The Effects of Moisture and Addition of Rigid Insulation to Exterior Walls on Building Envelope Performance

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Safavian, Henry
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In the northern prairie climate of Manitoba, the thermal resistance provided by the building envelope is an on-going concern. To improve thermal performance, some existing buildings need to be renovated. One common approach to achieve a higher thermal resistance for building enclosures is the addition of rigid insulation, such as extruded polystyrene foam (XPS), on the exterior side of a building without any changes to the inside of the structure. One challenge of this strategy is the possibility of having a damaged or absent vapour barrier, and the effect this may have on the building envelope performance. The field-based study presented in this thesis was conducted on two identical wood frame test buildings with different insulation materials inside the wall cavities. One had fiberglass batt and other with dense pack cellulose. The test program was completed at the Alternative Village at the University of Manitoba. Combinations of external rigid insulation and vapour barrier systems were compared to a typical wall assembly over a heating season. Three different indoor RH levels were used to compare hygrothermal behaviour and the potential for condensation and mold growth within the wall cavity. It was found that the addition of XPS in fiberglass and cellulose wall assemblies rose the outer wall cavity temperature. Also, the outer RH within the wall cavity increased in the cellulose building, while the addition of XPS did not affect outer RH in the fiberglass wall assembly considerably. It was found, however, that for the fiberglass building with a non-consistent vapour barrier, the average outer RH increased dramatically to 80% and 90% in mid and high levels of indoor RH, respectively. Similarly, in cellulose wall assemblies with non-consistent vapour barrier, the average outer RH in high-level indoor RH rose considerably to 49% and 63% considering without and with XPS, respectively. Therefore, the potential for condensation and mold growth on outside sheathing increased in both buildings. In terms of heat flux density, both the addition of rigid insulation and having consistent vapour barrier improved the thermal resistance of fiberglass wall assemblies. That is, the heat flux density through the wall assembly with non-consistent vapour barrier was 2.6 to 2.8 times greater than the value through the wall assembly with XPS and continuous vapour barrier. Furthermore, even though vapour barrier system did not affect the heat flux noticeably in cellulose enclosure, the heat flux density decreased considerably due to the addition of rigid insulation.
fiberglass insulation, cellulose insulation, thermal performance, relative humidity