Roof windows in low-energy buildings - Analyses of demands and possibilities for future product development

Abstract

As part of an ambitious energy policy and strategy for reducing the use of fossil fuels in the European Union, all new buildings are required to consume `nearly zero-energy' by the end of 2020. This creates a strong need for research in cost-effective solutions and technology that can help balance the goal of a very low energy use with good daylighting and a healthy and comfortable indoor environment. Windows play a very large role in both the energy consumption and the indoor environment of buildings. Roof windows are a particularly efficient daylighting source, which certain types of houses depend on to receive sufficient daylighting in all parts. The development of roof windows with an overall improved performance for use in nearly zero-energy houses might therefore help considerably in the achievement of these goals in a cost-effective way. The main hypothesis of this research was that the current best standard-practice roof windows can be improved in a way that makes it easier and more cost-effective to realise nearly zero-energy houses with sufficient daylighting and thermal comfort throughout. This hypothesis was tested through a series of simulation-based investigations focusing on the effect of various combinations of glazing size, thermal properties of glazing, frame and junctions, and transmittance of light (LT) and solar energy (g-value) on energy use, daylighting and thermal comfort. The effect of roof and facade window parameters was first studied in rooms with windows of a certain slope and orientation to identify the demands and possibilities in various parts of the building. A glazing diagram was developed which made it possible to map and compare the various options that provide suffcient daylighting and thermal comfort. This showed that well-dimensioned facade windows with light transmittances of about 40-70% could provide suffcient daylighting without overheating in the climates of Rome and Copenhagen, as long as they were located in rooms with a reasonable layout for daylighting and appropriate solar-control coating was used on solar exposed glazing. The same was true for sloped and horizontal roof windows with any choice of light transmittance in both climates. Roof-window thermal properties needed for flexibility were then identied by studying the effect of these options on space-heating demand in rooms representing various parts of a 11/2-storey house with a simplied floor plan and no interaction of air or heat between zones. This showed how improved roof-window frame constructions and heat loss coeffcients of the glazing lower than current standard levels would make it possible to achieve nearly zero- energy consumption with a wider range of options providing sufficient daylighting and thermal comfort, and with increased use of rooms with sloped roof windows oriented north. In Copenhagen, such improvements were found critical for adequate flexibility in building design, while in Rome they were not. Due to the low utilisation of solar gains, such improvements were also generally needed for roof windows in Copenhagen with any orientation to reduce the impact of the choice of window size on space-heating demand. Comparison of options with and without dynamic shading in a loft room with sloped roof windows facing south in the two climates, showed that dynamic shading made room for considerably more daylighting without overheating than using optimal solar-control coating on its own. However, in both cases, illuminances of 300 lx in 75% of the space could be achieved in 50-63% of the daylight hours, with no more than 40-100 h of excessive temperatures as defined by the Adaptive Thermal Comfort model. Moreover, as an option for reducing the optimum space-heating demand, dynamic shading showed limited potential in Copenhagen, while it could have some potential in Rome.  Finally, the performance and cost-effectiveness of various options for improvement were studied for two large single-family houses in Copenhagen with typical floor plans and sloped (Case A) and horizontal (Case B) roof windows. The scope for investment in improved roof windows was identied on the basis of the cost of the insulation not needed in the houses to meet nearly  zero-energy requirements with the improved roof windows installed instead of the options that are current best standard-practice. For the specific improvements investigated, this revealed examples of savings in insulation costs that would allow users to pay EUR 50-320 more per m2 improved roof window than for the products that are best standard practice today. Of these amounts EUR 50-60 were due to improvements in the glazed part alone, EUR 100-300 were due to improvements in the frame constructions, while EUR 320 were due to a relatively simple improvement in the horizontal roof windows, where the addition of a 3-pane glazing at the bottom of the light well considerably reduced the overall heat losses. If manufacturers can make such improvements available at prices within these scopes for investment, nearly zero-energy houses with sufficient daylighting and thermal comfort throughout could be realised in an easier and more cost-effective way.