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Targeting three typical external thermal insulation systems, this paper puts forward a new principle (change of capacitive reactance of high-frequency electromagnetic wave sensors) for measuring the temperature and humidity in the interface between different layers of exterior walls. Following this method, several other ways such as thermal calculations, software simulations and model analyses are compared with each other to evaluate the actual thermal insulation and heat transfer effects of the walls, discuss the causes of thermal insulation defects and the solutions to remedy these defects. The research findings provide a valuable reference for the design, construction, application, evaluation and repair of energy-saving walls in hot summer and cold winter areas.
external thermal insulation (ETI), exterior wall, moisture content, coupled heat and moisture, thermal insulation effect.
Several field tests were performed on the three ETICS of thin-plastered, capturing the variations in indices like the internal temperature field and moisture content. Then, the coupling between the temperature and humidity were analyzed based on the test data.
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The thermal conductivity coefficient and other thermal parameters were extracted from the energy-saving design data in the design drawings (Table 1). For coal gangue sintered hollow brick, the air interlayer was neglected in the heat transfer calculation. The heat resistance refers to the comprehensive thermal resistance. If the actual thickness deviates from the designed thickness, the actual thickness should prevail.
The leakage areas were first detected by an FLIR B360 infrared thermal imager, and the subsequent temperature and moisture measurements were carried out around these typical areas. The detection philosophy is as follows: Leakage takes place easily after the failure of the waterproof layer on the wall surface; in this case, the dampness may penetrate into the thermal insulation layer, which is porous and easy to absorb water. Due to the large thermal conductivity and heat capacity of water, the temperature distribution on the surface of defected parts will differ greatly from that on the surface of normal parts. This difference can be pinpointed by the infrared thermal image [4]. As shown in Figure 1, the exterior walls with Type RPPPs and Type GHBIM were subjected to different degrees of water intrusion (Figures 1a and 1b), while the exterior wall with Type RWB was basically not intruded by external moisture (Figure 1c).
(a) Facade of Type RPPPs
(b) facade of Type RPPPs
(c) facade of Type RPPPs
Figure 1. Thermal images for typical areas of three different ETICS
The temperature measurement was relatively simple. Several NiCr-Ni thermocouple sensors were pasted between
the wall structure layer and the insulation layer, and connected to an Ahlborn MU-56901 desktop data acquisition instrument. In this way, the temperature data could be obtained automatically at regular intervals. The real-time temperatures obtained from the positions of the sensors were automatically saved into the laptop (Figure 2a).
The temperature measuring points were arranged as follows. In view of the small size of NiCr-Ni thermocouples (diameter: 0.5mm), the sensors were placed through drilled holes (or prewired) at the interface between the wall structure layer (either clay sintered hollow brick or coal gangue sintered hollow brick) and the insulation layer and that between the insulation layer and the crack resistant mortar layer. The temperature sensing end was wrapped up with plasticine to ensure the contact-type temperature measurement, while the pores and fissures were sealed up with glass glue to prevent moisture entry and eliminate environmental impact.
3.1 Inspection of wall condensation
The internal condensation of the envelope structure is very dangerous and easy to occur. Hence, the three ETICS were inspected carefully when the indoor air conditioner was turned on and turned off. No internal condensation was observed in any of the systems, which fully reveals the advantages of ETI structure. Below is the calculation process of Type GHBIM in Table 1:
After looking up the table, it was calculated that R0=0.743 m2
K/W and the wall’s resistance to vapor permeability H0=2,891.03 m2
h
Pa/g.
The relative humidity of the indoor and outdoor environments was measured as 44% and 61%, respectively.
The partial pressures of water vapor in indoor and outdoor airs were calculated as:
${{R}_{m}}=\frac{\sum\limits_{D=1}^{MAX}{\sum\limits_{t=1}^{24}{{{Q}_{mt}}}}}{\sum\limits_{D=1}^{MAX}{\sum\limits_{t=1}^{24}{Q_{mt}^{req}}}}$
K)
K)
K)/W
