Real R-Value of Exterior Insulated Wall Assemblies

real_r-value.pngThe recent drive towards sustainable building construction has placed new emphasis on the provision of durable wall assemblies that provide a high effective resistance to heat flow (R-Value).

The authors’ practice focuses on large multiresidential, commercial and institutional buildings constructed of concrete, steel, masonry and glazing systems. In these types of buildings thermal performance has not historically been treated as a high priority item. Now, however, the requirements of sustainability programs such as LEED are requiring architects to design wall systems that provide high levels of thermal resistance. Architects are often shocked at the difference between effective R-value of a proposed opaque wall assembly and the nominal R-value of installed insulation materials. The difference is a result of the thermal bridges associated with structural elements and connections that pass through the building thermal envelope.

The authors have undertaken analyses, using the modeling program THERM, to numerically evaluate the effective thermal resistance of some typical wall assemblies
used in high-rise residential buildings. We evaluated the impact of slab edge detailing
and a variety of secondary structural elements needed to support the cladding. We have developed a method of presenting the information in a manner that architects can
practically use to determine actual insulation thicknesses required to obtain the overall
walls’ desired thermal performance.

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