Masonry Wall Interior Insulation Retrofit Embedded Beam Simulations

Apr 02, 2012

There is a large stock of uninsulated mass masonry buildings; the retrofit of insulation is commonly considered to improve their energy performance. Some durability issues associated with interior insulation have been or are being addressed, such as interstitial condensation and freeze-thaw damage issues. However, another durability risk is the hygrothermal behavior of moisture-sensitive wood beams embedded in the load-bearing masonry. Interior insulation reduces the beam end temperatures, reduces available drying potential, and results in higher relative humidity conditions in the beam pocket: all of these factors pose a greater risk to durability.

Three-dimensional thermal simulations were performed to examine the effect of interior insulation on embedded wood members. Simulations were run for the cases both of large wood members (“beams”) and smaller dimension lumber members (“joists”). In addition, simulations were run of various methods that would increase heat flow to the beam ends; the resulting effect on overall heat loss was also examined. This was followed by one-dimensional hygrothermal simulations to gain greater insight into the beam end behavior, including airflow effects.

Results indicate that the methods to increase beam end temperature have mixed results. Metal spreader plates increase the temperatures at smaller joists, but they do not appear to be a worthwhile strategy in larger beams. The thinning of insulation near the embedded beam ends appears to have minimal thermal effects. Hygrothermal simulations give results that vary strongly based on starting assumptions and material properties: field monitoring to determine in-situ conditions is recommended instead.

Kohta Ueno (Building Science Corporation)
Presented at: 
Building Enclosure Science & Technology (BEST3) Conference
Published & professionally reviewed by: 
Building Science Corporation
Building Enclosure Technology & Environment Council (National Institute of Building Sciences)

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