Advanced Wall System Issues
Traditionally, wall systems have been rated for their energy efficiency either by calculating the thermal resistance (R-value) of the insulation material within the wall system or by full-scale testing of a so-called clear portion of the wall system. This type of testing does not include information on the energy efficiency of the corners, window frames, door frames, or other interfaces, which typically make up over 50 percent of the wall system area. Since they differ thermally and geometrically from the basic clear wall, their energy efficiency varies and is normally less than that of the clear wall.
Because the energy efficiency of a wall system is affected by much more than the insulation used or the performance of the clear wall, consumers and designers have had to compare wall system choices using limited criteria. We are striving to develop new methods for accurately and comprehensively rating the overall energy efficiency of whole wall systems. We are also developing Internet-based calculation tools that simplify the computation required to determine energy efficiency.
Calculating the thermal performance of thermally massive walls has
been difficult. The steady-state R-value traditionally used
to measure the thermal performance of a wall does not accurately
reflect the dynamic thermal performance of massive building envelope
systems. Whole-building energy simulations for buildings containing
massive wall systems are also problematic. For example, the
computer model DOE-2 uses a one-dimensional calculation engine,
which is inaccurate in simulations of complex building envelope
assemblies. To enable these computer models to perform whole-building
energy simulations for complex building envelope assemblies, simplified
one-dimensional descriptions of complex walls must be developed.
Currently, the standard modeling process is to replace complex material
configurations with one-dimensional multi-layer structures with
similar R-values and material arrangements. Unfortunately,
such simplifications cannot accurately represent the complicated
two- and three-dimensional dynamic heat transfer that can be observed
in most massive-wall assemblies. To demonstrate the benefits
of these assemblies, thermal-performance analyses must properly
reflect the effects of thermal insulation and mass distribution
inside the wall. The recently developed equivalent-wall theory
has allowed more accurate analysis of a high-mass wall's energy