Improved Windows for Cold Climates (0)

Improved Windows for Cold Climates
In this paper the possibilities of improving the energy performance of windows in cold climates are exam -ined. The background for using windows with improved energy performance is the need to reduce the en-ergy consumption in buildings. Since the heat loss through windows often represents half the total heat loss from houses , much energy can be saved by developing and using better windows with respect to energy performance. The main purpose of having windows in houses is that they provide daylight and view, but windows also provide solar gain that can be utilized as a contribution to the space heating in the building . Therefore the windows also have a positive influence on the energy balance of buildings.
To evaluate the possibilities for developing better windows with respect to energy performance when used in Nordic and arctic climates, seven different window types have been examined in terms of the net energy gain and simulations of the energy consumption in buildings with focus on domestic houses.
A large part of the energy consumption in countries in Nordic and Arctic climates is used for space heating in buildings. In typical buildings the windows are responsible for a considerable part of the heat losses. Therefore there is a large potential for energy savings by developing and using windows with improved energy performance.
Traditionally evaluation of the energy performance of windows has focussed on the thermal transmittance, but as windows differ from the rest of the building envelope by allowing solar energy to enter the building, the total solar energy transmittance is equally important . In the heating season in cold climates the solar gain through windows can be utilized for space heating which results in a corresponding reduction in the energy production that is often based on fossil fuels . A suitable quantity for evaluating the energy perform-ance of windows in a simple and direct way is therefore the net energy gain, which is the solar gain minus the heat loss during the heating season. Especially in arctic climates where the heating season covers the whole year there is a large potential for exploiting the solar gain during the summer season. Furthermore the presence of snow increases the solar radiation because of the reflection.
In this paper the energy saving potentials for different window types have been examined by determining the net energy gains in Danish and Greenlandic climates. Furthermore the windows have been evaluated by performing building simulations of the heating demand in typical single-family houses in Denmark and Greenland . The examined windows are typical new windows from Nordic countries and new proposals of improved windows with low thermal transmittance and high total solar energy transmittance.
The results show that net energy gain can be increased considerably by reducing the frame width, which results in a larger transparent area causing a larger solar gain but still maintaining a low thermal trans-mittance. Using three layers of glass with large gaps, using very slim frame profiles, and omitting the edge constructions that normally causes thermal bridges achieve this. Applying shutters or low emissivity coated roller blinds incorporated in the glazing that are activated during night time can improve the energy per-formance of windows.
The results from this work show that it is possible to develop windows with a positive net energy in a fairly simple way, which means that it contributes to the space heating of the building.
Based on the calculations of the net energy gain and the heating consumption of seven different windows it is concluded that there are good possibilities for developing windows with improved energy performance for cold and arctic climates. The windows type 3,4,5 and 7 result in the highest net energy gains and the lowest energy consumptions in the houses.
For type 7 the good result is due to the very low thermal transmittance. An unfortunate effect of the combi-nation of the wide frame profile and the three- layer glazing is that the total solar energy transmittance is quite low resulting in a low solar gain. The good results for window type 3,4 and 5 show that the g- value has a significant influence on the energy performance. A simple and efficient way to improve the g-value is by increasing the glazing area by reducing the frame width. In the new developed window type 4 this is implemented with a frame width of only 25 mm and still keeping a low U-value. The 3- layer glazing with large gaps ensures that use of edge constructions that normally results in a thermal bridge can be avoided. Since the windows with low U-value and high g-value result in positive net energy gain they will contribute to the space heating of the houses. During periods with sunny days the high solar gain can cause overheat-ing problems. Therefore there is a need for developing windows with integrated solar shading devices .