HOW VAPOR RESISTANCE PROPERTIES OF COATINGS AFFECT EXTERIOR WALL MOISTURE PERFORMANCE Garth D. Hall, Senior Architect, AIA, Kenneth M. Lies, Principal, AIA, and Sarah K. Flock, Architect III Raths, Raths, and Johnson, Inc. Willowbrook, Illinois, USA Abstract: Vapor resistance properties of architectural coatings that are used on or within an exterior wall can directly influence the moisture performance of the assembly. Through the use of computer modeling, the effect that various coating applications have on common exterior wall assemblies are studied using varying indoor and outdoor climatic conditions. Such analysis is useful in the selection of appropriate coatings for both new and retrofit construction, and also in diagnosing performance problems in existing construction. Coatings are selected for use on or within building wall assemblies for many purposes, including creating aesthetics, minimizing maintenance, or providing resistance to weather. For example, elastomeric coatings applied to a stucco or concrete cladding can help fill and bridge cracking thereby improving water penetration resistance, as well as the overall appearance to the building facade. On the interior surfaces of an exterior wall, coatings have been traditionally selected for their aesthetic appearance and ease of cleaning. In selecting a coating material, many different attributes of the materials should be considered and tradeoffs made based upon project criteria and restraints. Often times little or no consideration is given to the possible effect that a coating or film membrane may have on moisture migration which may lead to moisture accumulation within the wall assembly. For example, low permeance paints installed on the interior face of gypsum wallboard may provide the desired aesthetics and durability properties but, unfortunately, also function as an unintended vapor retarder. In cold (heating) climates, depending on interior climatic conditions, this type of installation may not adversely affect the performance of the wall and, in fact, enhance the moisture performance of an exterior wall. However, this installation coupled with an exterior coating with low vapor permeance will reduce drying potential of the internal wall assembly. Careful consideration of the vapor permeance properties of wall coatings is important in the design and material selection process, for it can greatly impact the potential for moisture related problems or significantly benefit the performance of the exterior wall assembly. This paper compares the vapor permeance properties of coating materials commonly used in wall construction, as well as the potential effect of moisture accumulation within the assembly, depending on placement and exposure to different climate conditions. Our analysis examines a prescribed wall construction in different climate zones with interior and exterior coatings of various vapor permeance characteristics modeled using a hydrothermal simulation computer program. The models incorporate the initial moisture content of the wall materials typical of new construction to illustrate wall performance as related to moisture accumulation and drying potential of various assemblies. 1
MOISTURE VAPOR MOVEMENT AND VAPOR PERMEANCE Water can be found in three states: liquid, solid, and vapor. Depending on its state, it can be moved or transported through or within an exterior wall assembly through various mechanisms. Building materials have physical properties which impact liquid moisture movement and accumulation including absorption, sorption, liquid water permeance, air flow, and vapor permeance. In vapor form, water is moved by differences in vapor pressure and air flow. Water vapor migration through a material by differences in vapor pressure across the material is a process known as diffusion. Vapor pressures are elevated by temperature and the amount of moisture vapor in the air. As a result, the water vapor will want to move from the warm and more humid side of a building envelope (higher vapor pressure) to the cold and dry side (lower vapor pressure) as the water vapor molecules seek to reach equilibrium (Figure 1). Figure 1. Moisture movement through vapor diffusion in heating climate. Understanding the direction of this movement is relatively straightforward in extreme climates such as northern Minnesota or southern Florida where the direction of the vapor drive is consistent throughout most of the year. However, in mixed climates, such as Nashville, Tennessee, the warm side and cold side vary by season (Figure 2). For these climates, performing a moisture vapor analysis and developing designs can be more complex and difficult. 2
Figure 2. Comparison of WUFI models for Nashville, Tennessee with an exterior coating of 8 perms and an interior coating of 1 perm in winter months and summer months illustrates the affect of a mixed climate. All materials have different resistance properties to the flow of vapor through them. For example, some coatings have been developed that are breathable which means that they provide little resistance to the movement of moisture vapor, while others are considered impermeable. Vapor permeance of a material is formally defined as, the time rate of water vapor transmission through unit area of a flat material induced by unit water vapor pressure difference between its two surfaces. 1 This property is commonly given in units referred to as perms. In simple terms, it is the rate that water vapor moves through a material under specific controlled steady state test conditions. The vapor permeance of a material is given for a prescribed thickness of the material. The permeability of a material standardizes the value according to a particular thickness, specifically one inch thickness for English units or one centimeter for metric units. Vapor permeability is described as the rate of water vapor transmission per unit area of a prescribed vapor pressure differential and is commonly expressed as perm-inches of thickness. Vapor permeance of coatings can be measured using several test procedures and analytical methods. Historically, vapor permeance of materials has been evaluated by the ASTM E96, Water Vapor Transmission of Materials. This test method was originally published in 1954 and serves to measure the rate of vapor transmission through either a dry cup method or wet cup method. Different perm values may be possible depending on the material and which of these two methods was used. For this reason, when comparing perm values of different coatings it is important to recognize the method under which a product was tested and results were reported. For example, hygroscopic materials store moisture and therefore, behave differently under the wet cup method. The characteristics of hygroscopic materials result in a vapor permeance range that is dependent on the relative humidity of the surrounding environment. This can portray a material as more vapor permeable than may be achievable in the environmental conditions of a particular wall construction in which the 3
product is used. Liquid applied coatings must be adapted to be tested under the ASTM E 96 protocol. ASTM specifically evaluates permeance of organic coatings with the D1653 method. This method provides techniques for the coating material to be adapted into a sheet format in order to perform similar test methods to those in the E96. Design approaches and tools to address moisture vapor and hygrothermal behavior of exterior building walls have evolved over the past several decades. The initial recognition of the need to design for moisture vapor was triggered by the wall insulation manufacturers after experiences of exterior paint failures on wood framed buildings in cold climates 2. As a result, a simple prescriptive requirement of placing a vapor retarder (1 perm or less) on the warm side of insulation was universally adopted by the model building codes and accepted in industry design standards. As wall assemblies became more complex and building science research evolved, ASHRAE and other organizations recognized the benefits of a more formal steady state moisture analysis which would take into account more variables including thermal and vapor resistance properties of the wall assembly materials, as well as interior and exterior design temperatures and humidity conditions. This type of analysis was useful in giving a designer a general idea of potential wall behavior, but was also not very precise due to inherent limitations and assumptions. Today, there are sophisticated hygrothermal modeling computer programs available such as WUFI which is a heat and moisture transfer model program that was jointly developed by the Fraunhofer Institute for Building Physics and the Oak Ridge National Laboratory. This program can be used to evaluate moisture transport as well as estimate drying times of various wall assemblies. It considers not only the physical properties of the wall assembly but also hourly climatic conditions that include solar radiation, precipitation, wind, air flow; moisture contents of materials and water intrusion. However, caution must be used when modeling exterior walls using computer simulations. The complexity of modeling exterior walls has made it difficult to develop a computer program that looks at the combined effects of all of the moisture transport mechanisms. Therefore, when using any model, the results are based on limited transport mechanisms and ignore other possible contributors. Several of the conditions affecting exterior walls that are not taken into account in the WUFI program are water infiltration and air movement. Additionally, air leakage to the exterior or interior can positively or negatively impact wall performance, depending on the indoor and outdoor environment. Furthermore, accurate vapor permeance data is necessary to create a model that correctly reflects the performance of the building envelope. The quality of the data and the specific test method used to obtain the permeance values are important in gaining accurate results through modeling. WALL DESIGN CONSIDERATIONS Thoughtful design considerations need to be made regarding the selection and placement of materials with high vapor resistance or moisture problems can result. For example, materials such as a self-adhered asphalt impregnated weather-resistive barrier is considered a vapor impermeable product which, by current definition, has a vapor resistance value less than 0.3 perm. In cold climates, moisture problems may arise on a wall assembly where this type on material is used behind the cladding or when it is coupled with a vapor impermeable product such as polyethylene interior wall board. The following are some general design rules and design considerations regarding the selection and placement of materials for exterior wall assemblies. 4
Improper location of a vapor impermeable material: Interior and exterior climatic conditions directly affect the need for and placement of vapor impermeable products. In regions which are predominantly heating climates, such as the northern United States and Canada, vapor impermeable products are often needed and/or required by code on the interior (warm side) of the wall construction to prevent heated interior air from condensing and allowing moisture accumulation within materials located within the wall toward the colder exterior air. However, in regions which are predominantly cooling climates such as Florida or Texas, vapor impermeable products are often needed and/or required on the exterior (warm side) of the wall construction to prevent warm humid exterior air from condensing and accumulating within materials toward the colder, conditioned interior air (Figure 3). Figure 3. An interior coating creates an improperly placed vapor impermeable material in a cooling climate, resulting in staining on the back side of the coating. Use of double vapor impermeable materials: Wall constructions that include a vapor impermeable material such as a coating or weather-resistive barrier on the exterior of the wall construction, as well as a vapor impermeable product such as vinyl wallpaper on the interior of the wall construction should not be used without careful analysis. These two products coupled together in an exterior wall assembly can drastically impact the ability for drying within the wall assembly either to the exterior or the interior. Moisture storage of building components: Hygroscopic building components such as brick and concrete masonry can absorb and retain water due to rain wetting. When heated by solar radiation this moisture is forced toward the interior, creating an inward vapor drive. In hot humid cooling climates, a vapor impermeable product applied on the interior surface of the wall can result in moisture accumulation. Moisture content of materials: Many construction materials such as wood, plaster and concrete have high levels of moisture at the time of construction which need to be permitted to dry out. Problems can result in a wall assembly that has materials with high vapor resistance placed within the assembly that 5
inhibit drying of this built-in moisture and cause problems before the total building construction is even completed. The same assembly can also reduce the drying potential of any incidental water intrusion that occurs within the wall assembly. Detrimental effects can occur in the absence of liquid water. Sustained elevated humidity can increase the water content of materials and cause deterioration of the materials. Elevated humidity as low as 70 percent has been shown to support the formation of some biological growth without the presence of liquid water 3. AFFECTS OF COATING SELECTION USING COMPUTER MODELING The potential effects and behavior of surface applied coatings were studied using the WUFI hygrothermal modeling program. Our study considered the exterior and/or interior placement of coatings with varying vapor permeance properties in three different climatic zones. The following hygrothermal models demonstrate how changes in the vapor permeance properties of coatings applied to the exterior and interior surfaces of an exterior wall assembly can affect the humidity and moisture accumulation within the wall. Selected for comparative purposes, were vapor permeance values representative of typical interior and exterior wall coatings. Exterior elastomeric coating products may range from 2-40 perms. On the interior, latex paints with primers, can result in values as low at 2.0 perms 3 and with multiple coats, can result in values near1.0 perm. The same wall assembly was used for all of the models. From exterior to interior the construction, consists of 7 /8-inch three-coat Portland cement stucco, #15 asphalt saturated felt weather-resistive barrier, ½-inch gypsum sheathing, 3-½ inch unfaced batt insulation within a stud cavity, and ½-inch interior gypsum. All materials of the wall construction were modeled according to the material properties and weather data included in the computer program database. Models were evaluated for a duration of three years, with the model beginning and ending on October 1. Vapor permeance of the interior and exterior surfaces was modified within the parameters of the program without modifying individual material properties. Study #1A Miami, Florida Exterior Coating: None Interior Coating: None Our first model examines the wall construction without an interior or exterior coating. This model will serve as a control study for the Miami, Florida locale. The water content of the exterior stucco varies in relation to rain events but maintains a water content between 0 and 10 percent. Although the relative humidity of the stucco does reach 100% during wetting, with a water content around 5 percent, it does not remain there for long periods. The exterior gypsum wall sheathing experiences the largest range of humidity fluctuations and moisture content, but this is because the initial built in moisture is elevated. Beginning at approximately 5 percent water content, resulting in a RH around 90 percent, the sheathing dries out in the first few weeks and then maintains RH levels between 30-60 percent and a water content less than 1 percent. Additionally, the relative humidity of the interior gypsum begins at 80 percent, with a water content around 2 percent but, after the initial drying, is maintained between 30-60 percent, and a water content that remains below 1 percent. The results of this model show that all layers of the wall are able to reduce the initial moisture content within the first few weeks. No apparent accumulation or water content percentages in excess of 2 percent were noted in the exterior sheathing or interior drywall after the initial drying during the three year simulation. 6
Study #1B Miami, Florida Interior Coating: Vinyl wallpaper 1 perm This model examines the wall construction in Miami, Florida using an exterior vapor permeance of a typical exterior paint or elastomeric type coating and a low permeance coating, such as vinyl wallpaper on the interior. The water content of the exterior stucco still varies in relation to rain events, and maintains a water content between 0 and 10 percent. However, with the addition of the interior and exterior coatings, the relative humidity remains slightly elevated throughout the entire modeling period, and has a slightly higher moisture content than exhibited in Study #1A. Beginning at water content around 4 percent based on the initial built-in moisture, resulting in a RH above 90 percent, the sheathing dries out in the first few weeks and then maintains RH levels between 30-60 percent and a water content less than 1 percent. This is similar to the levels maintained during the model with no coatings. However, the interior gypsum drywall dries initially from the built-in moisture content, but then rapidly increases in moisture content during the summer months. The model shows water content in the interior drywall as high as 40 percent. The results of the model suggest the relative humidity of the drywall would be between 90 percent and 100 percent between June and August each year, and at 100 percent accumulating moisture beginning in August and lasting six months for the first year of the simulation, resulting in water contents around 40 percent. The moisture content of all materials in this model are higher at the end of the simulation than in the control model (Study #1A). Additionally, the moisture content of the interior drywall remains elevated for 6 months out of the year. Sustained high humidity and moisture accumulation can lead to formation of biological growth and deterioration of materials. Study #1C Miami, Florida Interior Coating: Typical primed and painted drywall 3 perms This model examines the wall construction in Miami, Florida using the same exterior vapor permeance but uses an interior vapor permeance representing the low end latex paint and primer values. The performance of this model is much better than with an interior coating of 1 perm, although high humidity and moisture accumulation still occurs at the interior drywall. However, a maximum water content around 40 percent in the interior drywall is reduced to around 10 percent, and the conditions are sustained at this maximum for a shorter duration than the previous model. 7
Figure 4. Example WUFI output illustrating water content percentages and relative humidities of a stucco cladding with an exterior coating of 8 perms and an interior coating of 1 perm (Study #1B). Study #1D Miami, Florida Interior Coating: Typical primed and painted drywall 8 perms This model examines the wall construction in Miami, Florida using the same exterior vapor permeance but uses an interior vapor permeance at the upper range for latex paint and primers. Again, many of the water contents are at a maximum at the beginning of the simulation due to the initial built-in moisture levels. The interior drywall starts in the 90 percent humidity range but begins dropping immediately and, within two weeks, is dropping below 80 percent. The value stays between 25 and 70 percent for much of the year during the simulation, reaching only 80 percent for short time periods between August and October before dropping back down 70 percent. The water content levels are near the lowest levels observed in the first model where no coatings were used on either the interior or exterior surfaces. The modeling of wall permeance variations in Miami illustrates how low vapor permeance at the interior surface contributes to elevated moisture levels and moisture-related problems. Elevated moisture levels within the wall occur at interior vapor presences of 3 perms, a value common to multi-coat applications of latex paints. This finding is consistent with the experience of the authors investigating buildings in hot humid climates. The modeling suggests that this wall construction, in this climate, benefits from being able to dry to the interior. The next set of modeling studies the same wall construction in Edmonton, Canada. The climate in Edmonton is considered a cold climate, and has in the past been required to include a vapor retarder by model building and energy codes. Due to the cold climate and code provisions, the first model is intended to represent 8
the typical solution used in residential construction to meet the code requirement: inclusion of a polyethylene sheet membrane on the warm side of the wall. However, since this paper is looking at surface coatings, the vapor retarder is modeled as a surface coating, such as low permeance paint. Study Exterior Interior Start End Min. Max. Coating Coating 1A No coating No coating 0.62 0.29 0.15 0.62 1B 8 perms 1 perms 0.62 14.67 0.30 14.72 1C 8 perms 3 perms 0.62 3.63 0.17 3.99 1D 8 perms 8 perms 0.62 0.51 0.14 1.35 Table 1. Water Content (lb/ft 3 ) of Interior Gypsum Board (3 Year Cycle) Miami, Florida Study #2A Edmonton, Canada Interior Coating: 0.3 perms Equivalent of polyethylene vapor retarder The model shows a drying of the wall materials from the initial built in moisture within two weeks of modeling. Although the model shows relative humidity in the wall materials remains high, within the first 10 days the moisture content of the exterior gypsum sheathing drops close to the minimum content. Over the length of the entire simulation, the moisture content of the exterior wall sheathing decreases from the initial water content of 2.18 lbs/ft 3 (approximately 5 percent) to a 0.74 lbs/ft 3 (approximately 1 percent). Study #2B Edmonton, Canada Interior Coating: 1 perm Low permeance paint coating or vapor retarder The modification in the interior surface to increase the permeance (although still meeting the traditional definition of vapor retarder) appears to make little difference in the performance of this wall configuration in Edmonton. The maximum moisture content of the exterior gypsum sheathing does increase from the 2.18 lbs/ft 3 in Study #2A to 2.87 lbs/ft 3 in this case, still just above 5 percent water content by weight. Similar to the results from the simulations run for Miami, the difference between these two permeance values appears legible when looking at the corresponding moisture content of the wall materials. Study #2C Edmonton, Canada Interior Coating: Typical acrylic exterior coating 8 perms Further modification at the interior surface to increase the permeance to 8 perms makes a large difference in the maximum water content of the exterior gypsum sheathing in Edmonton. The maximum moisture content of the exterior gypsum sheathing increase to 8.72 lbs/ft 3 (roughly 17 percent water content) from the 2.18 lbs/ft 3 in Study #2A. In the model, the water content of the exterior sheathing remains between 10 and 17 percent water 9
content for approximately four months. However, the increased drying capacity of the wall is similar to the results from the simulations run for Miami that, although the maximum water content is higher, the wall still shows the ability to dry. Study Exterior Interior Start End Min. Max. Coating Coating 2A No coating No coating 2.18 0.74 0.32 2.63 2B 8 perms 1 perm 2.18 0.65 0.35 2.87 2C 8 perms 3 perms 2.18 0.44 0.28 8.72 Table 2. Water Content (lb/ft 3 ) of Exterior Gypsum Board (3 Year Cycle) Edmonton, Canada CONCLUSION The models presented in this paper are useful in illustrating how seemingly minor changes in the vapor permeance of wall coatings affect overall wall performance. Designers and builders must understand material properties, and how the properties of one material effects the whole wall assembly. Manufacturers must also appreciate that coating applications can impact wall performance and provide accurate data regarding vapor resistance properties of materials for use by designers and builders. Hygrothermal modeling in the design phase for new construction is useful in evaluating the proposed wall assembly. In remediation efforts, modeling proposed changes to existing walls can help predict, and prevent problems due to changes in wall properties by addition or changes to interior or exterior coatings. Our modeling suggests that the use of weather data derived from the location of the building is useful in understanding the effects of local weather conditions on wall performance. A wall design that performs well under certain indoor and outdoor environmental conditions may experience moisture related problems in a different climate. Modeling shows that in Minneapolis, a city many would consider located in a very cold weather climate and a location that traditionally would have required a vapor retarder having a maximum perm rating of 1.0, actually may perform better without a vapor impermeable (less than 0.3 perm) product on the interior. Designers, owners, and builders, can all benefit from modeling exterior walls with actual material properties prior to construction or remediation of exterior walls to verify design assumptions. The computer modeling indicated that undesirable amounts of moisture will accumulate in problematic walls, despite having even low amounts of initial moisture in materials. Moisture accumulation can occur in exterior walls in both cold (heating) climates as well as hot (cooling) climates. Conversely, high initial moisture in materials can be effectively reduced to normal ranges by drying in properly designed wall assemblies. This is not to suggest that moisture related problems will never occur in these cases, since other factors such as time, temperature and other sources of moisture, such as water intrusion, will negate the rate of drying. 1 Heinz Trechsel, Moisture Analysis and Condensation Control in Building Envelopes, pg. xxiv, 2001. 2 William B. Rose, Water in Buildings An Architect s Guide to Moisture and Mold, pg. 58, 2005. 3 Lewis G. Harriman III, G.W. Brundett, R. Kittler, Humidity Control Design Guide For Commercial and Institutional Buildings, pg. 112, 2001. 10