Controlling moisture intrusion from blowing rain is a fundamental part of designing a functional and durable building. Driving rain is typically the largest source of bulk moisture for above-grade building assemblies.1 Inadequate wind-driven rain resistance can lead to moisture intrusion in building assemblies that are intended to stay dry, causing problems for building owners and its occupants.
Excessive moisture creates unfavorable health effects and damage to moisture-sensitive building materials. Damage resulting from excessive moisture in buildings may include mold growth, disintegration of moisture-sensitive materials (gypsum, for example), rot of wooden materials, masonry spalling from freeze/thaw cycles, corrosion of steel reinforcing and components, coating damage, efflorescence, and reduction of the insulating value (R-value) of thermal insulation. Figures 1 and 2 illustrate some of the damaging effects of excessive moisture.
This article focuses on test methods used to assess the ability of exterior wall coating systems to resist moisture intrusion from rainwater penetration. When it comes to diagnosing moisture intrusion problems, it is important to recognize and consider sources besides wind-driven rain — such as roof defects, missing sealants and water vapor migration. But for this article, we will focus on testing for rainwater penetration.
COATINGS AND WALL DESIGN
When rain strikes a building wall, it will begin to run off the wall unobstructed, but if there are surface defects such as pinholes or cracks, water can infiltrate because of the kinetic energy of the water, pressure, gravity and other forces. Because of their porous nature, unprotected masonry and concrete walls can absorb the rainwater and become a reservoir for retaining it. For some wall designs, however, water penetration from rain is not a problem because of designed drainage systems within the wall assemblies.
Wall assemblies may have moisture management systems consisting of drainage planes with moisture barriers, through-wall flashings and weep holes that are able to accommodate the water without any adverse effects. In contrast, other wall designs depend heavily on the use of protective coatings to control moisture intrusion from rainwater penetration.
A barrier wall such as single-wythe concrete masonry unit (CMU) does not have any redundancy within the wall assembly to control rainwater penetration, as illustrated in Figure 3. Once the water penetrates the protective exterior coating, water will enter the wall, dampening the block and possibly spreading to the insulation and interior finishes.
Exterior coatings are a critical component for the successful performance of barrier-type wall assemblies. Wind-driven rain resistance is essential for protecting these wall types, especially in areas with high amounts of rainfall. When selecting an exterior coating, the designer must consider several factors such as the wall type, climate, orientation and material properties.
If wind-driven rain resistance is essential, test results for wind-driven rain resistance can be found on the product data sheet (PDS) for some pigmented coatings and clear penetrants. High-build coatings (those that are applied in one or two coats to a dry film thickness of 10-15 mils) and elastomeric products typically have test results for wind-driven rain — but not all masonry and concrete coating products are tested for wind-driven rain. It is not common to see wind-driven rain test data for thin film acrylic products (those with 3-5 mils dry film thickness [DFT]). Test results are typically shown under the “Performance Data” or “Physical Properties” section of a PDS.
WATERPROOF, WEATHERPROOF AND WATER REPELLENT
It should be noted that a product with good wind-driven rain resistance does not mean it is a waterproof coating. The term “waterproof” should not be used interchangeably with “wind-driven rain resistance.” Waterproofing typically comes in the form of a sheet-applied or liquid-applied material and is defined as resistance to passage of water under hydrostatic pressure. For example, most foundation waterproofing (coatings used below grade) or linings are considered waterproofing. Waterproofing is further defined in ASTM D1079, Standard Terminology Relating to Roofing and Waterproofing.
Pigmented coatings used above grade on masonry or concrete structures are typically referred to as “weatherproof” or “weather resistant.” Clear penetrants used on integrally colored masonry are referred to as “water repellents.” “Weatherproof” or “water repellent” typically refers to the resistance of the passage of water not subjected to hydrostatic pressure. Waterproof systems are typically not required for above-grade applications.
LABORATORY TESTING OF WIND-DRIVEN RAIN RESISTANCE
ASTM D6904, Standard Practice for Resistance to Wind-Driven Rain of Exterior Coatings Applied on Masonry, is the most common type of laboratory test referenced for above-ground masonry and concrete coatings. In the coating industry, wind-driven rain resistance is the ability to resist water penetration through a dried film, erosion of the coating or coating degradation on a concrete test panel when exposed to water spray and pressure. Wear or coating degradation is assessed either visually or by weight loss/gain, when using the appropriate wind-driven rain resistance testing chamber.
According to ASTM D6904, which references Federal Specification TT-C-555B, Federal Specification of Coating, Textured (For Interior and Exterior Masonry Surface), a specified coating system (to include a base layer consisting of block filler and a second layer of masonry paint) should prevent the water from penetrating into the required masonry concrete panels under very specific testing conditions, and show no visible water leaks during testing. Water leaks are evident by the color change that occurs when the concrete block becomes wet.
The testing conditions require that the water flow rate, when using a fish-tail nozzle, should be 60 to 70 gallons per hour, with a five-unit difference in the atmospheric pressure inside the testing chamber relative to the outside standard pressure. This difference in atmospheric pressure is meant to produce an “equivalent dynamic pressure at 98 miles per hour wind velocity.”
Even though the ASTM references a primer and topcoat system, testing in our laboratory has included a single coating layer (a primer), a topcoat system and a multilayer topcoat system. The material or materials are applied to masonry concrete panels at a required wet film thickness and allowed to dry at laboratory conditions for a specific time, usually seven days. The wet film thickness of the system, either one coat or multiple layers, can be specified by the coating manufacturer or by the specification requiring the wind-driven rain results. All the parameters for testing — including the number of layers, type of material applied, the film thickness, conditions of cure time, duration of cure time and length of test exposure (numbers of hours in the wind-driven rain testing chamber) — are documented and reported with the test results. Testing is performed in either duplicate or triplicate.
The concrete masonry panels measure approximately 8 inches by 16 inches by 2 inches. The concrete test panels coated with the specified product(s) are clamped to the exterior of the wind-driven rain testing chamber and subjected to water exposure for the specified length of time. The ASTM method recommends a water exposure time of 24 hours, while the Federal Specification recommends a 48-hour exposure time, unless water penetration occurs sooner.
The standard does not have pass/fail criteria and leaves it to the discretion of the manufacturer and user. Acceptance criteria for wind-driven rain data can include: total weight loss or gain, blistering, softening, and other visual changes in the coating film, such as penetration of the water to the back of the concrete panels. Most coating manufacturers require a maximum limit of weight loss (due to erosion) or gain (due to absorption) of 0.2 pounds. Figure 4 shows the wind-driven rain test chamber without test panels, and Figure 5 shows the test panels while the test is in progress.
The initial purpose of ASTM D6904 was to determine if a clear non-pigmented sealer or penetrating sealer prevented water penetration and degradation of the dried film when applied to concrete or ceramic tile. However, the method has been used frequently to test a wider variety of coating materials than originally intended. Even though many types of materials have been evaluated, the text of the standard recommends that the film under test be a clear penetrating sealer or a waterproofing primer. Currently, coating manufacturers are using pigmented primers as sealers or other pigmented coating systems to resist water penetration when testing for wind-driven rain resistance on concrete panels. The results are useful for comparing the performance of the different materials.
On a related note, wind-driven rain resistance testing is required by code for building fenestration products such as glazing, windows, rainscreen systems and doors. Several test methods from the American Architectural Manufacturers Association (AAMA) and ASTM regarding testing of building fenestration products are widely used. Although this article doesn’t address testing of fenestrations, it is important to recognize that many elaborate testing methods and devices are available for testing fenestrations in a test facility or in the field. The same principle used to test fenestrations can be used to test exterior coating systems as described below.
FIELD TESTING OF WIND-DRIVEN RAIN RESISTANCE
Rilem Tube Test
A simple diagnostic method to check for wind-driven rain resistance is a water absorption test commonly known as the Rilem tube test. A European association headquartered in Paris, France, Rilem(Reunion Internationale des Laboratoires D’Essais et de Recherches sur les Materiaux et les Constructions) developed a test method to measure the quantity of water absorbed by the surface of a masonry material over a definite period of time. Test Method 11.4, developed by a technical committee within Rilem, provides detailed instructions on how to perform the test.3
One form of the test that measures the horizontal transport of water or its resistance to wind-driven rain is illustrated in Figure 6. The test apparatus consists of a clear plastic tube that is graduated from 0 to 5 milliliters. The tube is connected to a small reservoir that has a circular brim. The brim of the reservoir is affixed to the wall surface using putty. The putty has the consistency of plumber’s putty and creates a watertight seal between the wall surface and the Rilem tube. The area of absorption within the Rilem tube where it joins the wall is approximately 1 square inch.
Water is added at the pipe opening until it reaches the 0 graduation mark. When the tube is filled to the 0 graduation mark, it exerts approximately 0.17 psi of pressure on the wall, which is equivalent to 96.4 mph. The quantity of water absorbed is read directly from the tube and recorded. Generally, the test is checked every five minutes for a period of 20 minutes to determine how much of the water absorbs into the wall being tested. Tests should not be conducted in the rain because rainwater fills the pores of the wall surface and impacts the absorption rate. The rate at which water is absorbed is an indication of the wind-driven rain resistance of the wall.
Manufacturers of water repellents can provide acceptance criteria for their products. According to the SWR Institute’s Clear Water Repellent Manual,4 typically, if the water level drops no more than 20 percent of the original height during the 20-minute test period (that is, if the tube is filled to the 0.0 mL mark at the beginning of the test, the water level should be no lower than 1 mL after 20 minutes), the wind-driven rain resistance of the wall is adequate.
Typically, tests are done on both the block and mortar for masonry structures. Although each test only requires approximately 20 minutes to complete, testing an entire wall can take several hours since each test covers a very small area at a time. In order to expedite the testing, it is common to install multiple Rilem tubes at a time.
Generally, each type of masonry block is tested at different elevations to get a representative evaluation of the wall’s resistance to wind-driven rain. The test can be performed on both pigmented or clear penetrants, but we have used the test primarily on integrally colored surfaces as shown in Figure 7. In a failure analysis, the test helps to identify a point of moisture entry, and to determine the absorption properties of the wall surface and the effectiveness of any applied coatings or water repellents.
Sealed Test Chamber
ASTM test method C1601, Standard Test Method for Field Determination of Water Penetration of Masonry Wall Surfaces, is a field test that can be conducted to measure the amount of water penetration into a wall. According to ASTM C1601, this test is used to determine the surface water penetration quantitatively at a single location.
This test involves the use of a pressure/test chamber with a 12-square-foot test area that is mechanically anchored to the wall (Figure 8). The test chamber is sealed to the face of the wall with plumber’s putty or gasket material to form a watertight seal. Water is pumped into the test chamber and through a spray pipe at the top of the chamber, applying a uniform amount of water across the wall. Air pressure is applied simultaneously to create a force resembling wind-driven rain. The water is collected and recirculated.
The test is recommended to be conducted for a minimum of four hours. The volume of water lost through the wall, air pressure and water flow rate is recorded at a minimum of five-minute intervals throughout the test period. The rate at which water is absorbed into the wall is an indication of the wall’s resistance to wind-driven rain.
ASTM E514, Standard Test Method for Water Penetration and Leakage Through Masonry, is a similar test. The ASTM E514 test method measures through-wall penetration, whereas Test Method C1601 measures surface water penetration. A similar method involving a sealed test chamber, ASTM E1105, Standard Test Method for Field Determination of Water Penetration of Installed Exterior Windows, Skylights, Doors, and Curtain Walls, by Uniform or Cyclic Static Air Pressure Difference, is used to test wind-driven rain resistance in building fenestration products.
Less sophisticated tests can be done to evaluate the exterior coating system’s resistance to wind-driven rain. One common sense way to qualitatively assess the weatherproof properties of a wall is to observe the interior side after a blowing rain. The wall on the windward side can be inspected for leaks on the interior. This especially works well on single-wythe walls were the interior side of the barrier wall material (for example, CMU) is not concealed or covered. Leaks around mortar joints, darkening of the face or visible water on the interior side of the windward wall are obvious signs there are deficiencies in the wind-driven rain resistance.
If leaks during rain events are detected, more sophisticated tests can be done to diagnose the moisture intrusion.
While not complying with an ASTM standard, evaluations of the coating can be done by using water spray on the exterior surface for 30 to 60 minutes. This can be done with lawn sprinklers attached to ladders, home-made spray racks (Figure 9) or even a garden hose mounted to a ladder.2
We have also used a spray nozzle compliant with AAMA 501.2, Quality Assurance and Diagnostic Water Leakage Field Check of Installed Storefronts, Curtainwalls and Sloped Glazing Systems, to test coating systems for wind-driven rain resistance. Although AAMA 501.2 is primarily used to test fenestration products, the same principle can be applied to coatings.
The water spray test can be conducted on pigmented coating systems or on integral-color surfaces with integral water-repellent systems. Figure 10 shows a water spray test being conducted on integral-color half-high units. In this case, we determined the wall had poor wind-driven rain resistance. Water was observed to stream down the interior wall within five minutes of the water spray beginning, as illustrated in Figure 11. After about 10 minutes of spray, water began to pool onto the floor, indicating obvious deficiencies in the wind-driven rain resistance.
The results of a water spray test may not be as obvious as those illustrated in the photos. The interior side of the wall may be concealed with insulation or interior finishes, eliminating the ability to view bulk water entry through the barrier wall. In this case, moisture readings can be obtained on the exterior face of the wall using non-destructive moisture meters.
Radio frequency devices work well in this case by providing the capability to “read through” the exterior coating system without damage to the coating. The moisture content of the substrate is obtained pre- and post-water spray to determine if there is an increase in the moisture content. Coating systems with adequate wind-driven rain resistance should not show a considerable increase in moisture content after water spray.
Although these methods do not comply with an ASTM standard, we have found that if water penetration is visible within an hour — or the moisture content increases — there are problems with the wind-driven rain resistance of the exterior coating. Infrared thermal images of the exterior and interior surfaces in the test area are helpful in determining whether moisture intrusion is occurring through the coating system after a spray test.
Coating systems with adequate wind-driven rain resistance should not exhibit blistering, wrinkling or signs of moisture-related deficiencies. However, the opposite is not true. That is, if water intrusion is not visible, or the moisture content does not increase, it cannot be concluded that the coating possesses ample wind-driven rain resistance because the test variables are not adequately controlled. These “garden hose” tests can identify if the wind-driven rain resistance created by the coating is poor, but they do not confirm that it is good.2
The coating system or water repellent plays a key role in controlling moisture intrusion in barrier wall type assemblies. Unwanted problems can occur when the coating system does not control the absorption of water during rain events. There are several test methods available to test a coating system’s resistance to wind-driven rain.
Testing can be performed both in the laboratory and in the field during the mock-up stage, after construction, or during failure analysis. Some of the test methods, such as AAMA 501.2 and ASTM E1105, are primarily used to test the wind-driven rain resistance of fenestrations, but the same test principles can apply when testing coating or water repellent systems.
High-build elastomeric coating systems are a common type of pigmented coating used on masonry and concrete walls to control wind-driven rain. A wide range of water-repellent systems are also available for integral-color surfaces with products that have 15 percent solids or more. A trade-off, though, with some high-build coatings or high-solids water repellents involves lower permeance of the coating system.The fact is building walls are going to get wet at some point during its life from a roof leak, uncontrolled air leakage, water vapor diffusion or other source. There is a delicate balance between coating permeance to allow water vapor to escape, and the coating’s ability to control wind-driven rain. The designer must consider this balance when selecting the appropriate coating system for the structure. D+D
- Building Science Corporation, “BSD-013: Rain Control in Buildings,” Aug. 23, 2011.
- Trimber, K.A., K.J. Brown, and K.D. Knight, “Coatings for Commercial Structures and Building Deficiencies that Affect Performance” ASM Handbook, Volume 5B, Protective Organic Coatings, Kenneth B. Tator, ed. ASM International, 2015.
- Rilem Test Method No 11.4, Measurement of Water Absorption Under Low Pressure. Compiled by Frances Gale, September 1987.
- SWR Institute, Clear Water Repellent Manual.