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Applied Building Science

Analysis for High-Performance Enclosures

Alan Kristy Horizontal

An elementary understanding of science tells us that water flows downhill, that air moves from high to low pressure, and that heat transfers more readily through materials with high conductivity. Though it may seem so, these fundamental concepts are not sufficient to predict the movement of heat and moisture through our complex building enclosures assemblies, subject to varying indoor and outdoor conditions. We need to use building science to find the optimal design of enclosure assemblies to support both high-performance building operations and long-term durability. In last month’s column, we explored the importance of material compatibility in enclosure design. This month we will dive deeper, with a look at the application of using building science to inform enclosure design and specifications.

Building science combines physics, material science, meteorology, construction technology, architecture and engineering. This science is used to improve building enclosures (the horizontal and vertical assemblies, above and below grade, that physically separate interior environments from the external elements). Building enclosures control rain penetration, noise, fire, light, solar radiation, and the flow of air, heat, and water vapor, and at the same time must be durable, economical and architecturally expressive. More stringent building codes and increased demand for resilient, high-performance, low-carbon buildings, has placed greater emphasis on enclosures to support energy efficiency, daylighting, indoor air quality, thermal comfort and reduced carbon footprints.

Architects can improve enclosure design through applied building science to better control moisture, heat and air flow, and solar radiation. They can ensure a sound and functional enclosure by understanding the critical locations that require thoughtful design strategies (e.g., wall assembly, air/weather barriers, insulation, cladding, glazing, shading). Some of these design considerations include:

  • Moisture flow occurs when there is a source of moisture (rain and other precipitation), a path for it the travel through and a force to move it. Moisture control typically includes a water shedding surface (roofing, cladding, flashings) and a moisture barrier (sheathing membrane, drainage cavity membrane).
  • Air flow control with a barrier that is continuous, air impermeable, durable and stiff, integrated from below grade, up vertical systems and across horizontal elements such as plazas and roofs. Uncontrolled air leakage can contribute to energy loss, occupant discomfort, and condensation.
  • Vapor diffusion control with a vapor barrier that impedes the flow from high vapor pressure to low vapor pressure (warm to cool). Vapor barrier continuity is not as critical, unless it is also acting as the air barrier, except when this system is meant to reduce flooring failures (below a slab on ground) or is part of a temporary roofing layer during construction.
  • Heat flow in an enclosure occurs through the conduction of materials (thermal bridging), convection (mechanical ventilation, stack effect, air leakage), and radiation (through roofs, windows, cladding). Heat flow through the enclosure can lead to energy loss, occupant discomfort, and material deterioration. 

Building science can also be used to address fire and smoke spread with the proper placement of firestops around wall assembly gaps, openings, and penetrations, as well as noise and vibration control (with proper insulation and the interruption of noise/vibration transfer pathways). Two of the primary tools deployed by building scientists for moisture movement and heat transfer are hygrothermal and thermal analysis.

WUFI analysis of wall assembly. Image courtesy of WSP. 

Hygrothermal analysis is used to map heat and moisture transfer through envelope assemblies. The analysis models vapor diffusion and determines the relative humidity within an assembly, highlighting condensation risks. This informs when interventions, such as vapor barriers, should be installed and where to place them within the assembly to reduce the risk of condensation. Vapor diffusion is modeled using hygrothermal modeling tools such as WUFI (Wärme Und Feuchte Instationär—German for heat and moisture transiency). WUFI models the one- and two-dimensional heat and moisture transportation through assemblies, such as walls and roofs, tracing the variation in temperature, water content and relative humidly across the layers and thicknesses of proposed materials in critical assemblies. The software has a built-in and customizable user database that calculates wetting and drying, capillary action, heat transfer and vapor diffusion based on assumed interior conditions under mechanical load and outside conditions based on weather files. It can be used as a comparison tool to examine options, allowing the user to review locations of important layers of air tightness.

Thermal analysis is used to model heat transfer through enclosure assemblies, and is especially helpful when modeling areas with thermal bridges and identifying where the dew point will occur in an assembly. This analysis informs the designer on where to add thermal breaks or where additional insulation is needed. Thermal analysis is also helpful to understand the overall thermal performance of assemblies, which can feed into whole building energy models, enhancing the accuracy of results. Thermal analysis is conducted using tools, such as THERM by Lawrence Berkeley National Laboratory (LBNL), that model two-dimensional heat-transfer through building components. THERM uses a CAD underlay to trace over architectural details and define material properties using standard and user-customized databases. It uses finite element analysis to calculate heat transfer by conduction based on conductance values for enclosure materials. THERM is more accurate (in many cases) than one-dimensional heat transfer hand calculations, as it calculates interactions between systems, such as the interface between windows and walls. Other software tools are also used to examine 3-dimensional heat transfer.

THERM analysis of window sill. Image courtesy of WSP.

Solar radiation analysis (with tools like Sol-Air) to examine surface temperatures can complement a THERM analysis by more accurately modeling the exterior boundary temperature condition and its impact on heat transfer, in addition to its more direct applications for sizing expansion joints in materials or calculating the potential for solar energy systems.

Intuition and elementary science can only carry us so far. A deep understanding of building science, and the proper application of analysis tools such as WUFI and THERM, are an important compliment to the design and technical skills of the architect as they create beautiful, durable and sustainable buildings. Building science analysis can optimize building enclosures, enhance building performance and reduce risks for architects, contractors and owners. The ever-expanding knowledge-base in the marketplace, and in institutions offering building science programs in the United States will only continue to improve the built environment.   

Alan Scott, FAIA, LEED Fellow, LEED AP BD+C, O+M, WELL AP, CEM, is an architect with 30 years of experience in sustainable building design. He is a Senior Associate with WSP in Portland, Ore. Kristy Kwong, EIT, LEED GA, ENV SP, is a building enclosures designer with WSP in Los Angeles, CA. To learn more, visit and follow Scott on Twitter @alanscott_faia