The design of high-performance buildings requires careful attention to detail to create enduring structures with low energy use, especially when it comes to modern building enclosures. Applied building science is needed to predict the movement of heat and moisture through building enclosures assemblies. This analysis helps find the optimal design of enclosure assemblies to support both high-performance building operations and long-term durability. The following example illustrates this, showing how a simple building science analysis was used to solve a common metal building exterior wall condition.
Much research and literature on vapor retarders is available to help designers understand building code requirements and mitigate condensation potential due to moisture diffusion. However, some energy codes require increased insulation levels and when designers choose standard metal building wall assemblies and add insulation to meet these requirements, without consideration for moisture performance, potential condensation issues arise impacting the durability and insulation effectiveness of the assemblies.
Figure 1: Double-layered, filled-cavity metal building wall detail (original design)
We were recently contacted by a designer requesting a WUFI simulation to evaluate the moisture performance of a metal building wall system proposed for a warehouse to be built in California. Though WUFI (Wärme Und Feuchte Instationär—German for heat and moisture transiency) is a very useful tool to study wetting and drying potential, this analysis request for a metal building wall system is unusual.
Upon review of the proposed system, the designer’s concern became apparent. The metal cladding system included an insulation layer draped over the girts and covered with a laminated facing, as a condition for the manufacturer warranty.
To meet thermal resistance requirements of the local energy code, the designer proposed an additional layer of insulation placed between the girts (Figure 1). Such a system is identified in ASHRAE 90.1 as a typical double-layered filled-cavity metal building insulation system. The laminated facing is a Type I vapor retarder with a vapor permeance of the laminated facing is 0.02 perms. Such a vapor retarder sandwiched between two layers of insulation prompted the designer’s concern for the moisture performance of the assembly. Though a WUFI simulation was requested, we recommended a simple one-dimensional static temperature and vapor pressure gradient calculation to determine the dew point location.
Figure 2: Vapor pressure gradient profile
The warehouse will be located in California Climate Zone 2 with an outdoor design temperature of 28 F and indoor design conditions of 72 F at 50% RH. Figure 2 shows the saturated and actual vapor pressure gradients across the various material layer interfaces and the location of condensation, as expected on the warmer side of the laminated facing. In addition, the temperature gradient across the wall section showed that the laminated facing is below the dew point of conditioned indoor air, if it were to leak through the wall assembly. An alternate approach was needed to avoid the laminated facing acting as a vapor retarder. Though continuous insulation on the exterior would overcome this problem, it was not considered viable due to cost and added complexity to the design/installation processes.
The Metal Building Manufacturers Association energy compliance guide for metal building contractors identifies a similar situation and recommends the use of a perforated laminated facing (Figure 3). The designer worked with the product manufacturer to specify perforations at 1 foot on-center, resulting in a vapor permeable laminated facing with a perm rating of 10. In addition, the cavity insulation was specified with a foil faced vapor retarder having a vapor permeance of 0.5 (Type II vapor retarder). The revised design detail was analyzed to confirm that there are no overlaps of the vapor pressure or temperature gradients, thus mitigating any potential for condensation due to diffusion or air leakage.
Figure 3: MBMA recommended approach for double-layer, cavity-filled metal building wall assembly.
Ideal vapor retarder placement within metal building wall assemblies is dependent on the type, R-value and number of layers of insulation in the assembly. Though vapor diffusion is not the major source of condensation, using vapor pressure gradient and temperature gradient calculations, designers can identify the location of condensation due to diffusion and air leakage. Such an analysis is a cost-effective means to quickly compare design alternatives, avoiding detailed moisture simulation using WUFI or other complex analysis. Applying the right type of analysis at the right time can optimize building enclosures, enhance energy performance and reduce risks for architects, contractors and owners.
Alan Scott, FAIA, LEED Fellow, LEED AP BD+C, O+M, WELL AP, CEM, is an architect with over 30 years of experience in sustainable building design. He is a senior consultant with Intertek Building Science Solutions in Portland, Ore. Krishnan Gowri, Ph.D., LEED AP, Fellow ASHRAE, BEMP, is a senior consultant with Intertek Building Science Solutions in Tacoma, Wash., with over 25 years of experience as a building scientist, energy modeler and energy code expert. To learn more, visit www.intertek.com/building/building-sciences/ and follow Scott on Twitter @alanscott_faia.