.

Fig. 7: Wall dampness and dampness time
© ZAE Bayern

Fig. 8: Reduced thermal radiation when sun protection is attached on the inside
© ZAE Bayern
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Low-e in old and new buildings

In the future, low-e materials will play an increasingly important role not only in “textile architecture”, but also in conventional old and new buildings. In order to estimate the market potential, the benefits and limitations for old and new buildings were investigated.

Old buildings

Applying a low-e coating to a wall on the inside or outside reduces the corresponding heat transfer coefficient. The effect on the U-value is greater the lower the insulation standard of the construction. A relative decrease of 47% in the U-value can be achieved for a fabric; the figure is 17% for a typical old building and only 6% for well-insulated new buildings. The possibility of achieving considerable effects in old buildings with relatively simple measures is attractive. While coating a facade on the outside is relatively uncomplicated, applying a coating on the inside of an old building should be carefully planned. Reflection of thermal radiation back into the room does indeed increase the perceived comfort, but also leads to a decrease in the wall temperature. In the case of poorly insulated walls, this can lead to the temperature dropping below the dew point and thus to moisture damage. According to calculations carried out by the researchers, the heat transfer coefficient of the wall should be better than 1.8 W/(m² K) in order to avoid this sort of problem, which corresponds to a brick wall with a thickness of about 20 cm.

New buildings

Logically, the energy benefits of low-e facades are reduced if the building in question is well insulated. Nonetheless, they can still be beneficial on low-energy houses: For well-insulated buildings, thermal radiation of the exterior wall leads to temperatures which are often below the dew point, especially on clear, cold nights. The condensation can lead to algae growth on the facade, causing the facade to appear mossy. A low-e coating reduces radiation heat exchange with the cold night sky and reduces condensation formation significantly. This prevents algae growing on the facade and helps to avoid moisture damage. Fig. 7 shows the amount of condensation and the dampness time for a thermal insulating composite facade as a function of the emissivity. Drastic reductions in condensation are achieved with emissivities below 0.3.

Bangkok airport

The new airport in Bangkok (Fig. 1) is the only building so far where fabrics with low-e coatings have been used. The airport is characterised by its modern steel-glass-membrane construction. Tubelike structures, the so-called concourses, provide access between the terminal and the rest of the airport’s facilities. Over a total length of over 3,100 m, three-layer membrane roof surfaces alternate with glazed side surfaces. To reduce the incoming thermal radiation, the inner, visible underside of the membrane construction, which consists of a glass fabric, has a low-e coating. At the time, the planners didn’t yet have a free choice of colour. The fabric has a silvery-grey appearance and an emissivity of 0.4. In total, almost 100,000 m² of the aluminised glass fabric were installed. To protect the sensitive aluminium layer, TAG Composites & Carpets GmbH applied a fluoropolymer from Polymade ITT GmbH, and ZAE Bayern conducted quality assurance regarding the infrared-optical characteristic values. More stable coatings with a lower emissivity and – important in architectural design – a free choice of colour appearances were developed as part of the “Low-e fabrics” project.

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