As the focus of building construction progressively shifts toward sustainability with an emphasis on energy-efficient design, more importance is being placed on the building envelope's performance. Part of any building's performance is how it deals with heat, moisture and condensation control.
Understanding the movement of heat and moisture in buildings
“Modern construction relies on tight building envelopes to obtain higher energy efficiencies,” says Christopher Costanza, RA, AIA, LEED AP, architect at 9X30 Design Architecture LLP, Rochester, N.Y. “The problem with this is any hole within that envelope combined with a positive indoor pressure in winter or negative indoor pressure in summer will concentrate water vapor at a point where condensation can occur at or inside the building envelope and lead to premature system failure.”
Today’s super-insulated assemblies can create dangerous levels of entrapped moisture that go undetected by traditional dew-point analysis. Dewpoint analysis does not allow for capillary moisture transport or sorption capacity of component materials. Both characteristics reduce the risk of damage due to condensation. Dew-point analysis does not incorporate short-term conditions such as rain events and solar radiation.
For a more thorough analysis of how their designs react to heat and moisture, architects are using more complete, advanced, quantitative data via computer-aided hygrothermal analysis. Producing data that would be difficult to calculate otherwise, hygrothermal analysis can prevent a building with a design flaw from the outset, but can also study existing buildings.
Hygrothermal Analysis
Hygrothermal analysis examines the intrinsic movement of heat, air and moisture flow through building enclosure assemblies (walls, roofs, floors, below-grade). Hygrothermal model results include temperature, humidity and moisture content data at materials and at interfaces throughout the assembly. Architects use this iterative process to simulate the performance of assemblies so that the future durability and risk associated with different assembly designs, materials, and climate or exposure can be evaluated and assessed.
Users can accurately visualize and assess factors such as surface condensation and mold growth potential, material degradation, and the wetting and drying potential of the building envelope. For instance, condensation risk can be analyzed with different insulation or vapor retarder properties. Even the influence of driving rain on cladding degradation can be predicted. Advanced features address surface heat transfer, permeance, short-wave radiation absorptivity, long-wave radiation emissivity, rain loads and moisture absorbance.
With hygrothermal analysis’ simple, straightforward reporting, users can determine a design’s expected performance, highlight problem areas for revision by the principal designers and even predict potential long-term problems in novel, un-tried assemblies. Different repair and retrofit strategies for alternative assemblies, climate variations and moisture loadings allow comparison and ranking of different designs. All of this produces a quick understanding of hygrothermal outcomes and their implications to building performance.
Because metal’s material properties are different than those of other building materials it is particularly important to analyze its properties for hygrothermal performance. “Metal has an extreme value for each material property needed for the analysis,” says Jasha Kistler, enclosure consultant, The Facade Group LLC, Portland, Ore. “For example, it has no moisture storage function, no liquid transport coefficient, zero permeability, very high thermal conductivity with negligible moisture or temperature dependency, and typically a high density compared to other building materials.”
ASHRAE Standard 160
To ensure consistency among computer tools designed for hygrothermal analysis reporting, the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) published ASHRAE Standard 160, “Criteria for Moisture-Control Design Analysis in Buildings,” in January 2009.
The standard provides performance-based procedures and criteria for moisture design analysis for buildings. It sets criteria for moisture design loads, moisture analysis methods, and satisfactory building performance. The standard can be used for the design analysis of the building envelope or to help guide specifications for HVAC equipment and controls. Eventually, it should form the basis for prescriptive moisture design rules based on a uniform set of design assumptions and loads. Intended to help reduce building failures in service, provide consistency in design approach and recommendations, the standard offers more flexibility in design for moisture control and better ability to incorporate new materials, and provide greater transparency by requiring reporting of design assumptions.
Hygrothermal analysis aims to prevent errors in building assemblies, like this metal panel system seam. (Image courtesy of Christopher Costanza, 9×30 Design)
According to ASHRAE, this standard originated because results obtained from earlier thermal and moisture models were extremely sensitive to the assumed moisture boundary conditions. For instance, during winter in cold climates, the moisture conditions in walls depend greatly on the indoor humidity conditions. Thus, a consistent approach to moisture design demands a consistent framework for design assumptions, or assumed “loads.” The question whether design features such as vapor retarders or ventilation systems are necessary cannot be answered objectively unless there is a consensus definition of the interior and exterior moisture boundary conditions that the building is expected to be able to sustain without negative consequences to itself or its inhabitants. This standard formulates design assumptions for moisture design analysis and criteria for acceptable performance.
According to ASHRAE, a design analysis involves the determination of the probability of failure, and treats all design parameters and loads as stochastic variables. However, sufficient data are often not available to make a full statistical treatment practical. Instead, where only limited data exist, a moisture design protocol must be based on a combination of statistical data and professional judgment. Another judgment involves the choice of an acceptable probability of the occurrence of damage. Although it is common to impose very stringent criteria for structural design because of safety concerns, moisture damage usually occurs over a long period of time and usually has less catastrophic, although sometimes costly, consequences. An international consensus has emerged that the analysis should be predicated on loads that will not be exceeded 90 percent of the time. This standard adopts this approach.
In a moisture analysis for building envelope design, the choice of indoor environmental conditions is extremely important, especially for buildings in cold climates. This standard opts for a design indoor climate definition that is based on engineering principles, is independent of construction, and reflects the influence of ventilation and air-conditioning equipment and controls that may or may not be part of the building design. In buildings where indoor humidity and temperature are explicitly controlled, the building envelope performance should be evaluated with the intended indoor design conditions. In residential buildings, indoor humidity is rarely explicitly controlled, so default design assumptions are needed for these buildings. In general, the standard encourages designers to use their own design parameter values if they are known and part of the design. If they are unknown or not included in the design, the standard provides default values for those loads and parameters.
Software Solutions
No one can do hygrothermal analysis on a napkin. Analyzing heat flows and moisture migration in buildings is not easy. State-of-the-art software like WUFI, HygIRC and Delphin can assist with this struggle.
Menu-driven WUFI was originally developed for use by building science researchers. However, in recent years an increasing number of architects have also started using the program hoping to answer design questions such as, “Will this design work?” and “Why did this wall assembly fail?” WUFI is a family of software products that allows realistic calculation of the transient coupled 1- and 2-D heat and moisture transport in walls and other multi-layer building components exposed to natural weather. WUFI is an acronym for Wärme Und Feuchte Instationär-which, translated, means heat and moisture transiency. Originally developed by the Fraunhofer Institute for Building Physics for use in central Europe, WUFI was later adapted by the Oak Ridge National Laboratory for North American climates and construction practices. The software simulates the movement of moisture and heat on, into, and through complex building assemblies that you build by choosing components from an internal data library.
Results from simulations include graphs showing the moisture content of the entire assembly as well as each layer within the system. Graphs are also available showing the temperature and relative humidity data for the simulations. There is also the option of graphing isopleths, an alternative metric generated by the simulation.
HygIRC is a Canadian program developed for research purposes by the Institute for Research in Construction (IRC) of the National Research Council (NRC) of Canada. HygIRC is a parametric program that computes 1-D hygrothermal performance of building assemblies. The program was developed as part of the MEWS (Moisture Management for Exterior Wall Systems) Research Program as a way to analyze the building physics. Though it was initially designed for research purposes, HygIRC is slowly being introduced into the commercial sector.
Delphin is a simulation program for the coupled heat, moisture and matter transport in porous building materials. The Institute for Building Climatology at Dresden University of Technology’ faculty of architecture developed it. Delphin calculates thermal bridges and evaluates hygrothermal problem areas such as surface condensation and interstitial condensation. It can also design and evaluate inside insulation systems, evaluate ventilated facade systems and ventilated roofs, and determine transient calculation of annual heating energy demand.
To improve your understanding of hygrothermal analysis, many hygrothermal software companies offer free downloads. Courses, seminars and workshops on this topic are frequently held to learn more about it.