometry was found in north-oriented rooms than for south-north-oriented rooms.

From comparison of calculations based on the use of the climate-dependent target with calculations based on CBDM, it was found that for the two orientations to have comparable daylight availability over time under realistic sun and sky conditions, either the glazing-to-floor ratio towards the south must be decreased (less ambitious target) or the glazing-to-floor ratio towards the north must be increased (more ambitious target). For south-oriented rooms, the use of the more ambitious target corresponds to the use of glazing-to-floor ratios as found by using the climate-dependent target. When the less ambitious target is used, glazing-to-floor ratios to fulfil the daylight target are found that are smaller than optimal from the perspective of heating demand. As the risk of overheating is close to insignificant in north-oriented rooms, the use of larger glazing-to-floor ratios poses no problem, so the choice to go for the more ambitious target for daylight availability seems an easy one to make. If low glazing U-values are used, the larger glazing-to-floor ratios needed to achieve the more ambitious daylight target might actually help reduce space heating demand. However, for high glazing U-values, larger glazing-to-floor ratios mean a significant increase in space heating demand and the use of a climate-dependent target might be a good compromise if we do not need the same amount of daylight in north and south-oriented rooms.

Apart from more daylight availability, the use of a climate-based daylight evaluation increases the choice of room geometry for both orientations. With the ambitious daylight target this advantage is still present, but less pronounced. It could, however, be concluded that the use of CBDM is needed to illustrate the ‘real’ space of solutions for both orientations. However, further studies, including on visual comfort in south-oriented rooms and the possible health effect of daylight, will be needed to determine how comparable targets for north- and south-oriented rooms can be set in homes. At this stage, the use of a climate-dependent target might also be considered a valid approach because architects and designers do not always have the knowledge or expert tools to calculate the available daylight using CBDM in the early design phases.

5.1.4 Usability of charts illustrating the ‘space of solutions’

The selection of beneficial window solutions in terms of space heating demand, thermal environment and daylight availability requires knowledge about the properties of the specific products as well as the various geometrical factors related to their application (design), such as window size, room geometry and orientation. The parametric analyses and the charts illustrating the space of solutions are an invitation to an open discussion of the link between various design and performance parameters as well as the options and potential conflicts related to window design in ‘nearly zero-energy’ houses. Furthermore, the research described in this thesis is an example of an approach by which window solutions with minimum space heating demand can be chosen in a space of solutions for each geometry defined by targets for minimum daylight availability and overheating by using these charts.

The charts also highlight potential design conflicts in deep or narrow south-oriented rooms, because either thermal comfort or daylight must be compromised when only side-lit windows are used. Conflict situations like this can lead to a discussion on the performance parameters and the chosen targets in the charts. In principle, these can be tested for sensitivity to e.g. different insulation thicknesses, different user patterns and adaptive models for thermal comfort, different ventilation systems, and different daylight targets. In the Danish climate, it was possible to have window design fulfilling the targets for daylight and overheating without the use of mechanical cooling and with a moderate venting rate. However, especially in warmer climates, where mechanical cooling is needed to avoid overheating, the value of daylight compared to the energy used for cooling may give rise to several discussions, such as whether it is reasonable to dimension the window sizes in south-oriented rooms on the basis of targets for daylight availability under overcast situations.

The research described in this thesis also evaluated daylight as an independent performance parameter, rather than expressed in terms of a reduction in energy used for artificial lighting. As the energy consumption in residential buildings decreases, however, energy used for artificial lighting might come to represent a large share of the total energy consumption if no appropriate energy-efficient lighting and control system is used.