By Jonathan Smegal, M.A.Sc. and Aaron Grin, M.A.Sc., P.Eng.
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This short report summarizes a multi-phase research project initiated by Gauvin 2000 Construction Ltd., Balanced Solutions (now Building Science Consulting Inc.), the University of Waterloo Building Engineering Group, and Building Science Corporation. Contributions of materials and funding for this project were received from Dow Building Solutions/Dow Chemical Company, James Hardie Building Products, DuPont, and Owens Corning.
During the Vancouver-area “leaky condo crisis” an unusually large number of moisture-related problems, not only in condos but also in other building types, prompted extensive public attention and ultimately many government and industry investigations. Among other things, questions were raised about the relative performance of different wall assemblies built before, during, and after the leaky condo crisis in the coastal Pacific Northwest climate.
The Vancouver Test Hut Project was initiated in 2005 to provide side-by-side, real-world performance data collected under Pacific Northwest coastal conditions, for a range of wall assemblies using different materials, ventilation gaps, and insulation strategies (interior vs. exterior).
Materials and Methods:
Building Science Consulting Inc. (BSCI) designed and instrumented a 900 square foot Test Hut facility, which was built in a rooftop location with full exposure to weather conditions in Coquitlam, B.C., one of the municipalities comprising Metro Vancouver. The Test Hut has a total of 28 wall test panels (seven per cardinal direction) and 6 roof test panels (three on each of the north and south roof slopes). In addition to weather exposure, intentional controlled wetting events can be carried out using wetting systems incorporated into the test panels during construction. An integrated monitoring and control system measures and records performance data, controls interior temperature and humidity conditions, and is connected to the internet for remote monitoring and control.
The research plan has included four phases to date:
- Phase I (1st Year): The initial set of test walls was constructed to represent conventional construction practices from the previous 40 years, as well as one wall with continuous exterior insulation. The role of specific assembly layers was investigated. During this phase, indoor relative humidity (RH) was allowed to fluctuate naturally, with a constant indoor air temperature of 20°C. Controlled wetting was used to simulate leaks on the interior surface of the sheathing.
- Phase II (2nd & 3rd Year): Phase II was a continuation of Phase I, with the RH increased to 50%.
- Phase III (4th & 5th Year): A new set of similar test walls was constructed. Indoor temperature remained at 20°C, and RH was reduced to 40%. Wetting apparatuses were used on both the interior and the exterior of the wood sheathing in each assembly, in order to simulate different types of leaks. The focus in Phase III was on the durability of the wall systems when subjected to exterior wetting of the sheathing.
- Phase IV (6th and 7th Year): This phase extends the investigation of assemblies with continuous exterior insulation, by adding new wall assemblies and increasing moisture loads. Data analysis for this phase is currently being completed.
The specific test walls for each phase are available on buildingscience.com (Phases 1-3 and Phase 4). Walls were deconstructed at the end of Phase II and again at the end of Phase III to observe the condition of the assembly components.
To date, research has yielded a number of important findings. For example:
- Both insulation levels and airtightness affect moisture-related durability. It is now well-known that as buildings become more highly insulated, less heat moves through their walls and the walls’ potential for inward drying decreases. It is less well-known that airtightness can be a factor. When relative humidity at the test hut was uncontrolled, the airtight environment led to high average RH. Under these conditions, wall assemblies typical of the 1960s (with R8 kraft-faced batt insulation) had higher than expected moisture contents, because the interior relative humidity was higher than what would have been expected of typical residential construction in the 1960s.
- Polyethylene barriers are a double-edged sword. The widespread adoption of polyethylene vapor barriers has contributed to improvements in airtightness and therefore also contributed to increased levels of interior moisture. Poly barriers, when well sealed, offer protection against the diffusion of elevated indoor relative humidity; however, they also restrict inward drying of incidental water from air leakage or rainwater intrusion.
- Existing strategies do work, but can be improved. Common practices in Vancouver have come to include a rainscreen with a large ventilation gap (current codes indicate 10 mm [0.4 in.]) and a class I interior vapor control layer. This combination works well as long as there are no air leaks: the gap allows the assembly to dry to the outside, reducing the risks associated with using poly on the interior. However, the risk of air leaks remains, and the use of poly on the inside of the assembly means that any incidental moisture in the wall cavity must dry to the exterior. As well, the cost of building with large drained and ventilated claddings is significant.
- There is a strong alternative design. The data shows that using continuous exterior insulation with a small gap of several millimetres can meet a number of different needs. This alternative wall design is cost-effective, energy-efficient, and durable. However, if the insulating sheathing is vapor impermeable, then the assembly must, of course, be able to dry to the inside.
Conclusions and Recommendations:
Although the Pacific Northwest is a challenging climate, the data suggests that several wall assemblies can be used there without significant durability risks. Standard rainscreen walls that comply with current Vancouver-area building codes are likely to be safe; however, the data suggests that different designs could also be successful. Two areas that should be further researched are smaller ventilation gaps and the use of continuous exterior insulation. The development of strong design recommendations for a wider variety of wall assemblies would allow architects and builders to confidently choose the best strategy to suit the specific requirements of each project.
- Smegal, J., Lstiburek, J., Straube, J. and Grin, A. 2012. RR-1207: Vancouver Field Exposure Facility: Phase III Exterior Insulation Analysis. Westford, MA: BSC. Available at buildingscience.com.
- Lstiburek, J. 2012. “BSI-058: Parthenon, Eh?” Available at buildingscience.com.
- The Vancouver Test Hut Facility. Webpage on buildingscience.com.
- SolPlan Review. “Performance of Insulated Walls: Effect of Continuous Exterior Insulation”. March 2013. Available here