The case analysis of dew condensation on the inner surface of insulated glass in construction engineering.

Insulating glass is widely used in residences, hospitals, schools, shopping malls, and other places that require indoor environments due to its good optical and thermal properties. According to domestic surveys, the failure rate of insulating glass after two years of use is 3% to 5%, and 63% of the failure cases are due to condensation on the inner surface of insulating glass. The dew point of insulating glass refers to the temperature when the humidity of the air sealed in the air layer reaches saturation. When the temperature of the surface layer is lower than this temperature, the water vapor in the air layer will condense or frost on the inner surface of the glass. The international standard "Insulating Glass" clearly stipulates that the dew point of insulating glass should be ≥ -40°C and the expected service life should be at least 15a. According to this regulation, the insulating glass used in buildings should not have condensation or frosting on the inner layer during daily use. The reason for this phenomenon can be attributed to the rise in the dew point of the inner layer of the insulating glass. Therefore, the dew point has become a necessary indicator to judge the quality of insulating glass. This article starts from the actual case of dew condensation failure of insulating glass in engineering and detects the dew point of the failed product on site.

1. The Project Overview

The project is a high-end residential commercial housing complex in New York City, American, with a total construction area of about 900,000. There are 18 residential buildings in total, the highest building height is 90.275 m, the volume ratio is 1.81, and the greening rate is 36%. The project started construction in 2012 and was completed in 2014. It was delivered in the same year and the occupancy rate reached 80%. The light-transmitting parts of the project mainly use insulating glass with a thickness of 6+12A+6 (mm), 6+0,76+6+12A+6 (mm), and 5+0.76+5+9A+5 (mm). The length and width are between 540 mm and 2220 mm, which belongs to a super large glass. 

2. The Problem Description

After moving in July 2014, the owners found that part of the insulating glass was fogged, and gradually there was a clear phenomenon of water droplets hanging on the wall. The insulating glass where this phenomenon occurs is distributed on the balcony, master bedroom, second bedroom, study room, and kitchen of each floor. This phenomenon occurs in the insulating glass used in north-south facing houses, and it occurs randomly on the insulating glass used in the opening fan and the fixed fan, and there is no rule to follow. The owner has cleaned the outer surface of the insulating glass many times, but the phenomenon has not improved. And with time, this phenomenon has become more and more serious. Gradually, part of the insulating glass has lost its sense of transparency, seriously affecting the lighting in the building.

Since the houses purchased by these residents are all well-decorated houses, the developers provide quality assurance for the glass of the houses. In this regard, both the developer and the community property owner expressed their willingness to replace the atomized insulating glass for free. But in the process of using the house, because there are no rules to follow, the owners are restless, worrying that there are also problems with other insulating glass, and there are even potential safety hazards. The developer has not encountered the same problem before and passed the inspection when the house is handed over. The developer actively helps the owner to deal with this problem and does not want to hurt the brand because of the quality of the insulating glass. An on-site inspection will find that there is indeed fogging and frosting on the inner surface of some insulating glass. There are no obvious impact marks on the inner and outer surfaces of the glass with fogging and frosting, and no damage is caused by an external force.

3. The Reason Analysis

The schematic diagram of the insulated glass structure is shown in Figure 1. If there is visible moisture in the insulated glass on site, it means that the insulating glass is invalid. To find out the cause of the problem of the insulated glass in the residential area, the business commissioned a third-party testing company to provide technical support. The technicians first conducted an on-site survey of the insulated glass in the community and found that this phenomenon was consistent with the dew condensation and frosting that occurred in the dew point test of the insulating glass. The unqualified dew point is largely related to the manufacturing environment and use environment of the glass, and is inseparable from the edge materials such as glass processing technology, desiccant, and sealant. The quality of the edge sealing material directly affects the service life of the insulated glass. In the change of temperature and humidity environment, the surrounding water vapor continuously penetrates the insulated glass cavity, causing the desiccant to absorb water until it fails.

^{Figure 1 The schematic diagram of insulated glass structure 1}

The dew point does not meet the standard and can be analyzed for the following reasons:

(1) Sealant. 

According to the insulating glass spacer material and sealing method, the safety insulating glass for the building is divided into polysulfide sealing groove aluminum type double-channel sealing insulating glass, silicon trunk rubber sealing groove aluminum type double-channel sealing insulating glass, polyurethane sealing groove aluminum Double-channel sealed insulating glass, composite butyl rubber strip sealed insulating glass, hot-melt butyl rubber sealed groove aluminum type. Nowadays, the safety insulating glass used in construction is widely used as polysulfide seal groove aluminum type double-channel sealed insulating glass and silicon trunk rubber seal groove aluminum type double-channel sealed insulating glass. In the use of sealant, the quality of the sealant for insulating glass is not good or the thickness and width of the sealant are too small, which will affect the bonding degree of the sealant. In many cases, workers do not pay enough attention to the construction, the operation is not standardized when using manual gluing equipment, and the lack of sizing technology leads to the uneven and discontinuous coating of the colloid, which will lead to turbidity, discoloration, and a large number of air bubbles in the sealant. , pulverization, discontinuity, glue flow, and the phenomenon of fogging due to too high volatile matter content in the glue. The viscosity of the sealant and the glass will be greatly reduced so that the insulating glass cavity is no longer sealed, and a large amount of water vapor penetrates the insulating glass cavity, resulting in failure of the insulating glass, poor sealing effect, and short service life.

^{Figure 2 The insulating glass air tightness testing 1}

(2) Processing technology. 

Insulating glass for construction is a product in which two or more pieces of glass are evenly separated by effective support and bonded and sealed around so that a dry gas space is formed between the glass layers. The control of the production and construction process is not strict, and there are stains on the glass surface or spacers, resulting in a decrease in adhesion after lamination. Improper operation by the operator will directly lead to poor lamination effect and sealing failure of the insulating glass. After a period of use, the insulating glass will crack or degum, which will lead to condensation and fogging of the insulating glass over time.

^{Figure 3 The insulating glass outer laminated plate pressure structure 1}

(3) Desiccant. 

In the choice of desiccant, if an unqualified molecular sieve is used as the desiccant, the result is likely to cause salting out in the spacer, which will corrode the spacer, cause the molecular sieve to fail, and the moisture content will increase. The water absorption capacity of the desiccant also affects the moisture content in the hollow glass cavity to a certain extent. Or during the processing, the desiccant is exposed to the air for too long, which makes the desiccant invalid or partially invalid. The factory does not pay attention to process supervision. The desiccant absorbs too much water, and the hollow cavity has failed.

^{Figure 4 The insulated glass molecular sieve desiccant filling machine 1}

4. On-site Investigation and Detection Analysis

To check the dew and frost phenomenon of the insulating glass in the community, the technicians first inspected the use of the sealant on site and found that there were a large number of air bubbles, pulverization, and discontinuity in the sealant strip, and sealant in the outer channel was uniform and neat and did not exceed the glass. edge. However, the thickness and width of the inner sealant are too small. A sealant that does not meet the specified technical requirements affects its full bonding and sealing effect with the glass. Secondly, the processing technology and the glass surface were observed. The outer surface of the glass has no obvious scratches, the inner surface of the glass has a little stain, and there is no flow of sealant. Since the project has been put into use for some time, it is impossible to determine whether the stain is the pollution when the insulating glass was assembled at the factory, or whether it was caused by the penetration of water vapor due to the decreased sealing performance in the later stage.

In addition, it is impossible to directly verify the failure of the desiccant without destroying it. It is only recommended to send a part of the desiccant for inspection in the later stage to test the durability of the water-air seal. On-site visual inspection of the spacer material shows that there is no distortion, the surface is smooth and clean, and there are no stains, spots, or flaky oxidation. The hollow cavity is free of foreign objects.

Finally, a test plan was formulated according to the site conditions. Since it is difficult to accurately identify the production or delivery batches of the glass used in the project installation, the dew point project determined the number of sampling groups to be 1901 pieces according to the test plan and the actual operating conditions on-site. The test volume covered Most floors of the complex. The technicians marked the glass of the dew point test sample on-site,

^{Figure 5 The Insulating glass air tightness manual inspection 1}

Inject ethanol into the dew point meter, and then add dry ice to lower the temperature below -60°C. After keeping the contact time, remove the dew point meter and observe the surface immediately. If there is frost or condensation, continuously increase the test temperature until -40°C o Thickness and quantity are 1528 pieces of glass of 6 + 12A+6 (mm), 437 pieces of glass of 6+0,76+6 + 12A+6 (mm), 5+0,76+5 + 9A+5 ( mm) glass 16 pieces. Due to the consideration of potential safety hazards at the high level, the operation of technicians, the placement of equipment, and all tests are carried out indoors. In terms of staffing, 2 groups of technicians were arranged, each with a total of 4 sets of equipment. Each tester is responsible for one instrument and equipment, and the coordinating personnel is appointed by the entrusting party. The two groups of personnel enter the site in turn. It is estimated that 4 devices enter the site at the same time. Every day (working hours 9:00—17:00) can test about 220 pieces, under the condition of ensuring the work intensity and the accuracy of the results, it will take about 18 days to complete the test. After arriving at the scene, each device is equipped with 1 auxiliary person. Before the test, the entrusting party shall number the glass of each window and door in the project, and describe the position of each numbered glass. Door and window drawings with numbers and descriptions shall be handed over to each tester. The specific test sequence will be fully discussed by both parties before entering the venue. After the test is completed, the results are summarized in Table 1.

^{Table 1 Summary of dew point detection results}

Among the 1,608 samples in Building 19, 1,484 had a dew point of <-40°C and 124 had a dew point of ≥-40°C. Condensation and frost appeared, and the unqualified rate reached 7.71%. 20 Among the 288 samples in Building No. 2, 245 samples had a dew point <-40°C, and 43 samples had a dew point >-40°C. Condensation and frost appeared, and the unqualified rate reached 14.93%. Among the 1,896 samples installed in total, 1,729 samples had a dew point of <-40°C, and 167 samples had a dew point of ≥-40°C. Condensation and frosting occurred, and the unqualified rate reached 9.66%. On-site testing of 90 uninstalled samples showed that the dew points were all <-40°C, and there was no condensation or frosting. If there is frost or condensation on the inner surface of the glass sample, the dew point is ≥ 40°C after inspection, which cannot meet the standard. It can be seen that the occurrence of frost and condensation on insulating glass is directly related to the failure of the dew point of the product to meet the standard.

5. The Solutions

After a large number of on-site tests, the project found that the quality problem of the insulating glass caused serious dew and frost during use. The developer also replaced the insulating glass for the owners free of charge.

Judging from the on-site situation, even in high-end residential areas, after two to three years, some insulating glass has the problem that the dew point does not meet the standard. question. Therefore, it is suggested that the project purchaser should pay attention to the following issues:

(1) Product qualification.

Since the product certification of architectural (safety) hollow glass products, hollow glass manufacturers have been forced to carry out product certification. Enterprises must silk screen or paste the 3c logo on the product body for sale, or on its minimum outer packaging and accompanying documents ( Such as the certificate of conformity) is added with the 3c mark. When purchasing products, first check whether there is a 3c mark, and check whether the purchased product is within the competence of the company that has passed the compulsory certification through the Internet based on the company information, factory number, or product certification certificate and whether the certification certificate is valid. At the same time, for the main materials such as sealant and desiccant that affect the life of the product, you should confirm the supplier with the manufacturer, and check the certificate to confirm the relevant qualification of the supplier.

(2) Admission inspection. 

When entering the site, check the appearance quality of the product. The insulating glass must not have any stains, inclusions, and sealant splashes that hinder the perspective. Part of the samples can be properly taken and sent to a third-party laboratory for verification of their quality. The inspection company can conduct quality control inspections on the newly entered insulating glass. Most of the failures of insulating glass are due to the continuous penetration of water vapor from the edge into the hollow cavity in the environment, and the desiccant will eventually lose its water vapor adsorption capacity due to the continuous adsorption of water molecules. However, due to the change in ambient temperature, the thermal expansion and contraction of the gas in the hollow cavity make the sealant under stress for a long time, and the ultraviolet rays and water vapor in the environment accelerate the aging of the sealant and accelerate the water vapor entering the hollow cavity. speed, so that the insulating glass cannot reach the expected service life. For example, in this project, the dew point did not meet the standard after only two years of use. For the long-term service life of the insulating glass, it can be sealed and durable in the laboratory. Performance, gas sealing durability, water vapor permeability of edge sealing materials, and other related tests to verify the ability to insulate glass sealant to block water vapor penetration (try to choose automatic insulated glass sealing equipment), molecular sieve adsorption capacity (try to choose automatic molecular sieve irrigation installation equipment), the level of process control in the production of insulating glass (try to choose the automatic insulating glass production line with screw plate pressure and glass side inflatable function); solve the worries of customers.

6. The Epilogue

After a series of on-site and professional technical personnel investigations and guidance, the project has not had any quality problems with insulating glass. However, since the project started testing after the problem was discovered, it was an "after-the-fact" treatment. Due to the limitations of the site conditions, the specific cause of failure could not be thoroughly investigated. The on-site dew point detection only represents the dew point performance of the product in the current state (after several years of installation and use of the project), and cannot explain whether the problem is caused by the aging of insulating glass over time. At present, in the production process of insulating glass, manufacturers of different scales have a relatively large gap in the production process level, and there is a certain gap in level, resulting in a difference in the quality of the final product, and the profit-seeking behavior of some enterprises has made the quality of insulating glass, so it is recommended to carry out a systematic quality inspection before the material enters the market, and test the water and air sealing durability of the product and other indicators that affect the long-term performance of the product, to improve the quality of the insulating glass as a whole and reduce the damage caused by the insulating glass.

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