At UL FGG KSKE two studies dealing with the influence of glazing properties on daylight levels and energy balance of a building were conducted. Our primary goal was to find out to what extend lower U value of windows influences daylight levels in buildings. In addition, we monitored energy balance of

buildings (transmission losses and solar gains) and compared benefits and deficiencies (can lower heating energy use compensate lower daylight levels).

In the first study [1] students collaborated in a joint comparative analysis carried out in the framework of seminars at two subjects: Building, Environment, Energy (UL FGG UNI — Construction course, 4th year) and Bioclimatic building (UL FGG VSS — Construction course, 3rd year) in academic year 2006/2007. The study comprised student-selected buildings for which the influence of changed glazing characteristics on internal environment and energy balance of buildings were calculated. The choice of buildings included a wide selection of types and geometries of buildings, from small residential houses, schools and larger public to office buildings (Fig. 1). Random selection of buildings (surface- to-volume ratios were among 0.10 and 1.00) covered current architectural production and ensured that the results were general enough to be statistically valid. For each of the 27 buildings students calculated yearly energy balance in a building (according to EN 832, all at the same location) and daylighting in a room of a chosen building (21. 3., 21.6., 21.9. and 21. 12. at 12:00 under standard CIE overcast sky) for two variants of commercially available window glazing:

1. double glazing, U = 1.40 W/m2K, g = 0.65, tv = 0.70

2. triple glazing, U = 0.60 W/m2K, g = 0.50, tv = 0.50

All the other input data remained the same (location, geometry of building, sizes of windows, U values of non-transparent parts, orientations, etc.). Daylighting calculations were carried out with a computer

tool SPOT1M v3.1 Sensor Placement + Optimization Tool. Heating energy demand according to EN 832 was calculated with computer tool GEnV 1.1.

In the case of 27 student-selected buildings, change of window glazing resulted in the average reduction of specific heating energy demand (Qn/Au) by 14.4%. At the same time the reduction of average illuminance level (Eav-eq) in rooms decreased by immense 25.3%. The reduction of the ratio of heating to daylight is almost 1:2 and is in some cases even higher. For instance, case P17-TS reached 0.7% reduction of heating energy demand and 29.2% reduction of daylight level in a chosen room. Similar result was obtained in case P19-TS where heating energy demand was reduced by 1.5% and the reduction of daylight was 27.8%. Only in three cases (P1-TS, P2-TS and P16-TS) the heating energy demand reduction was higher than the reduction of daylight levels in rooms (Fig. 2).

After students executed calculations of energy consumption and internal illuminance levels in their buildings for both types of selected glazing, they were expected to make a decision on the final configuration of their buildings external envelope in regards to their findings. In the end they had to make an argumented decision whether they were going to use triple or double glazing. In such a way students were encouraged to make an executive design decision on the basis of their own findings (executed calculations of energy demand for heating and internal illumination levels) and knowledge about building physics and internal environment that they accumulated during seminar course and lectures in their respective subjects. With such an approach to seminar work students are not just given information but are expected to use the tools and knowledge at their disposal to come up with the best solution for a certain case on the basis of their own experience. A valuable lesson for future civil engineers is learned — that interventions in one area of building design can influence other aspects of the building as well, and for optimal results a holistic approach is necessary.

Very similar results were obtained in an independent parallel study [2] carried out by the staff of KSKE. We studied a hypothetical single room object 4m x 6m x 2.5m with one south oriented window positioned on the shorter wall, for which we changed size and glazing characteristics. The window size varied from 14% (1/7 — K1 configuration) of floor area, 20% of floor area (K2 configuration), to 42% (glazing of entire wall — K3 configuration) of floor area. Two variants of window glazing characteristics were chosen:

1. double glazing, U = 1.12 W/m2K, g = 0.61, tv = 0.76

2. triple glazing, U = 0.74 W/m2K, g = 0.50, tv = 0.66

The U value of the non-transparent building envelope was U = 0.15 W/m2K. For easier comparison in both studies the same computer tools were used.

As expected, the analyses showed distinctly negative influence of triple glazing on daylighting of spaces compared to double glazing. Surprisingly, use of heating energy was minimally reduced when triple glazing was used. Energy balance of the K1 configuration with the smallest window opening (Az = 1/7 of floor area) was almost the same if double (Qn/Au = 68.33 kWh/m2a) or triple glazing (Qn/Au = 67.80 kWh/m2a) was used. The difference was only 0.78 %. At the same time the illuminance level (Eav) fell by 13.1 % when triple glazing was used. The reason for small differences of energy demand is the g factor and decreased solar gains (Qs), which are 18% lower when triple glazing is used, but the transmission losses (Qt) are reduced by only 9.2% (Fig. 3).

The actual values of heating demand reduction and illuminance levels deterioration differ compared to the first study, but the ratio between them is identical (when triple glazing is used, daylighting level is reduced by a factor 2 regarding heating energy demand). The results of both studies show a trend of extreme deterioration of daylighting in buildings and relatively small reduction of heating energy demand when triple glazing is used.

In the third configuration (K3 — window size is 42% of floor area) the size of glazing is 20% of the entire fa? ade area, which, compared to the first configuration, presents three times larger window area. As expected, transmission losses dramatically increase and double glazed configuration has 17.4% higher losses than the triple-glazed one. Regarding solar gains the situation is reversed; in the case of triple glazing the gains are 18.8% lower. In the context of the entire building heating energy demand is

by 7.0% higher when double glazing is used, but at the same time the average illuminance level is reduced by 14.6% (Fig. 4).

It is obvious that major part of heat losses originate from ventilation, when well insulated non­transparent envelope (U = 0.15 W/m2K) and quality glazing (U = 1.12 W/m2K, g = 0.61, tv = 0.76) are used. In the first configuration with double glazing 57 % of the entire heat losses belong to ventilation losses (Qv).

Reducing U value of glazing in buildings, which have 10% — 20% of glazed fa? ade area in the present state-of-art is not reasonable. The quantity of transmitted daylight disproportionally decreases compared to heating energy demand. Furthermore, the reduction of g and Tv factors has negative impact not only on solar gains and daylighting, but also on health and use of energy for electric lighting. Especially prolonged use of artificial lighting is the factor that additionally speaks in favor of window glazing with higher visual and solar transmittance.

2. Conclusion

Students collaborated in joint comparative analyses of the presented problem. They used 27 randomly selected buildings of various building types (small residential buildings, schools, and larger office buildings), geometries and configurations. For each building they calculated yearly daylighting under standard overcast CIE sky and heating energy consumption according to EN 832. The buildings were glazed with two variants of glazing systems (double glazing: U value 1.40 W/m2K, g = 0.65, tv = 0.70 and triple glazing: 0.60 W/m2K, g = 0.50, tv = 0.50). All other input data remained the same. The average of 27 buildings showed that by changing double-glazing for triple glazing heating demand of buildings reduced by 14.4%, but at the same time it deteriorated daylight levels in the reference rooms by dramatic 25.3% (Fig. 1). The proportions between the heating reduction and daylight levels were in some cases even more drastic and reached close to 50% of daylight reduction. Only in 3 cases the reduction of heating energy demand was larger than the reduction of daylight levels. A parallel analysis that was carried out independently, as an internal study at KSKE, showed similar results.

Both analyses showed the trend of drastic reduction of daylight levels in the reference rooms and at the same time small or even in some cases negligible improvement of the heating energy balance. The study showed that in moderate climate at the moment the technical maximum is double glazing with U value in the range of 1.1 W/m2K and transmissions about tv = 0.76 and g = 0.61. Both values are given for clean glass and perpendicular incidence angle. In reality we have to allow for correction factors and can expect even more evident differences. Daylight represents a very important element of wellbeing of users and directly influences the amount of electrical energy used for artificial lighting. At the same time lower g value reduces the possibility of solar heat gains. Considering the present characteristics of commercially available triple glazing we can justly ask ourselves, if wide use of such glazing can be justified.

References

[1] A. Krainer, M. Kosir, Z Kristl,. Analyses of glazing characteristics influence on internal environment in 27 randomly chosen buildings — partly realized by the students in the framework of the seminars Building — Environment-Energy (UL FGG UNI — construction, 4th year) and Bioclimatic buildings (UL FGG VSS — construction, 3rd year), UL FGG KSKE; 2007.

[2] A. Krainer, M. Kosir, Parametrical study of U value influence on daylghting and energy balance of a hypothetical one-space building — internal publication, UL FGG KSKE; 2007.

[3] A. Krainer, M. Kosir, Z. Kristl, M. Dovjak, Passive house versus bioclimatic house. Gradbeni vestnik letnik 57 (2008) 3, 58-68.

[4] L. Heschong, Windows and Classrooms: A Study of Student Performance and the Indoor Environment. Fair Oaks, Heschong Mahone Group. California Energy Commission, 2003.

[5] L. Heschong, Windows and Offices: A Study of Office Worker Performance and the Indoor Environment. Heschong Mahone Group. Fair Oaks, California Energy Commission, 2003.