Single float glass transmits the majority of solar radiation between 315 and 2500 nm and absorbs other wavelengths. In real-time situations non-perpendicular incidence angles of radiation, double or triple glazing, additional low-E coatings, glass coloring and layer of dust and dirt on the surface result in much lower transmission of solar radiation than declared. When taking into account dirt on glass in a city environment (e. g. correction factor 0.8) and solar incidence angle typical of temperate climates (e. g. correction factor 0.8), transmission of single glass for visible light decreases from 89% to 57%

(89 % x 0.8 x 0.8 = 57 %). To decrease thermal transmission of windows, double or triple glazing is used. U value of a double glazing with air filling is usually about 3.00 W/m2K. If the window system is improved by a low-E coating with Argon filling, U value drops to 1.16 W/m2K. However, lower U value also causes lower transmittance for the visual part of solar spectrum (tv = 78 %) and lower transmittance for the whole solar spectrum (g = 63 %). Further improvements of U values bring us to the use of triple glazing (double low-E coating and Argon filling (U = 0.60 W/m2K) of Krypton filling (U = 0.58 W/m2K)). As mentioned above, further lowering of U values cause even lower transmittance for solar radiation.

Students also discussed the concept of passive house in relation to inside environment quality and compared the passive and the bioclimatic concepts. The goal of the passive house is the reduction of heating energy use to less than 15 kWh/m2a. To reach this goal, glazing — which interests us most — has to be triple. This consequently reduces the dynamic communication between the inside and the outside environment. In the philosophy of the passive house design the reduced daylighting and cutting off the direct contact with external environment is viewed as collateral damage. But the concept of alienating people from natural environment is according to many studies harmful. The external environment is not hostile; on the contrary, it has simulative effects on body and mind. Daylight provides quality lighting, stimulates the sense of sight and is an important communication between the internal and external space [3]. Constant changes of light improve concentration and responsiveness. The same goes for hearing and the sense of smell. The bioclimatic concept, on the other hand, is based on simultaneous adaptation to external conditions and internal needs. The closer the building is able to follow these two profiles (temperature dynamics, solar and thermal radiation, relative humidity and air stratification), the more efficient it is. The unstable model represents the dynamic structure, which functions in real time. The goal of the above-described interventions in the framework of bioclimatic design is a healthy living and working environment with low energy use and not low energy use with physiological minimum.

Glazing properties have direct influence on the level of daylight in living and working environment and on energy balance of buildings. Low daylight levels have proven negative influences on comfort, health and efficiency of people as well as on energy used for lighting and cooling of spaces. Studies

carried out in the 70-ies in the USA showed possible energy savings for lighting of office spaces in the range between 15% and 20% if enough daylight was available (also regarding quality factors) [3]. Lately the advantages and positive effects of daylight on efficiency and sales increase were proven in the HGM study [4, 5] carried out in 2003. Of course, lower U value of glazing decreases transmission losses through the building envelope, but when designing non-transparent parts with U values 0.2 W/m2K or lower, the majority of heat losses are produced due to ventilation, not because of heat transmission. Because of the above-mentioned reasons and complex influences on the functioning of the entire building system, window properties are not a trivial question and deserve a systematic analysis.