Factors affecting the performance of ventilation cavities in highly insulated assemblies

Abstract

The article presents experimental studies of typical Finnish highly insulated (HI) envelopes with thermal resistance values ( R value) for the wall and roof inside the ventilation cavity between 7.7 and 8.1 m2K/W and 13 m2K/W, respectively. The conditions in the ventilation cavities were studied by using typical and increased R values for the exterior part of the cavity, which were 0.18 m2K/W and 1.57 m2K/W in the walls, and 0.13 m2K/W and 2.13 m2K/W for the roof. With higher exterior R values of 1.57 m2K/W and 2.13 m2K/W, the cavity temperature increased only after closing the inlet gap of the cavities. If the cavity inlet was closed, the restriction of the outlet gap from 20–25 mm to 10 mm had no significant effect on the temperatures. A closed ventilation inlet resulted in increased absolute humidity in the cavity, which indicates that the restriction of cavity ventilation should be made with care to avoid impairing the drying-out ability. The computational analysis showed that the optimal air change rates in the wall and roof cavities of HI structures were 4–40 1/h and 20 1/h, respectively. The conventional 22-mm-thick wood cladding enables safe cavity conditions in HI walls if the vapor barrier is vapor tight and other moisture sources are low. A lower heat flux and additional heat loss caused by cloudless sky at night support the observation that HI roofs have a higher moisture risk. In HI roofs, a conventional exterior R value of 0.13 m2K/W should at least be increased to the range of 0.3–0.4 m2K/W, which is achieved, for example, by a 20-mm-thick mineral wool board under the roofing. The use of mold-resistant materials in the ventilation cavity is recommended to mitigate the possible ramifications of the moisture behavior of HI roofs.

Hygrothermal performance of highly insulated external walls subjected to indoor air exfiltration

“…By selecting less vulnerable exterior sheathing materials, the MI values can be kept below 1 (Figure 9). In winter, the MI levels decrease due to subzero temperatures, and in summer, the high temperatures at the exterior section of a wall prevent mold growth, based on the measurements in the ventilation cavities of exterior walls (Viljanen et al, 2021).…”

“…As solar irradiance is negligible in Finland during the cold season (e.g. Viljanen et al, 2021), the results associated with cold weather are applicable for all the directions of the facade.…”

“…These measures aimed to minimize uncontrolled airflow, which is a major concern in exfiltration tests (Li et al, 2007). The ventilation cavity and the siding were omitted from the walls assuming a ventilation rate of the order 100-200 air changes per hour behind the fac xade (Viljanen et al, 2021). approximately 40 and 30 mg/h, respectively.…”

“…The location of the intended outflow of air was occasionally verified using a tracer gas (Sensistor XRS9012). In all the three tests, the PD across the walls was monitored with a Beck 984Q pressure transmitter, whose precision is at most ±1 Pa. During the first eight phases of the MCON1 test, the exfiltration rates were measured with an orifice plate pipe (see also Viljanen et al (2021)). The PDs across the reduction of the pipe and across the test wall were measured simultaneously using PD meters (Furness Controls FCO318).…”

“…For example, in Finland, the benchmark value for the thermal transmittance (U) of an external wall is 0.17 W/m 2 K. However, even lower values such as 0.12–0.14 W/m 2 K are widely used to minimize the energy consumption of buildings and meet strict energy efficiency requirements. The performance of highly insulated (HI) structures has been studied by, for example, Vinha et al (2013), Gullbrekken et al (2015), Pihelo et al (2016), Nelson (2017), Gradeci et al (2018), Viljanen and Lu (2019), and Viljanen et al (2021). Such studies have typically focused on the diffusion of water vapor to the colder outer parts of a HI structure.…”