Study and Countermeasures for Condensation and Crystallisation of Shadow Box Glass for

Curtain Wall

Based on a large number of façade project cases and testing specimens, this paper studies the cause of condensation and crystallisation problems in the shadow box of curtain walls. This study also proposes countermeasures for preventing these problems. Investigations and references are offered for future curtain wall designs and production of similar façade construction projects.

The use of monolithic glass for the shadow box of curtain wall is quite common in curtain wall projects in Hong Kong. As a coastal city, Hong Kong is located in the subtropics, the salty and humid environment, plus the large temperature difference between day and night, condensation problems with the shadow box occur easily. The worst of which, crystallisation to the glass surface is found, which is irreversible.

Condensation and Crystallisation

When obvious condensation appears in the shadow box glass, it affects the appearance of the facade, In the process of curtain wall maintenance, observation and preliminary analysis have been done by Jangho engineers to the demounted shadow box glass. Most of the condensation can be wiped and cleaned, the glass can be reused in the facade.

However, a small part of glass could not be completely cleaned. It was found that there was white attachment on the inner surface of glass, which was rough to the touch. Engineers extracted the sample and sent it to the laboratory for testing, with the test report revealing that the white attachment is a crystalline solid, the main chemical composition being silicide, which is similar with glass. Based on the report, it is believed that the generation of the white attachment is probably due to the weathering effect of glass in a warm and wet environments. The fluctuations in humidity and temperature with the content of carbon dioxide in the air circulation could intensify the weathering process.

Figure 2 shows the materials test report provided by the laboratory. The chemical composition was studied through the scanning electron microscopy images (SEM) and Elemental analysis (EDX). Meanwhile Jangho conducted statistical analysis on the large amounts of maintenance cases，it was found that most of the condensation cases occurred especially when monolithic clear glass was used in spandrel.

Initiation of Condensation

Jangho conducted analysis of the shadow box, and checked the shadow box assembly, ventilation holes and that its size are set as per design. The setting of ventilation is a standardised design of the curtain wall, which is good for the evaporation and discharge of condensation. However, engineers found that some ventilation holes were partially or even completely blocked by sealant, resulting in a poor ventilation effect. Moisture accumulated in the chamber could not be dissipated and thus provides favourable humid conditions for condensation to form. Moreover, in the aspect of thermal analysis, glass condensation is related to the dew-point temperature of the cavity and the surface temperature of the inner side of the shadow box glass, which promotes the usage of energy-saving glass unit for shadow box.

Causes of Crystallisation

As we know, the major composition of architectural glass is SiO2, Na2O, CaO and a small amount of sodium silicate. Sodium silicate is soluble silicate, which is easily soluble in water. When water adheres to the glass surface, the sodium silicate on the surface is destroyed by hydrolysis, producing sodium hydroxide and silicon dioxide.

This precipitated silica is just the crystalline solid which was observed during testing.

The hydrolysed sodium hydroxide, in turn, reacts with carbon dioxide in the air to produce sodium carbonate, which adheres to the glass surface. Sodium carbonate is good in water absorption and non-volatile, it would turn into alkaline solution after deliquescence. When the temperature and humidity change, the concentration of alkaline liquid increases, and these relatively high concentrations of lye in contact with the glass would erode the glass surface, thus the white crystals are formed.

As the temperature fluctuates between day and night, condensation water on the glass surface happens in cycles of evaporationcondensation. As shown in Figure 4, this cycle of evaporationcondensation accelerates the chemical reactions, eventually produce the crystalline solid and the reaction is irreversible.

Jangho conducted a simulation experiment to support the analysis and also to find a solution to the problem. Fig. 5 shows the simulation experiment specimen. 27 specimens are set for comparison, aiming to determine the influence and its degree on the generation of condensation in respect of different situations: glass configuration, ventilation hole size and position, facade orientation.

At the same time, indoor and outdoor temperature and humidity, the glass’ internal surface temperature and humidity were recorded to provide data for dew point temperature analysis. The unitised curtain wall system was used in the test, and the test period was carried out for 12 months, one month for data acquisition. The test field was carried out at roof floor of a high-rise building in Singapore, which is a country with a warm and rainy climate. The following table shows the description of the experimental samples.

Conclusion

Based on the test results and analysis, Jangho proposes the following solutions:

1. Ventilation holes. The test results show that in the case of nonventilation, condensation occurs extremely easily in the shadow box and is difficult to volatilise. Consequently, ventilation holes should be designed in shadow box and ensured that they are not blocked, so that the moisture in the shadow box can be quickly discharged, hence the humidity degree in the chamber can be effectively reduced.

2. Use coated glass. The simulation test showed that no obvious condensation traces were observed in the shadow box in case of using coated glass. Analysis shows that the coating layer could cut off the direct contact between water vapor and the glass surface, so as to avoid the chemical reaction between the water and glass when evaporation-condensation cycle takes place in the chamber.

3. Use IGU glass unit. The simulation test showed that no obvious condensation traces were observed in the shadow box in case of using IGU glass unit. It has same effect as using the coated glass. Thanks to the lower U-value of IGU glass, which greatly reduces the probability of condensation of glass, effectively reducing the number of evaporation-condensation cycles, so as to avoid the chemical reaction between the water and glass. Crystallisation is avoided on the glass surface.