Windows

Windows have to fulfil functional as well as esthetical tasks. They dominate the appearance of a building with their form, structure and placement on the envelope.

Design criteria, function

In addition to the classical functions of windows, like the protection against the weather, daylighting, supply of fresh air and the connection to the outside, they have meet thermal and acoustical requirements.

Protection against the weather: The requirements concerning mechanical stress, joint permeability and resistance to heavy rain are defined within the national German code DIN18055 [1]. The joint permeability means the air leakage through the window and frame and is defined by the joint permeability coefficient. The coefficient describes the airflow, which is exchanged between the interior and exterior per meter joint length at a reference barometric pressure dif-ference of 10 Pa. The requirements towards heavy rain demands that no water enters the interior when simultaneously exposed to rain and wind.

Daylight: A sufficient level of daylight reduces the energy demand and also provides visual comfort to the user. To guarantee a sufficient level of daylight, the windows have to meet minimal requirements in size and orientation. Further parameters of influence are the room geometry and the surrounding buildings (see Daylight, Tool Shading by Overhangs). Energetic disadvantages have to be omitted when optimising a buildings day lighting potential by enlargement of the glazing area. The energy savings due to reduced amount of electrical lighting an possibly solar gains have to be weight against the increased transmission loss.

Ventilation: Windows are used for natural ventilation of rooms. They have to provide sufficient controlled air exchange and have to minimise the infiltration and exfiltration caused by uncontrolled air exchange through a careful design (see Ventilation).

Usage of solar energy: In minimising the heating energy demand of buildings windows can play an important role in optimising the passive solar gains. A positive energy balance, i.e. higher passive solar gains than transmission heat losses, is only accomplished by windows with low U-values and high solar heat gain coefficients (SHGC). An ideally oriented building and suitable designed shading devices are a necessary condition to accomplish this goal. South orientated windows are preferable. The energy input into the building with this orientation is smaller compared to east or west orientated windows and is easily controllable by simple shading devices, because in summer the incident angle of the solar radiation is very high. Whereas in winter the low standing sun maximises the solar energy gain and reduces therefore the heat energy demand of the building.

Sound insulation: Noise imposes a health risk for human beings. Long-term exposure greatly increases the risk for heart and circulatory diseases. To protect the human being from external noise, windows have to fulfil requirements concerning airborne sound insulation defined in the national German code DIN 4109 [2]. Parameters influencing the sound insulation properties of a window are on the one side the sound insulation properties of the window pane and frame itself, and on the other side the airtight construction of the seal of the casement and the joints. The sound insulation of the glazing increases with increasing thickness (weight per area), damping capacity of the space between the panes, and the number of panes.

Thermal insulation: Heat transmission and airtightness (Glossary) characterise the thermal insulation qualities of a window during winter. The heat transmission is defined by the overall heat transmission coefficient UW which again is defined by the heat transmission coefficients of the glass Ug and the frame Uf and of a length related heat transmission coefficient G The airtightness is characterised by the joint permeability coefficient a. The thermal transmission coefficient decreases with increasing numbers of panes, decreasing thermal conductivity of the fluid in the space between the panes and with decreasing emissivity of the coating of a pane. The thermal insulation during summer is also influenced by the solar heat gain coefficient of the window pane. An excess of solar energy input has to be avoided using additional shading devices.

Security: security guidelines are divided into active and passive measures. Passive security deals with the protection of the human being during the usage (Opening, closing, cleaning). Active security means the protection against theft or direct attacks.

Parameters related to building physics:

The physical properties (lighting, heating, ventilation) are governed by the optical and thermal properties of the glazing and the frame. Figure 1 shows the different heat transfer mechanisms.

Figure 1: Thermal transfer mechanisms of a double pane window.

The short-wave solar radiation (wavelength 380 to 780 nm) is partly reflected and absorpted at the window panes. The remaining fraction of the solar radiation is being transmitted to the buildings interior. The heat transfer from the interior to the inner pane occurs via long-wave infrared radiation (wavelength 8 to 12 mm) and convection. The heat energy absorbed at the inward surface of the inner pane is being delivered to the outward surface of the inner pane via heat conduction. In the inter-pane space the heat transfer occurs via convection and longwave radiative exchange, as it is also the case between the outward surface of the outer pane and the surrounding.

Implementation types:

Windows are produced as various types differing in pane thickness, pane number, pane material, inter-pane distance and gas filling, pane coating and frame construction. The properties of a selection of implementation types are given in Table 1.

The thickness of the panes is defined by the structural stability, safety and acoustical requirements. The material or the combination of materials of the individual panes is chosen according to the safety requirements concerning active security and fire protection.

Number of panes, inter pane distance: The number of panes and the distance between them is regulated by thermal insulation and noise protection standards. The quality of noise protection steadily increases with increasing inter pane space. This doesn't apply for thermal insulation quality, for which an optimal distance between the panes exists, where the UG-value is minimal. An further increase of the distance would result in an increase of the UG-value.

Fluid in the inter pane space: The thermal and sound insulating properties of the inter pane space can be altered when the air is substituted by another gas. The use of inert gases (argon krypton) compared to the usage of air reduces the UG-value because of the reduced thermal conductivity and improves the sound insulation as a consequence of the increased area weight of the glazing. The SHGC value is decreased by this procedure as well.

Pane coating: An Improvement of the thermal insulation properties of the glazing can be achieved by coatings with low emissivity coefficients . The coating reduces the radiative part of the heat transfer mechanism and hence the UG-value of the glazing but also the SHGC-value.

Construction of the frame: Window frames are usually made out of wood or plastic. The frame material, insulation layers and the construction influence the Uf-value of the frame. Table 2 shows selected configurations of window frames.

Smart windows: Currently, several innovative window designs, so called "smart windows", are on the verge or even at the start of mass production. Smart windows possess special optical and thermal properties (e.g. electrochromic, thermochromic or holographic). The idea behind the development of these window types is to vary the SHGC depended of a control variable (e.g. Voltage, Temperature) or to redirect the path of the sun rays as needed through light diverting techniques.

Shading devices:

Shading devices act as protection against the sun, glaring, weather, as temporary thermal insulation or against theft and as any combination out of these functions.

To avoid glaring or increased solar cooling loads, windows have to be equipped with shading devices. These can either be fixed or movable devices or sun protection glazing. The use of permanent solutions should be avoided because of the increased electrical lighting needs and the decreased potential to utilise passive solar gains during the heating and transitional period.

Shading devices can be mounted external, internal or in the inter-pane space and can be operated manually or automatically. Externally mounted elements are shading the glazing and prohibit the penetration of solar radiation to the interior. These elements can be realised as movable or fixed constructions. Fixed devices can block the solar radiation of a high risen sun during summer without significantly increasing the heat energy demand by allowing the penetration of solar radiation during winter. Internal elements partly reflect the solar radiation transmitted through the glazing back to the outside. The remaining portion is absorbed and transferred to the interior via convection and infrared radiation. This can lead to an additional heating of the room. Movable elements can be opened or closed depending on the respective needs and hence provide the optimal mean to regulated the protection against the sun.

Literature

[1] DIN18055, Ausgabe 1981-10, "Fenster; Fugendurchlässigkeit, Schlagregendichtheit und mechanische Beanspruchung; Anforderungen und Prüfung", Beuth-Verlag.

[2] DIN 4109, Ausgabe 1989-11; "Schallschutz im Hochbau; Anforderungen und Nachweise", Beuth-Verlag.

[3] Der Energieberater, Deutscher Wirtschaftsdienst, Köln, 1997.

[4] RWE-Bauhandbuch, Ausgabe 12, Druck und Verlag GmbH, Oberhausen, 1998.