aBstraCt thermal modeling software can be used to account for issues with elevated condensa- Many building owners and developers predict the location of a dew point within a tion levels by pressurizing the building and are concerned with the implications of building enclosure assembly, and thereby, removing moisture through increased wall upgrades in the energy code, and spe- assist designers in determining the appro- section drying potential or by increasing cifically, the requirement for continuous priate placement and permeance of air and the temperature, which in turn allows for exterior insulation around the building vapor retarders when continuous exterior increased vapor storage in the air. This is enclosure. There is concern over the initial insulation is used, as well as what changes impractical for a variety of reasons, not the cost, the lack of information that they cur- may occur to the traditional position of the least of which is exposing building occupants rently have regarding the payback period, dew point. When permeance and psychrom- to greater discomfort. and the question, “Will this really make a etry are not considered in tandem with the A more appropriate method of vapor con- difference?” The more pertinent question, upgraded energy code requirements, how do trol is to design and construct the building however, may be, “How will this make a we stop ourselves from hurting ourselves? enclosure with hygrothermal control. This difference?” The answer is not necessarily creates the design necessity to manage vapor as direct as initially expected. The coupled INTRODUCTION transmission and condensation formation, use of hygrothermic modeling during design Consider a glass of iced tea on a hot utilizing various approaches, including air and performance testing of building enclo- summer day. Intuitively, we know that barriers, vapor retarders, and insulation in sures with continuous exterior insulation this glass is going to “sweat.” This same exterior walls. Building components need to during the construction of a facility provide phenomenon can occur in building wall be designed and constructed in the appropri- much-needed information on how these systems. As water vapor migrates through ate locations to manage where condensation upgraded energy requirements affect the the building enclosure and moves toward occurs within the wall assembly. The design hygrothermal performance of the exterior the colder components with surface tem- intent is not to prevent condensation, but walls of a building. As one may expect with peratures below the dew point temperature, to manage where it will occur in order to simulating the hygrothermal performance of condensation occurs on those surfaces. minimize the effect on the building or its the building enclosure with continuous exte- Condensation is the change in physical occupants. rior insulation, the thermal transmission state from vapor into liquid (water) when in The appropriate design and construc- through the exterior wall is substantially contact with a solid surface. Condensation tion of exterior wall systems has become decreased. Inherently, with this decrease in in building construction can be an undesir- something of a moving target as we attempt thermal transmission, there is a decrease in able condition, as it may cause discomfort to build lighter, more sustainable buildings. moisture transport as well. When moisture through humid interior conditions, biolog- The industry is consistently learning more content is reduced, there is, intrinsically, a ical growth, corrosion, and deterioration, about how wall configuration changes will decrease in thermal transmission; and from as well as decreased energy efficiency of affect performance and the proper roles that there, the cycle continues. mechanical and building enclosure systems. air barriers, vapor retarders, and insulation The author will demonstrate how hygro- HVAC systems can be adjusted to should play in the ongoing transition from 2 2 • I N T E R F A C E A P R I L 2 0 1 7 the mass-wall mindset to rainscreens and of the world, and whether or not those sys- with the transfer of that heat and mass. For drainage walls. In addition to the consider- tems would perform as well as wood stud example, temperature conditions in a build- ation for advanced HVAC systems, we now walls. Research completed during this study ing influence moisture transport. Inversely have the factors of permeance and diffusion accounts for in-depth information on system and simultaneously, high moisture levels to consider, which will be further discussed thermal performance of various insulation result in increased heat losses. The analysis in this study. configurations of stud walls by use of the of heat and moisture coupling is known as As one may expect with continuous insu- ASTM C236 method, which has now been “hygrothermics.” lation (CI), thermal transmission through replaced by ASTM C1363, Standard Test Warme und Feuchte instationär (WUFI) the exterior wall is substantially decreased Method for Thermal Performance of Building Pro 5.2[11] is the computer software used in when compared to walls where the insula- Materials and Envelope Assemblies by this study to anticipate the outcomes of the tion is not continuous (e.g., stud walls with Means of a Hot Box Apparatus. Information various wall systems under review by hygro- insulation between studs). Inherently, with from the testing of the assemblies contained thermal modeling. WUFI provides realistic, this decrease in thermal transmission, there in the aforementioned study has shown that dynamic simulations regarding transient, is also a decrease in moisture transport. continuous exterior insulation has realized coupled one-dimensional heat and moisture When moisture content is reduced, there is benefits regarding thermal transmission of transmission in multilayered wall systems also a decrease in thermal transmission. In wall systems. when they are exposed to various climat- other words, generally, when you improve A second study completed by ic conditions. Factors considered in the one, you improve the other. Mukhopadhyaya, Ping, Kumaran, and van simulation include, but are not limited to, As one of the updated provisions in Reenen[3] investigated the role of Class interstitial condensation, the influence of the 2015 International Energy Conservation I, II, and III vapor retarders in exterior driving rain, airflow in the assembly, and Code (IECC),[1,2] building enclosure testing wood-framed stucco walls utilizing a 2-D the effects of proposed designs on building has been incorporated as a key tool in the simulation tool, hygIRC-2D.[10] The above- enclosure systems. verification of performance of the building referenced paper highlights the importance As with all software, there are many via air leakage testing of the thermal enve- of placing a vapor retarder with the appro- input requirements for beginning the model, lope, which is now mandatory. In alignment priate permeance correctly in the wall sys- and the accuracy of the model is highly with this provision, another key to avoiding tem and that the permeance of the total wall dependent on the quality of the data input. performance issues and potentially costly impacts moisture collection in the building With WUFI, the major considerations are: failures in building design and construction components. 1) components of the wall assembly, 2) ori- has become more than just planning—it is This paper is a collateral extension of entation, 3) surface transfer coefficients, 4) establishing and validating that the exterior the previously mentioned studies and uti- initial conditions, 5) time horizon, 6) hygro- of the facility meets the owner’s objectives. lizes the technology of a hygrothermal mod- thermal special options, and 7) climate This can be done through the verifica- eling tool that simulates heat, vapor, and information. These configurations are fur- tion process of building enclosure commis- moisture in one dimension. This simulator ther expanded in subsequent paragraphs. sioning (BECx). It is through the holistic will be used to model various insulation and lens of building wall comparison throughout vapor retarder configurations in multiple Components of the Wall Assembly the commissioning process—encompassing climate types in North America. The intend- The basic wall assembly that was mod- design, construction, and testing—that this ed outcome is a study that addresses the eled in different configurations is as shown study will view performance and consider- impacts of the updated IECC prescriptive in Figure 1. ations for multiple methods of insulating insulation requirements and recommended The material properties used for each exterior walls based upon the IECC pre- configurations, and provides guidance on of the building enclosure components are scriptive requirements. The focus of this optimizing building enclosure performance. as outlined in the materials database in paper will be on exterior stud walls with the WUFI software. The properties are combinations of both CI and insulation Wufi® and simulation inPut as derived through detailed comparisons within the stud cavity. This study is based reQuirements between known conditions and measure- on computer-based hygrothermal simulation Hygrothermal modeling aids in the ments taken through Germany’s Institute and modeling. These modeled wall systems understanding of moisture will rely upon data from previously conduct- conditions and the effects ed field testing for validation,