Hygrothermal Simulation for the Building Surveyor

02/23/2011 14:18

Recent emphasis on sustainability has increased our desire to design more efficient new structures, leading to the development of more energy efficient envelope assemblies and the use of non-traditional and sometimes even novel assemblies – many of which do not have a substantial case history to commend them. Simultaneously, communities are turning to densification and urban renewal as viable options to reduce sprawl. When these two are combined, historic structures are often selected for redevelopment with new occupancies – often referred to as adaptive reuse. These two situations: high performance buildings and adaptive reuse of historic structures represent considerable challenges to the Building Surveyor. In order to meet these challenges, while considering the performance and durability of the proposed development work, computer modeling is becoming an increasingly useful tool. Understanding the complex interactions between heat, air and moisture movement within assemblies of new high performance buildings or adaptively re-used and modernized historic structures, is key to ensuring longevity and durability. These coupled interactions are dynamic, multi-dimensional phenomena and are not adequately modeled using one dimensional or static analysis. Known commonly as hygrothermal or HAM (heat air and moisture) simulation, modern numerical methods based on the finite element/finite volume principle are employed to predict the transient thermal and hygric behavior of multi-layer building assemblies exposed to natural weather and user defined interior conditions. The use of hygrothermal computer simulation programs is supported by various standards including ASTM E-241 (2000), WTA Guideline 6-2-01 and ASHRAE/ANSI 160 (2007). For the purposes of this brief, the focus will remain on programs with support in North America. WUFI®Pro (Wärme und Feuchte instationär) is a one-dimensional heat and moisture transfer model that is used to assess the hygrothermal behavior of multi-layer building envelope assemblies. Created at the Fraunhoffer Institute for Building Physics in Germany, a partnership with the Oakridge National Laboratory (ORNL) resulted in the development of a North American version. Currently, WUFI is in use in both Canada and the United States. It uses measured weather data, including driving rain and solar radiation, allowing realistic simulations of the behavior of assemblies under exposure to weather. Its calculations are based on detailed data regarding vapor diffusion and liquid transport in building materials and have been validated by measurements obtained in the laboratory and outdoor testing. One of the latest upgrades allows for the accounting of imperfections within the components and assemblies by introducing moisture sources and adding vented areas. WUFI can be used to test potential solutions, as well as assess deterioration mechanisms within building envelope assemblies. It can assist with estimating the drying times of materials having trapped or concealed construction moisture, investigating the danger of interstitial condensation, or studying the influence of driving rain on exterior building components. The program can also help to select repair and retrofit strategies with respect to the hygrothermal response of a particular assembly subjected to various climates. This allows the comparison of different designs with respect to total hygrothermal performance. WUFI is, in essence, designed to model only porous materials. Metals, glass and other effectively impermeable materials are cumbersome to model and require a certain manipulation of program settings. WUFI is only able to model one-dimensional systems. The interactions between bridged materials, such as insulation and framing, cannot be accounted for. Because of the limitations noted above, unless one is an expert in verification of input data, the model is limited to the value of “warning – change your design to one that is more safe”. This is sufficient and appropriate for most Building Surveying assignments. 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. The program was part of MEWS (Moisture Management for Exterior Wall Systems) Research Program as a way to analyze the building physics in low-rise, wood-frame construction. Initially designed for research purposes, HygIRC is slowly being introduced into the commercial sector. A major benefit of HygIRC is the ability to not only identify the area of greatest moisture build-up, but also the wetting period duration (Mukhopadhyaya et al 2003). HygIRC not only pinpoints a potential problem, it also aids in determining the magnitude and severity of that problem. The newness of this software means that its accuracy and reliability are still uncertain and can only be confirmed by long-term, controlled experiments. As such, the program is constantly undergoing adjustments to its algorithm and set-up. Another drawback is the RHT scale that is unique to this program. The RHT scale applies a linear weight to relative humidity and temperature; this assumption may not always be the case. Only through long-term, controlled experiments will the validity of the RHT assumptions be confirmed. Also, the RHT scale is not immediately intuitive. Unlike HygIRC’s contemporaries, which can give numbers and results that can be easily interpreted; the RHT scale requires a certain amount of background knowledge and understanding of how that scale was derived. Computer simulation programs give Building Surveyors and other design professionals the ability to model designs that would be difficult to calculate otherwise. Simulations quickly verify a design’s expected performance, highlight problem areas for revision, or predict potential long-term problems in novel, un-tried assemblies. While not a substitute for good judgment, judicious use of simulation programs can highlight potential deleterious side-effects and also pin-point potential improvements to proposed assemblies, increasing efficiency and reducing costs. -- Phil Parker MRICS Phil Parker MRICS Associate, Read Jones Christoffersen Phil Parker is RJC’s resident expert in heritage building conservation. He practices building science and restoration engineering, has a background in materials engineering and instrumentation, and extensive in-field experience. Mr. Parker’s expertise also includes facility planning, design, construction and management. Since joining RJC in 1996, Mr. Parker established the Nanaimo office in 1999, worked in project management for the Building Science and Structural Restoration Group in Victoria between 2001 and 2004, and was made an RJC Associate in 2009. Mr. Parker’s contribution to the industry has been recognized by his peers through awards of honor from the City of Vancouver and awards of merit from the Consulting Engineers Association of BC. Mr. Parker graduated with a degree in Civil Engineering from the University of British Columbia in 1994 and went on to complete the AIBC Building Envelope Education Program in 1999. In addition to being a published author, he is a guest lecturer and instructor at Camosun College in Victoria, and the British Columbia Institute of Technology. He is a member of the Royal Institution of Chartered Surveyors, a leading international organization for professionals in property, land, construction and related environmental issues.