Coating System Concerns for Single-Wythe Concrete Masonry Unit (CMU) Construction

Within a 20-mile radius of your home, you have seen one or more big box stores, strip malls, or other buildings with peeling paint on the exterior and/or interior surfaces. Failing paint and streaks of efflorescence are so common on buildings that it is hardly noticed until large sheets hang from the walls. While moisture intrusion is commonly the cause, and locating the source(s) is a huge challenge, the selection of the coating system also bears some responsibility.

Common CMU Coating System(s)

Exterior coating systems must be capable of withstanding wind-driven rain, and depending on the local climate, may also need a certain minimum permeance to allow moisture vapor from the interior of the wall to escape through the film. Common coating systems applied to the exterior of CMU walls are based on 100% acrylic resins for both block fillers and finish coats. Polyvinyl acetate (PVA) resins should be avoided on exterior walls as they can de-grade in the damp alkaline environments that are common with building walls. While acrylic materials with PVA resins are used on interior walls, peeling can occur when the interior wall becomes damp from moisture intrusion.

Elastomeric coatings are also commonly used on building exteriors for their ability to flex, bridge small cracks, and withstand wind-driven rain. A trade-off, though, involves lower permeance of many elastomerics, which can potentially create problems after multiple repaints. A review of these and other coatings for CMU can be found in recent Durability +Design articles by Jason Helsel, “Concrete Solutions: Coatings Answers for a Highly Complex Substrate;” and by Steven Reinstadtler, “Concrete Infrastructure Protection: Trends and Solutions.”

The focus of this article is not to address specific coating types, but instead to identify issues that should be considered when making coating selections, and to discuss some new opportunities for the coatings industry now that thin-film coatings are included as air barriers in the new International Energy Conservation Code (IECC).

SSPC Guide For Testing The Suitability Of Coatings As Air Barriers

SSPC committee C.8.4 on Air Barriers, chaired by Kevin Knight of Edifice Tutorial, has completed the first draft of a Guide entitled “Laboratory Testing to Evaluate Thin-Film Coatings on CMU Assemblies.” When completed, the Guide will serve as the basis for testing thin-film coatings to determine their suitability to serve as air barriers (and weather barriers) on building enclosures. Proposed properties to be tested are as follows. Acceptance criteria are also included in the draft guide for each of the performance tests:

  • Peel strength – ASTM C794
  • Crack bridging – ASTM C1305
  • Chemical resistance (alkali, muriatic acid, commercial cleaner) – ASTM D543
  • Rate of burn – ASTM D635
  • Low temperature flexibility before and after UV weathering – ASTM D1970
  • Sealability – ASTM D1970
  • Freeze-thaw resistance – ASTM D2243
  • Water penetration, static – ASTM E331
  • Water penetration, dynamic – AAMA 501.1
  • Pull off strength before and after UV weathering – ASTM D7234
  • Water vapor transmission – ASTM E96
  • Air permeance – ASTM E2178
  • Air leakage of wall assemblies without penetrations – ASTM E283
  • Air leakage of wall assemblies with penetrations – ASTM 2357
  • UV weathering (1000 hours) – ASTM G155
  • Nonvolatile content – ASTM C1250
  • Volatile content – ASTM D2369
  • Extensibility (standard conditions and heat aged) – ASTM C1522
  • Tensile strength, elongation and elasticity – ASTM D412
  • Wet film thickness – ASTM D4414
  • Dry film thickness – ASTM D3767

The Guide will provide a uniform means for evaluating the suitability of thin-film coatings as air and weather barriers. While most specifiers intuitively recognize the need for the exterior coating to keep the weather out, they do not always have an appreciation of the need for the coating to “breathe,” to have adequate permeance to allow moisture vapor to pass through the film.

coating system paint failure
Figures 1 and 2: Premature paint failure is an all-too-common occurrence on coated CMU walls. The new SSPC Guide, “Laboratory Testing to Evaluate Thin-Film Coatings on CMU Assemblies” is intended to help prevent these situations.

Testing Of Water Vapor Permeability

One of the common tests for determining the water vapor permeability of coatings is ASTM E96, “Standard Test Methods for Water Vapor Transmission of Materials.” The permeance of individual products can be found on Product Data Sheets or by talking with the coating manufacturer. When comparing permeance values between products, it is important to know the units being presented and the conditions of the test (e.g., wet cup, dry cup, temperature, etc.) in order for the comparisons to be apples-to-apples. The most common units are WVP US perm ratings.

While the permeance of the individual products may be known, the value of greater importance is the permeance of the entire system being applied. Unfortunately, this data is typically not available, and it cannot be derived using a mathematical formula that combines the values of the individual products. Instead, it must be determined by testing the entire system.

Permeance testing requires the careful application of the entire system to a uniform thickness within a tolerance of +/- 0.4 mils. It must be applied to a material that will allow it to be separated from the substrate, and one that is large enough to create three free films, each at least four inches in diameter. The films are exposed to the test conditions for a given length of time in order to determine the vapor permeance. The specific duration is dependent upon the time required for equilibration of the material under test, which is typically two to five weeks. The required length of time is determined by the consistency of the slope of the line generated from the data. The results are averaged in order to determine permeance within +/- 17%.

While it is possible to determine the permeance of new coating systems through testing, it is not possible to determine the permeance of an existing system. Even if the existing system has peeled completely to the substrate, it cannot be tested. The thickness of the field samples will not be +/- 0.4 mils, any contaminant in the film will impact the ability of water vapor to pass through the material, and the test requires that the three replicates be of chemically identical material. Aged, detached coating samples from the field can exhibit differences in chemical makeup due to degradation from exposure.

The Use Of Permeance When Designing Projects

How is permeance data used when selecting the systems to apply? Do you always want a “breathable” system on the exterior side of a single-wythe CMU wall? What about the interior side?

To begin to address these questions, following is information from a paper I co-authored with Kevin Brown (KTA) and Kevin Knight (Edifice Tutorial) for the SSPC 2010 conference. The text below addresses only vapor drive, but there are other sources of moisture intrusion discussed in the paper that can wreak havoc on coating systems such as air exfiltration; stack effect; wind load; mechanical pressurization; and defects in the building parapet; coping, flashing, gutters, etc.

“A ‘vapor drive’ is caused by the difference in relative humidity between the building interior (typically higher RH) and the exterior environment (typically lower RH), causing water molecules in the interior air to pass through molecules of the wall components. If the wall is not insulated, and given certain exterior ambient climatic conditions, temperature will cycle within the block and fall below the dew point, causing the vapor to condense, wetting the interior of the block and potentially leading to coatings problems. The degree of vapor drive is controlled by the porosity of the wall, together with environmental factors, especially:

Permeance testing requires lab conditions and absolute uniformity, but the results determine permeance within +/- 17%.

Moisture gradients – Moisture vapor will naturally move from a higher concentration to a lower concentration, until in balance. With high vapor pressure to the interior of the wall, and low vapor pressure to the exterior of the wall, vapor drive will be directed outward (and vice versa when the relative vapor pressures are reversed). The greater the difference of this vapor pressure or ‘concentration gradient,’ the greater is the vapor drive.

Temperature gradients – Moisture vapor will naturally move from the warm side of a wall to the cooler side. With higher temperatures to the interior of the wall and lower temperatures to the exterior of the wall, vapor drive will be directed outward (and vice versa when the differences in temperature are reversed). The greater the ‘temperature gradient’ (difference), the greater the vapor drive.

In other words, the movement of moisture via diffusion is a result of differences in vapor pressure that are related to the temperature and moisture content of the air on both sides of the wall. The mixed climates found across the country, both in the interior space and external to the building, must be taken into consideration when determining the direction of vapor diffusion and when designing the painting system.”

Based on the above, the environment both inside and outside of the building will influence the direction of the vapor drive. If that’s the case, if a system with high permeance is found to work successfully on the exterior of a CMU building in Maine, can that same system be expected to perform well in Florida where the direction of the vapor drive may be different? What about the permeance of the systems applied to the interior walls? What if the interior space isn’t conditioned? With countless variations of these questions, how are decisions made? One potential solution is a computer program known as WUFI. But if the coatings industry wants to take advantage of this design tool, it needs to do some legwork to help populate the WUFI database with coatings information.

WUFI

WUFI (Wärme und Feuchte Instationär or Transient Heat and Moisture) is a sophisticated computer program developed by the Holzkirchen, Germany branch of the Fraunhofer Institute for Building Physics (Fraunhofer IBP). It is handled in the US by Oak Ridge National Laboratory (ORNL). Wall components are entered into the program together with the local interior and exterior environments, and the program assesses the thermal properties of the components, the impact on heating losses, and the direction of vapor diffusion and liquid transport. That is, it can help to determine the effect of varying air and weather barrier placement on the interior and exterior of buildings based on building-specific construction details and local climates. Once populated with coatings data, it will help the designer to determine in advance whether to apply system A, B, or C to the exterior of the building and/or system X, Y, or Z to the interior.

Unfortunately, WUFI cannot be used for coatings design today, because coating system data is not currently included in the program. Kevin Knight has approached ORNL to discuss including coatings data in WUFI. ORNL is very receptive to this, and indicated that the required data is the dry film thicknesses of each coat in the system, vapor and air permeance of the system, and for exterior systems, the UV index for heat gain. Note that the data is brand-specific and if provided, will be included in the WUFI database for a nominal charge (approximately $600/system).

The inclusion of coating system data in WUFI should create opportunities for the coatings industry. It will improve the ability to make the appropriate coating system choices based on project-specific building conditions and climates. It will be a big step in incorporating coating systems in the push to improve the energy efficiency of buildings and in reducing the peeling and disbonding of coatings that is prevalent on many of our CMU buildings today. But the coatings industry needs to take action to provide the data required for WUFI. The second step is to confirm that the thicknesses tested and entered into WUFI are actually applied in the field, since the design decisions are coating thickness-dependent.

Conclusions

A lot has been written on selecting coatings for use on concrete buildings and single-wythe CMU, but there is a void when it comes to addressing the permeance of the systems for interior and exterior use based on the local climate (both inside and outside of the building). Now that thin film coatings are being recognized as air barriers, it is time for the industry to address these questions head on, and provide improved guidance on coating selection. While there may be a number of ways to address this void, one approach that may be viable is WUFI. The coatings industry needs to determine if this is a viable design tool, and if so, to provide the required data for the program.

As chair of the SSPC Commercial Coatings Committee, I will be happy to work with interested manufacturers to further investigate the merits of WUFI, and if found to be promising, spearhead a process for getting the required coatings data into the program.

 

Ken Trimber KTA-TatorKenneth A. Trimber is president of KTA-Tator Inc. He is a NACE Certified Coatings Inspector Level 3, is an SSPC Certified Protective Coatings Specialist, and is certified at a Level III coating inspection capability in accordance with ANSI N45.2.6. Trimber has 40 years of experience in coatings inspection, testing, and analysis, is a past president of the Society for Protective Coatings (SSPC), and is chairman of the SSPC committees on Surface Preparation, Visual Standards, and Containment. He is also past chairman of ASTM D1 on Paints and Related Coatings, Materials, and Applications. He is the author of The Industrial Lead Paint Removal Handbook and co-author of Volume 2 of the Handbook: Project Design. Trimber is chair of the newly formed SSPC Commercial Coatings Committee (Architectural, Commercial, Institutional). He was named Coatings Specialist of the Decade at the SSPC National Conference in 1990, and is also past technical editor of the Journal of Protective Coatings & Linings.

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