The calorific losses through the window are deducted from the transmitted power, which are characterised by the heat transfer coefficient of the glazing Ug, or of the entire window including the frame Uw. Double glazing coated and filled with heavy noble gases achieve a minimum Ug value of 1.0 W/m2K, triple glazing at best a Ug value of 0.4 W/m2K. Even at a glazing Ug value of 1.3 W/m2K, a wooden or plastic frame increases the window’s Uw value slightly. For passive house concepts, specially insulated expanded polystyrene frameworks must be used, so that the low glazing values of triple glazing are not worsened by the frame proportion.
The passive solar gain Q)u usable in the room results from the balance of losses and gains. The losses are calculated from the Uw value of the window of surface Aw and from the temperature difference between the room air T and the outside air To:
Q = Uw (T – To)-gG (7.9)
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From the available energy balance, an effective U-value Uf can be defined, which is often used for monthly or annual balance calculations with mean temperature differences and irradiances.
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Balanced over a heating season, about 400 kWh/m2a of solar irradiance is available on a south-facing facade in Germany. The mean temperature difference between the inside and outside of about 17°C, multiplied by the number of days in the heating season, results in the so-called heating degree day number, which on average in Germany is about 3500 Kelvin – days per year. The maximum usable energy per square metre of glazing surface for two- pane low-e coated glazing with a Uw value of 1 W/m2K and g = 0.65 is thus:
The amount of heat effectively usable in the room depends greatly, however, on the storage capability of the structural elements on the inside, since high passive solar heat gains can easily lead to overheating of the interior and thus do not contribute to covering the heating requirement. A detailed analysis of the dynamic storage behaviour of building elements can be found in section 7.3.
In the monthly balance procedure based on EN 832 for calculating the heating requirement, the efficiency of the solar irradiance transmitted by windows is indicated as a function of the relation of the monthly gains to the transmission and ventilation heat losses. For low energy buildings with an annual heating requirement between about 30 and 70 kWh/m2a the result is, for a heat-storing heavy building construction, a flat minimum of the heating requirement for a window area proportion on the south-facing facade of approximately 25%. In administrative buildings with mostly higher internal loads, the window area proportion should be lower still, to avoid overheating in summer. With a light building method with a small storage capacity, the minimum heating requirement is obtained for 0-20% of the window area proportion.
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Figure 7.4: Influence of the window area proportion on the heating requirement. |
