Hygrothermal Evaluation Comparison of Static and Dynamic Analysis Methods Quickly Displays Building Performance

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1 This article was originally published in MasonryEdge/the StoryPole Vol6 No1. Hygrothermal Evaluation Comparison of Static and Dynamic Analysis Methods Quickly Displays Building Performance by Andrew Dunlap AIA, CDT, LEED AP, NCARB To predict and prevent detrimental accumulation of condensation within enclosure systems, Hygrothermal Evaluations are performed. Material damage, corrosion, loss of insulating capacity, organic growth (mold) and poor air quality can result from uncontrolled condensation. These evaluations determine the enclosure s ability to MANAGE water transport through the system under varying conditions. Hygrothermal: pertaining to Humidity and Temperature Hygrothermal analysis tools, methods and software programs aid in designing enclo-sure systems that do not accumulate moisture, do not exceed the Critical Moisture Content (CMC), promote drying if wetting occurs and avoid unintended conse-quences of improper material selection. In today s design and construction field, elevated interior relative humidity may be desired for occupant comfort or may be necessary to properly accommodate the functions of the building. Facilities such as museums, hospitals, laboratories, libraries, arenas, manufacturing, computer centers, concert halls, green houses, entertainment, natatoriums and mixed use may require or experience elevated levels of relative humidity (RH) and must be evaluated to determine if water accumulation can occur within their proposed (or existing) enclosure systems. If evaluation by modeling indicates potential for accumulation of condensation, then revisions to the design must be implemented to control the condensation. In the case of existing facilities, remedial measures may be required. Evaluation Process Good professional practice stipulates that some level of hygrothermal evaluation be performed when designing enclosure systems that separate spaces with varying environ - ments. ASHRAE 160 Criteria for Moisture- Control Design Analysis in Buildings is a recent standard (2009) developed to provide performance based design criteria for predicting, mitigating or reducing moisture damage to the building envelope In some jurisdictions, the standard is referenced as an acceptable method to design enclosures. When a comprehensive evaluation is required, three distinctly different types of software, used simultaneously, are required. Each is designed to analyze specific types of condi tions and to provide specific types of information. Each has advantages and limitations and varies in complexity. Some are static, some dynamic. Use of multiple analyses concurrently allows evaluation of a system s hygrothermal performance from a multi-faceted point of view. Suitable data, knowledge and experience are necessary to obtain reliable answers. If not available, the evaluation will likely be inaccurate and may present more risk than benefit. The level of evaluation depends on required performance of the enclosure system(s). Not all buildings or occupancies require comprehensive evaluation. One cannot simply state the level of evaluation required for any particular facility. This determination is subject to multiple variables, such as geographic location, climate type, interior temperature and RH, enclosure systems and material used and the level of knowledge of the design professional. Depending on the facility s location, the definition of elevated interior RH may vary. In climate zones 4 and above, evaluation of enclosure systems should be considered in buildings that are actively humidified or that can expect an interior RH greater than 20%-25% during winter months. As the interior RH level increases, the level of evaluation will increase as well. It requires a knowledgeable and experienced technical architect, mechanical engineer or both to determine and ensure proper use of the software, determine the appropriate level of evaluation and to perform an accurate hygrothermal evaluation providing accurate results. Dewpoint calculation is one approach used to predict and prevent detrimental levels of condensation. Due to requirements to obtain product and weather data and effort to perform calculations, analysis was not often executed. Computer aided software has been developed to streamline this process. LEARNING OBJECTIVES Upon reading the article you will: 1 Compare and contrast three software programs used when performing hygrothermal evaluations. 2 Recognize the importance of material selection, properties and placement within an enclosure system relative to the facility s exterior climate and expected interior temperature and relative humidity levels. 3 Understand moisture management capabilities of the described wall systems for specific interior and exterior conditions assumed. Standard dewpoint calculation methods are still being used. However, utilizing advanced software and methods is becoming more prevalent. More advanced programs originally developed by laboratories to study enclosure system performance were typically developed and designed by scientists for scientific use. A wide selection of software readily available does not mean that they are easily understood or used correctly. The critical component is the user who must understand benefits and limitations of the software to accomplish accurate evaluations. Refer to ASTM MNL20: Moisture Control in Buildings: A Key Factor in Mold Prevention, ASTM MNL40: Moisture Analysis and Condensation Control in Building Envelopes and to the US Department of Energy website for Building Energy Software Tools Directory for an extensive list of available software. This article focuses on proprietary dewpoint calculation software, WUFI and THERM 5.2 to describe the evaluation process. SmithGroup s Thermal and Vapor Analysis Program (SG TVAP) Developed by Curt Songer, PE, SmithGroup, SG TVAP is a steady-state dewpoint calculation software utilized to predict locations of conden - sation that may occur within an enclosure system. It requires the R-value and Perm value of the system s materials to perform the calcula - tion. Plentiful historic weather data is available for use with the software. Due to the static nature of dewpoint analysis, the material properties are not altered due to change in temperature or moisture content. This method also does not account for thermal lag or thermal storage. It is a cumulative analysis (not consecu tive) that determines if the system Vol 6 No 1 MASONRY EDG E / the storypole Masonry Technology Innovation

2 Figure 1: Sample Output of SG TVAP Dewpoint Analysis Software. Graph includes temperature profile, dewpoint temperature profile through enclosure materials (color coded) at specific interior and exterior temperature and RH. experiences a net wetting or net drying for a one year cycle. If net wetting, then the amount for a one year cycle is predicted. Indication of expected condensation locations is represented at locations where the temperature profile line drops below the dewpoint temperature profile (shaded in black in the sample provided). Sample output from TVAP Dewpoint Calculation Software (Figure 1) Output from the dewpoint analysis of a relatively common Brick Veneer Cavity Wall on a Steel Stud Backup Wall is chosen to illustrate a common problem. Stud cavity is filled with batt insulation and the interior vapor barrier is located directly behind the interior gypsum wallboard. Besides thermal bridging caused by steel studs dramatically reducing the effectiveness of insulation, the effects of improper material selection can also be seen. (Thermal bridging caused by steel studs cannot be analyzed with dewpoint analysis software and will be discussed in more detail in the example evaluation.) The air/water barrier material located outside of the exterior sheathing was deliberately input as a vapor impermeable material to show detrimental effects of improper material selection. Many air/water barriers also have properties of vapor barriers. With an interior temperature of 72.5 F, interior RH of 50%, when utilizing dewpoint analysis software, substantial condensation can be predicted to occur within the batt insulation and exterior sheathing due to the vapor tight characteristic of the air/water barrier. This is indicated by the black shaded area where the temperature profile line drops below the dewpoint temperature profile line. For this specific condition, selection of a vapor permeable air/water barrier would have eliminated the likelihood of condensation from occurring. Proper material selection must be considered to prevent condensation from an unintended use of a vapor barrier. WUFI Wärme und Feuchte instationär in German (Translation: Transient Heat and Humidity) Developed and described by the Fraunhofer Institute for Building Physics (IBP) and Oak Ridge National Laboratory (ORNL), WUFI is a menu-driven PC program which allows realistic calculation of the transient coupled one-dimensional heat and moisture transport in multi-layer building components exposed to natural weather. It is based on the newest findings regarding vapor diffusion and liquid transport in building materials and has been validated by detailed comparison with measurements obtained in the laboratory and on outdoor testing fields. WUFI can be thought of as a continuously changing dewpoint analysis. The interior and exterior environments are constantly and more realistically changing throughout a specified period of time. It incorporates more precise hourly weather data including temperature, RH, precipitation, wind and solar radiation. It accounts for changes in material properties due to temperature and moisture content and also includes the effects of thermal lag and storage. Moisture movement is calculated in two directions, to the interior and exterior. Air and/or water leakage can be simulated by injecting a source into individual material layers. It considers the hygroscopic nature of materials through absorption/desorption (wetting/drying) which allows for materials to absorb and retain a certain amount of moisture. Simulations are typically performed over a five year period to determine if an enclosure system is accumulating moisture over an extended period of time. However, due to the complexity of material and weather data required for the software, data is often limited. WUFI produces various graphs of the individual materials or the entire system s temperature, water content and RH as a function of time. It also provides a real-time animation that simulates the change in system materials temperature, moisture content and RH as it runs through a given hourly weather tape. Sample output from WUFI (Figure 2) A screen capture of the WUFI animation paused at a winter condition illustrates a simulation performed on the same wall system as the dewpoint analysis in Figure 1: Brick Veneer Cavity Wall on a Steel Stud Backup Wall. Again, thermal bridging of metal studs cannot be analyzed. Unlike the results of the dewpoint analysis (indicating location of condensation), the WUFI diagram provides the amount of water content and the RH within the materials at any given time. The professional must determine if these levels are acceptable. Individual material layers are illustrated by light color-coded shading and are labeled below the graphs. For instance, on the far left, solid brick masonry is indicated by light red shading. Indication of temperature, RH and water content are overlaid onto material layers. In the top graph, the dark red line is the current temperature through the system. Light red shading signifies temperature history as the system runs through multiple yearly cycles. In the bottom graph, the dark blue line is the current water content, light blue shading indicates history. Also, in the bottom graph, the dark green line is the current RH. History is represented by light green shading. Interior conditions for this sample are similar to the previous dewpoint analysis. However, WUFI has the ability to fluctuate conditions, so for this sample, interior temperature was set to vary between 70 F to 75 F and 45% to 55% RH. Outside conditions continuously change as dictated by actual recorded weather data. This 2011 Vol 6 No 1 MASONRY EDG E / the storypole Masonry Technology Innovation 19

3 Hygrothermal Evaluation Figure 2: Screen Capture of WUFI Animation of Temperature, RH and Water Content of the materials (color coded) of an enclosure system as it actively simulates a specific time frame of interior and exterior temperature, RH, precipitation and solar radiation. occur within the materials to determine if it is detrimental. It is the professional s responsibility to determine if conditions experienced by the system are acceptable. To avoid consequences of an unintended vapor barrier, proper material selection must be accomplished to prevent elevated levels of material RH and water content. THERM 5.2 Developed and described by Lawrence Berkley National Laboratory (LBNL), THERM is a state-of-the-art, computer program for use by building component manufacturers, engineers, educators, students, architects and others interested in heat transfer. THERM models 2D conduction heat-transfer effects in building compo nents such as windows, walls, foundations, roofs and doors where thermal bridges are of concern. Heat-transfer analysis, based on the finiteelement method, allows for evaluation of a product or system s energy efficiency and local temperature patterns, which can help identify or may relate directly to problems with condensation, moisture damage and structural integrity. While it does not include the effects of moisture, surface temperatures can be compared to dewpoint temperatures determined by other means to verify if there is risk of condensation. Figure 3: Sample Output of THERM 5.2 Color Infrared Image. Temperature predictions can be identified at any pinpoint location throughout the modeled system and total product (or system) U-factors can be calculated. is a more accurate depiction of what is actually occurring in reality and should yield more accurate results. Red and green small horizontal triangles to the right and left of both the top and bottom graphs indicate interior and exterior temperature and RH conditions occurring at the point the simulation was paused. The clock on the upper right indicates time when simulation was paused. The bar directly below illustrates progress of the total simulation. Blue arrows between the two graphs, some to the interior and some to the exterior, represent the direction of moisture flow for individual material layers. Where the animation is paused, a slight elevation in water content can be seen in the exterior sheathing. The exterior sheathing and batt insulation experienced elevated levels of RH for some period of time. Raw data can be extracted and analyzed to determine how many consecutive hours various conditions Explanation of sample output from THERM 5.2 (Figure 3) THERM was developed to determine thermal performance of fenestration systems. How - ever, design professionals have found other uses for it, such as predicting how adjacent wall construction affects fenestration (and vice versa) and analyzing effects of thermal bridge conditions occurring in enclosure systems. Figure 3 includes models of a curtain wall head transition to a cavity wall. The two models compare effects of a typical fixed steel lintel (left) and an atypical thermally broken steel lintel (right). As indicated, the system utilizing a thermally broken lintel has warmer surface temperatures, is more efficient and can withstand higher interior RH with less risk of condensation Vol 6 No 1 MASONRY EDG E / the storypole Masonry Technology Innovation

4 Compare and Contrast One principal advantage of dewpoint calculation, its inherent simplicity, can also be seen as its primary disadvantage. Due to the static nature of the method, it must be viewed as a snap shot in time. Dewpoint calculations analyze systems at specific interior and exterior temperatures. Since this method does not account for the hygrothermal effect of material properties, it can become problematic and inaccurate. Professionals could assume they are analyzing a system at what appears to be the worst case scenario (winter or summer), when in fact it may not be the worst. In addition, dewpoint calculations generally do not account for thermal bridging of materials within enclosure systems, i.e. stud wall construction with insulation placed in the stud cavity. Advanced software such as WUFI and THERM have been developed to over come these limitations. These thermal analysis programs begin to include other aspects into the evaluation process of enclosure systems. WUFI provides comprehensive analysis through out continuous yearly cycles, assisting professionals from missing the worst case condition and resulting in a better and more accurate evaluation. Although it is a static analysis program, THERM can be utilized to evaluate effects of thermal bridging in enclosure systems. What can and cannot be accomplished Estimation/Prediction of moisture content of individual materials within a system: A common misconception is that all condensation is bad. This is not necessarily the case. Many enclosure systems can and do experience condensation within the system throughout a yearly weather cycle. Certain systems are able to manage condensation due to the hygroscopic nature of some materials. Materials are able to absorb and store/retain a certain amount of water without having a detrimental effect on the performance of the material/system. The system must not continue to accumulate moisture over time but to maintain a moisture balance. Provided the desorption is equal to or greater than absorption, detrimental effects from accumulation may not occur. Transient analysis software can determine if a moisture balance is maintained. This software can be used at all levels of the design process to determine, understand and prevent moisture accumulation within systems. How much water is too much for a given material? The simple answer is any amount of water above and beyond the critical moisture content for the specific material. The CMC is the point at which damage could start to occur within a material. There is not one particular value; it is different for various materials. One source of information for CMC can be found in the Proceedings of the Bugs, Mold and Rot II Workshop sponsored by the Building Environment and Thermal Envelope Council (BETEC) and the National Institute of Building Sciences (NIBS). Other general guidelines have been developed over time by various organizations, such as the developer of WUFI. Some of these guidelines can be found on the WUFI forum website. Because it is difficult to assess acceptable levels of moisture and data is not readily available, a safe practice is to choose wall systems that minimize moisture collection. Experience and careful consideration must be relied upon to determine the acceptability of moisture content within materials and systems. Estimation/Prediction of RH: Similar to the prediction of moisture content, transient hygrothermal analysis software can also predict the RH within various materials of an enclosure system. This capability can help predict and prevent microbial growth from occurring on or in certain materials. In order to sustain certain types of microbial growth, four criteria must be present: a nutrient source, optimum temperature, optimum RH and time (duration of the temperature and RH). The RH of a material changes as environmental conditions surrounding the material change. If temperature of a material lowers, but moisture content remains the same, it will result in a higher RH. Transient software can simultaneously calculate the RH, temperature and duration of both throughout an enclosure system. This data can be extracted and analyzed with newly developed add-on software to determine if these conditions are occurring at a location where there may be a nutrient source. If data analysis results suggest potential for microbial growth, the system must be redesigned and the analysis process would begin again. Figure 4 is an isopleth of a microbe that indicates the time, temperature and RH required Figure 4: Isopleth: A line connecting points on a graph that have equal values in relation to other specific variables. Shown is an isopleth of a common microbe for germination and growth. For example, according to Figure 4, the temperature must be between 68 F and 100 F and RH between 89% and 98% for at least one full day in order for germination to occur. As the temperature or RH range increases, the corresponding days required for germination must also increase. Additionally, in order for a growth rate of 2.0 mm/day, a temperature range of approximately 68 F to 100 F and an RH range of 89% to 100% must be maintained. If the temperature or RH ranges increase, the growth rate will decrease. THERM can be utilized to evaluate effects of thermal bridging in enclosure systems. Prediction of Freeze/Thaw Cycles: Another use for transient modeling is the prediction of the number of freeze/thaw cycles that may occur within materials. Clay masonry is often subjected to detrimental effects of freeze/ thaw cycles. After performing analysis with modeling software, raw data can be extracted and evaluated separately in software such as MS Excel to determine the number of cycles that may occur at a specific location within a particular enclosure system. Additionally, moisture content of the material at time and location of the cycles may be extracted. Software does not output yes/no answers, only data. Masonry materials can accommodate a certain amount of moisture without risk of damage. In order for masonry to be negatively affected by freeze/thaw cycles, moisture content at the time of the cycles must be greater than what the specific material can accommodate, the CMC. Transient modeling software cannot provide this number; it can only determine whether or not it has been 2011 Vol 6 No 1 MASONRY EDG E / the storypole Masonry Technology Innovation 21

5 Hygrothermal Evaluation Figure 6: Section Detail of Wall Type One Figure 7: THERM 5.2 Plan Detail Model of Wall Type One. Results in Infrared Mode show thermal bridging. Efficiency of insulation: Performance of many insulation products decreases as the moisture content increases. Ample moisture dependant R-value data is available. However, when performing a traditional dewpoint analysis, it can be difficult to determine what R-value to use. As a result, analysis can be quite inaccurate depending on the enclosure system being evaluated and type of insulation used. Transient modeling is beneficial for this. As transient simulation is a consecutive process, insulation conductivity is continuously recalculated and based on material moisture content at that particular moment. Depending on the makeup of the enclosure system, interior and exterior temperature/rh conditions and location of the insulation, moisture content can have dramatic fluctuations, corresponding to variations in thermal performance. If elevated levels of moisture content occur, performance loss is probable. Current simulation capabilities provide substantiation of performance loss. Recently developed add-on software can be used to predict the changes in thermal performance of the system shown on an average monthly basis through a year cycle. Figure 8: Output Results from SG TVAP for Wall Type One. The Temperature profile line does not drop below the Dewpoint temperature profile line. Condensation should not occur. exceeded. The only way to determine the CMC is through physical testing of specific brick in question. This information may be available from some manufacturers. Effects of Air/Moisture Sources/Sinks: Some of the more advanced modeling software include air and/or moisture sources/sinks within the enclosure assembly. These can be incorporated into the modeling to simulate leakage and/or ventilation of air and/or water. These can replicate the cladding component of a rainscreen system that is leaking a certain amount of air and/or water. This can also be helpful when evaluating effects of partially vented cavity wall systems. The potential drying effect the cavity has on performance can be more accurately shown with advanced software. Ability for the vented space to promote drying can be predicted or validated. Water sources can be included to show the effects of a leaking wall to determine if it will stay continually wet or if it has the ability to dry. Inclusion of water sources to simulate leaks may also have an effect on the RH of individual components of a system which can affect their thermal performance and ability to promote microbial growth. Similar to capabilities previously discussed, software can simulate effects and provide results of air/moisture sources/sinks. However, little guidance is available to determine the input value to replicate actual conditions. Example Evaluation Further illustrating characteristics and capabilities of hygrothermal analysis software, the following is an example of an evaluation of three different, but similar, commercial wall systems. Interior and exterior conditions are the same for each. The following conditions are assumed. Facility is in a northern climate (Climate Zone 5) with elevated levels of interior RH. Effective air barrier is provided. In all cases the vapor barrier can also be the air barrier. Includes the effect of moisture migration through diffusion, not through air leakage. Interior surface coating (paint) is considered to be a vapor transmitting material that does not impede water vapor from migrating in either direction. Temperature and RH Conditions Interior Temperature: 72.5 F, +/- 2.5 F Interior Relative Humidity: 50% set point, +/-5 Exterior Conditions: Recorded weather data for Detroit Wall Types: Wall Type One: Brick/Steel Stud Cavity Wall with Stud Space Insulation Wall Type Two: Brick/Block Cavity Wall with Cavity Insulation Wall Type Three: Brick/Steel Stud Cavity Wall with Stud Space Insulation and Cavity Insulation Vol 6 No 1 MASONRY EDG E / the storypole Masonry Technology Innovation

6 Many wall systems contain two zones, the wet zone and the dry zone. Typically, these zones are divided by a material that serves as the water (resistive) barrier. This material can sometimes also serve as the air and/or vapor barrier. Materials used in the dry zone generally cannot tolerate elevated levels of RH or moisture content. Materials used in the wet zone can be expected to be wet at some point and must be able to tolerate effects of elevated levels of RH and moisture content. Extruded polystyrene, spray polyurethane foam and some polyisocyanurates are insulation products generally considered acceptable for use in the wet zone of a cavity wall type construction provided they meet all applicable building codes and ASTM material standards. Multiple materials that can be used for the water barrier can often also be the air barrier. Generally these should be self adhered sheets or fluid applied materials. Composition of these materials can vary greatly and can significantly affect the water vapor permeance. Many of the liquid or sheet synthetic rubberized or polymer modified materials are typically vapor impermeable (barriers) whereas the acrylics and the spun or heat bonded polypropylene or olefin materials are often vapor permeable (breatheable). WALL TYPE ONE Brick / Steel Stud Cavity Wall with stud space insulation Wall Type One includes steel stud backup, continuous vapor permeable air/water barrier applied directly on exterior sheathing, noncontinuous batt insulation installed in the steel stud space and a vapor barrier installed directly behind the interior gypsum wall board. This system may not meet certain energy codes in certain climate zones. For example, a continuous layer of insulation with an R-value of 3.8 or more is required in climate zones 5-8 when using International Energy Conservation Code (IECC) 2006 as the energy code (Figure 6). It is critical to evaluate and understand both the plan and section views of any enclosure system. Figure 7 is an infrared plan detail of Wall Type One modeled in THERM 5.2 which illustrates thermal bridging effects of steel studs. If this were modeled in a section view (Figure 6), thermal bridging of the studs would not be evident. As can be expected, the effectiveness of insulation is dramatically reduced. As calculated by THERM 5.2, system U-factor for all components in the wall is approximately , which equates to an Effective R-value of approximately R11, less than half the calculated R-value of Figure 9: Screen Capture of WUFI Animation Film paused during a summer condition on Wall Type One. Elevated levels of water content and RH are present in the exterior sheathing and batt insulation located in the dry zone. Figure 10: Wall section at interface of exterior wall with floor slab. Infrared image produced from THERM model indicates thermal bridging due to the floor slab. Figure 11: Wall section at interface of exterior wall and roof slab. Infrared image produced from THERM model indicates thermal bridging due to the roof slab Vol 6 No 1 MASONRY EDG E / the storypole Masonry Technology Innovation 23

7 Hygrothermal Evaluation Figure 12: Section Detail of Wall Type Two: Brick and Block Cavity Wall Figure 13: THERM 5.2 Plan Detail Model of Wall Type Two. Results shown in Infrared Mode. No thermal bridging. approximately R23 at the center of the stud space. In addition, the system s ability to resist condensation at the surface is also reduced due to lower surface temperatures. Thermal bridging can also potentially cause staining on the interior surface of the wall. Figure 8 is the results graph of the SG TVAP analysis performed at a winter condition. Dewpoint analysis does not include effects of the thermal bridging from the steel studs. As indicated, the Temperature Profile Line does not drop below the Dewpoint Temperature Profile Line as they progress through individual materials. As a result, dewpoint analysis method would suggest that detrimental condensation should not be expected for the specific conditions in which the simulation was performed. Dewpoint calculation was intentionally only performed during a winter condition to illustrate that when used alone, it can sometimes be misleading. Figure 9 demonstrates areas of concern during summer months that would not have been discovered if relying solely on dewpoint calculation. WUFI simulation indicates highly elevated RH levels throughout exterior sheath - ing and batt insulation. Elevated levels of water content are also occurring in exterior sheathing and batt insulation. Both of these conditions are occurring in the dry zone of the wall system (to the right of the air/water barrier). Figure 14: Output Results for Wall Type Two from SG TVAP. The Temperature profile line does not drop below the Dewpoint temperature profile line. Condensation should not occur. It is critical to analyze end results of the WUFI simulation after it has computed multiple years of weather data. In this case, the wall system does not continue to accumulate water content over multiple yearly cycles. Exterior sheathing and batt insulation consistently become wetter during summer months and dryer in winter months. This is because the summer moisture drive is inward. Due to the placement of the interior vapor retarder, higher levels of summertime atmospheric moisture cannot dry to the building interior. It is not good practice to have "wet" materials in the dry zone even if they may dry out over a yearly cycle. Effectiveness of insulation is reduced due to elevated RH, becoming less efficient. If the RH level is maintained, it may promote and sustain microbial growth. Data can be analyzed to determine if optimum temperature and RH levels for microbial growth are present for a substantial period of time. Even if microbial growth is determined to not be a concern, accelerated deterioration of some materials can occur due to the elevated RH. Figure 15: Screen Capture of WUFI Animation Film paused during a winter condition. No indication of elevated levels of RH or moisture content in the dry zone Vol 6 No 1 MASONRY EDG E / the storypole Masonry Technology Innovation Wall Type One can also cause other problems not evident in static or dynamic hygrothermal

8 analysis programs. In Figures 10 and 11, effects of not using continuous insulation in the wall cavity can easily be observed thermal bridg ing at the floor and roof slabs and discontinuous installation of the interior vapor barrier. Depending on interior RH, thermal bridging may cause localized condensation, damaging materials within and adjacent to the wall system. Thermal bridging also reduces overall efficiency of the exterior enclosure s thermal performance. Buildings utilizing this type of wall system often have perimeter steel beams installed quite close to the exterior wall. These limit access needed to provide a quality installation of the vapor barrier and insulation directly below floor and roof slabs. This installation can be extremely difficult to achieve, if possible at all. In addition, due to the vapor barrier s location, it will inherently be discontinuous at the floor and roof slabs. The vapor barrier must rely on the concrete slab to maintain the continuity of the vapor barrier system. Depending on the building s interior RH level, this may not be acceptable. WALL TYPE TWO Brick/Block Cavity Wall with cavity insulation Wall Type Two is a typical Brick and Block Cavity wall with interior gypsum wall board. It includes a continuous air/water/vapor barrier applied directly on the exterior of the CMU wall, a continuous layer of 2" extruded polystyrene insulation, 2" air space and exterior veneer brick (Figure 12). Figure 13 Infrared plan detail of Wall Type Two as modeled in THERM 5.2. No thermal bridging is present as the insulation is continuous in the cavity. System U-factor for all components in the wall is approximately 0.067, which equates to an Effective R-value of approximately R15. This software does not include benefits provided by thermal mass of masonry in the Effective R-value calculation. Figure 14 Results graph of SG TVAP analysis performed at a winter condition. Temperature Profile Line does not drop below the Dewpoint Temperature Profile Line as they progress through individual materials of the wall system. Dewpoint analysis method would suggest that potential detrimental condensation should not be expected for specific conditions in which the simulation was performed. Note: there is a point at the back side of the masonry veneer in which the material temperature and dew - point temperature nearly cross. This minor Figure 16: Wall section at interface of exterior wall and floor slab. Infrared image produced from THERM model indicates no thermal bridging due to the floor line. amount of moisture is not considered detrimental as it is in the cavity of the wall system, the wet zone, which by its nature has some venting capabilities. Figure 15 Screen capture of WUFI animation paused at a winter condition. There is no indication of elevated RH or moisture content (green and blue lines and shading) in the dry zone (right of vapor barrier). Depending on material type, a certain amount of water content may always be present. For example, concrete layers that make up concrete block consistently show approximately 2 lbs/ft 3. The key is that water content does not continue to accumulate over time. Defining elevated may be difficult as it is dependent upon many factors, such as material in which RH is being evaluated, corresponding temperature and duration of the elevated condition. However, if occurring in a material that could support microbial growth, if the RH begins to approach 70% with a corresponding temperature of approximately 50 F to 100 F for an extended period of time, this could be considered elevated. Refer to figure 4 for other possible ranges. Note: A brick cavity wall utilizing a steel stud back wall could perform in a similar manner as this wall system as long as it also utilizes continuous insulation in the cavity only and continuous air/water/vapor barrier installed directly on the exterior sheathing of the steel stud wall assembly. However, the benefits of the thermal mass from the CMU backup wall would not be included. Figure 16 indicates the importance of utilizing continuous insulation installed in the cavity. Unlike Wall Type One, thermal bridging does not occur at floor and roof slabs as the cavity insulation is continuous, keeping slab edges warm. The vapor barrier is also not interrupted by the slabs with this type of wall. Brick relief angles are not included in figures 10 and 16. It is important to understand that if brick relief angles are required, they should also be installed in a manner that will thermally break the steel from interior to exterior. Cavity insulation must remain as continuous as possible. Because it is difficult to assess acceptable levels of moisture and data is not readily available, a safe practice is to choose wall systems that minimize moisture collection. Proprietary products are designed specifically for this application or it can be achieved by holding the steel angle away from the wall with intermittent steel clips allowing insulation to be installed behind the angle. No matter how this is achieved, additional and atypical detailing is required to ensure that thermal bridging is not occurring at relief angles, lintels or any other conditions that may produce a system breach. (Note: This type of detailing is not considered standard practice. However, in buildings with high humidity, it is critical to follow through and evaluate all atypical detail conditions beyond the typical wall system details, looking for potential areas of thermal bridging similar to what can be found at floor slabs, roof slabs, steel lintels and relief angles.) 2011 Vol 6 No 1 MASONRY EDG E / the storypole Masonry Technology Innovation 25

9 Hygrothermal Evaluation WALL TYPE THREE Brick/Steel Stud Cavity Wall with Stud Space Insulation and Cavity Insulation Wall Type Three, a cavity wall with continuous air/water/vapor barrier on exterior sheathing and continuous insulation installed in the cavity, sometimes created as an afterthought, begins as good design. Assuming there is no insulation installed in the stud space, the wall would perform similar to Wall Type Two, just not with the added benefit of thermal mass from the CMU backup. However, this wall type now includes insulation in the steel stud backup. Note: there is not a vapor barrier installed on the interior side of the insulated stud assembly. Often, someone has an idea to fill steel studs with insulation to gain additional thermal performance. However, the air/water/vapor barrier that is in the cavity is now effectively insulated from the heat of the interior. Without adding another vapor barrier on the interior side of the stud assembly, this condition can allow elevated interior RH to condense on the interior surface of the exterior sheathing. If a vapor barrier were installed on the interior side of the stud assembly, then similar issues as described in Wall Type One occur (Figure 18). Figure 19 indicates the infrared plan model of Wall Type Three. Due to the continuous layer of cavity wall insulation, effects of steel stud thermal bridging are not as significant as in Wall Type One. However, effectiveness of the batt insulation is still dramatically reduced. Batt insulation added to this system has an R-value of 19. The System U-factor for all components included is approximately 0.046, which equates to an Effective R-value of approximately 22. This is only R7 more than Wall Type Two, not an additional R19 as might be expected. Figure 18: Section Detail of Wall Type Three: Brick, Steel Stud Cavity Wall with Stud & Cavity Insulation Figure 19: THERM 5.2 Plan Detail Model of Wall Type Three. Results shown in Infrared Mode Figure 20 is the results graph of the SG TVAP analysis performed at a winter condition. As indicated, the Temperature Profile Line drops below the Dewpoint Temperature Profile Line as they progress through the system s individual materials. This would suggest that potential detrimental conden sa tion could be expected within batt insula tion and exterior sheathing for specific conditions in which the simulation was performed. Figure 21 is the screen capture of the WUFI animation for Wall Type Three paused at a winter condition. Simulation indicates elevated RH levels in exterior sheathing and batt insulation. Water content is also elevated in the exterior sheathing. Both of these conditions are occurring in the dry zone. Figure 20: Output Results from SG TVAP. The Temperature profile line drops below the Dewpoint temperature profile line. Condensation may occur within exterior sheathing and batt insulation. Similar to Wall Type One, it is critical to analyze the end results of the simulation after it has computed multiple years of weather data. In this case, the wall system does not continue to accumulate water content over multiple yearly cycles. However, exterior sheathing and batt insulation consistently become wetter during winter months and dryer in summer months. A primary benefit of using transient software is that there is no longer a need to perform multiple dewpoint calculations at multiple times throughout a year. Transient software essentially performs calculations at all times throughout a year. The difficulty with this simulation is determining if moisture content has exceeded the critical moisture content for the system. Again, it is not a good idea to have "wet" materials in the dry zone even if they may dry out throughout a yearly cycle. Effectiveness of the insulation is reduced due to elevated RH and material damage such as corrosion of steel studs may occur. It should be made clear; using steel stud backup walls is not the concern. The problem is when studs are filled with batt insulation and used in facilities that contain higher levels of interior RH. Unintended consequences Vol 6 No 1 MASONRY EDG E / the storypole Masonry Technology Innovation

10 Figure 21: Screen Capture of WUFI Animation Film paused during a winter condition. Elevated levels of water content and RH are present in exterior sheathing and batt insulation located in the dry zone of the wall system. of these decisions may result in substantial material and property damage. Future Development Sophistication of analysis programs has outpaced information that is readily available. When used carefully and correctly, they can provide more useful data than ever before. Additional, more accurate and comprehensive material and weather information is needed. More information is also needed for the CMC of various materials and various materials resistance to freeze/thaw damage. Organizations such as ASTM, ASHRAE and ORNL continually work to meet this demand. However, as the requirements for the evaluation process continues to increase, the demand for material data will increase as well. Manufacturers can assist this by proactively testing their materials and providing the necessary material data. How much water is too much? Professionals must first understand that analysis software cannot provide simple answers. Careful consideration is required to determine which systems will work for which applications. Hygrothermal evaluation can help us determine if our systems will perform without risk of detrimental condensation. In addition, they offer a method to compare multiple systems to provide the best possible solution for any given situation. There are sources of guidance and reference regarding various methods of hygro thermal evaluation from organizations such as ASHRAE, ASTM, ORNL and LBNL. There are varying opinions throughout the industry about the types of methods that are most accurate. However, the one item that is typically agreed upon is that evaluation is required. Predicting problems before they occur is a primary goal of this process. For more information and additional resources, visit n n n Andrew Dunlap, AIA, CDT, NCARB, LEED AP, is an associate with SmithGroup s Building Techology Studio. His work focuses on the analysis and development of exterior building envelopes, specializing in the thermal and vapor analysis of wall, window and roof systems. Dunlap serves as Vice President and Programs Committee Chair for the Building Enclosure Council Greater Detroit Chapter and as an adjunct professor at University of Detroit Mercy. He has also served on the Steering Committee for Development of a new degree program at the University, for what is now the Architectural Engineering Bachelor of Science program. Dunlap holds BS degrees in Mathematics and Architecture and a Masters of Architecture from the University of Detroit Mercy Andrew.Dunlap@SmithGroup.com 2011 Vol 6 No 1 MASONRY EDG E / the storypole Masonry Technology Innovation 27