Masonry Edge the Storypole Vol7 No1 : Page 22

KNOWLEDGE EDGE Mitigating Moisture on Masonry Walls Insulated on the Interior Design & Repair Techniques for Improved Performance by Norbert Krogstad Moisture damage was likely already happening to a lesser extent; however, it was not until these heavy rains that it became visible on the interior, taking the form of mold growth at the base behind baseboards ( Figure B ), around windows ( Figure C ), at electrical outlets and at other openings in the exterior walls. In more than one case when interior gypsum wallboard was removed to ﬁnd the moisture source, the exterior surface of the polyethylene sheeting was covered with a layer of water droplets ( Figure D ), batt insulation on the interior face of the concrete masonry (CMU) was saturated and the interior face of the CMU was very wet ( Figure E ). The cause of this moisture was due both to excessive water penetration and moisture migration within exterior wall systems. I n many parts of the country, especially the Midwest and Northeast, 2011 brought several long periods of soaking rain followed by warm weather. These heavy rains resulted in moisture problems in certain masonry wall systems that had gone unnoticed in many cases prior to this point. In particular, many double-wythe and single-wythe masonry wall systems that were insulated on the interior side with batt or rigid insulation installed between furring strips were susceptible. The interior surface of the insulation and furring strips was covered with polyethylene sheeting and gypsum wallboard ( Figure A ). In multi-wythe systems, these walls typically had narrow air spaces (although code minimum is 1'', actual air space width commonly varied between 3/4'' and 1 1/4'') between the veneer and backup wythes and did not include any coating on the exterior surface of the backup. This wall system, common in new mid-to high-rise, high-end, multi-family residential structures, is similar to multi-wythe mass wall systems used in many older walls, circa 1900. However, unlike this current wall system, older masonry walls typically had plaster on the interior and were not insulated, so they behaved diﬀerently. Causes of Moisture Problems Periods of heavy rainfall Upon reading the article you will be able to: LEARNING OBJECTIVES 3 Describe eﬀective wall systems for new construction and effective remedies for existing walls to minimize moisture problems. 2 Compare masonry wall assemblages for their ability to mitigate moisture problems/ susceptibility to damage as a result of moisture. 1 Identify consequences of unintended extended contact between water and components of a masonry wall system and adjacent systems. occurring prior to warm weather supply the moisture needed for this phenomenon to occur. Unlike short periods of hard rains where much of the water ﬂows oﬀ the surface of the masonry, long periods of rain allow masonry walls to absorb considerable moisture. Redistribution due to evaporation and condensation occurs during warm sunny weather following periods of rain. The temperature of the masonry is increased by direct solar radiation and warm exterior temperatures. This causes moisture within the masonry walls to evaporate into the air within the wall system. The warm, moist air ﬁlls the wall system. Because the interiors of these buildings are air-conditioned during warm weather, gypsum wallboard and polyethylene sheeting are cool. Moisture from the warm, moist air will condense on the exterior surface of the polyethylene sheeting resulting in a layer of water droplets. These water droplets will ﬂow down the exterior surface of the polyethylene sheeting to locations where the membrane is discontinuous, such as at the ﬂoor. If the gypsum wallboard is in contact with the ﬂoor, moisture can be absorbed into the end of it. Even if the wallboard is not in contact with the ﬂoor, high moisture will result in condensation on the cool baseboard trim. This trim also slows evaporation to the interior, allowing mold to grow on the organic material behind the trim at the base of the wall ( Figure F ). A similar problem occurs around the perimeter of windows and at other openings, such as for outlets. In all of these locations, polyethylene sheeting is often installed discontinuously. This allows moist air within the wall system to come in contact with cool surfaces, resulting in condensation. Wet organic materials, such as the paper on the gypsum wallboard or the surface of the wood trim, are food sources for mold. 22 MASONRY EDG E / thestorypole Vol 7 No 1 Masonry Technology | Innovation masonryedge.com

KNOWLEDGE EDGE

Norbert Krogstad

Mitigating Moisture on Masonry Walls Insulated on the Interior Design & Repair Techniques for Improved Performance In many parts of the country, especially the Midwest and Northeast, 2011 brought several long periods of soaking rain followed by warm weather. These heavy rains resulted in moisture problems in certain masonry wall systems that had gone unnoticed in many cases prior to this point. In particular, many double-wythe and single-wythe masonry wall systems that were insulated on the interior side with batt or rigid insulation installed between furring strips were susceptible. The interior surface of the insulation and furring strips was covered with polyethylene sheeting and gypsum wallboard (Figure A). In multi-wythe systems, these walls typically had narrow air spaces (although code minimum is 1'', actual air space width commonly varied between 3/4'' and 1 1/4'') between the veneer and backup wythes and did not include any coating on the exterior surface of the backup. This wall system, common in new mid- to high-rise, high-end, multifamily residential structures, is similar to multi-wythe mass wall systems used in many older walls, circa 1900. However, unlike this current wall system, older masonry walls typically had plaster on the interior and were not insulated, so they behaved differently. Moisture damage was likely already happening to a lesser extent; however, it was not until these heavy rains that it became visible on the interior, taking the form of mold growth at the base behind baseboards (Figure B), around windows (Figure C), at electrical outlets and at other openings in the exterior walls. In more than one case when interior gypsum wallboard was removed to find the moisture source, the exterior surface of the polyethylene sheeting was covered with a layer of water droplets (Figure D), batt insulation on the interior face of the concrete masonry (CMU) was saturated and the interior face of the CMU was very wet (Figure E). The cause of this moisture was due both to excessive water penetration and moisture migration within exterior wall systems. Causes of Moisture Problems Periods of heavy rainfall occurring prior to warm weather supply the moisture needed for this phenomenon to occur. Unlike short periods of hard rains where much of the water flows off the surface of the masonry, long periods of rain allow masonry walls to absorb considerable moisture. Redistribution due to evaporation and condensation occurs during warm sunny weather following periods of rain. The temperature of the masonry is increased by direct solar radiation and warm exterior temperatures. This causes moisture within the masonry walls to evaporate into the air within the wall system. The warm, moist air fills the wall system. Because the interiors of these buildings are air-conditioned during warm weather, gypsum wallboard and polyethylene sheeting are cool. Moisture from the warm, moist air will condense on the exterior surface of the polyethylene sheeting resulting in a layer of water droplets. These water droplets will flow down the exterior surface of the polyethylene sheeting to locations where the membrane is discontinuous, such as at the floor. If the gypsum wallboard is in contact with the floor, moisture can be absorbed into the end of it. Even if the wallboard is not in contact with the floor, high moisture will result in condensation on the cool baseboard trim. This trim also slows evaporation to the interior, allowing mold to grow on the organic material behind the trim at the base of the wall (Figure F). A similar problem occurs around the perimeter of windows and at other openings, such as for outlets. In all of these locations, polyethylene sheeting is often installed discontinuously. This allows moist air within the wall system to come in contact with cool surfaces, resulting in condensation. Wet organic materials, such as the paper on the gypsum wallboard or the surface of the wood trim, are food sources for mold. Moisture within the wall system not only results in mold growth at discontinuities in the sheeting, but also can lead to deterioration of other elements within the wall system. Water condensing on the exterior face of polyethylene sheeting will wet the fiberglass batt insulation and furring. In the case of wood furring, this wetting can result in decay and mold growth (Figure G). In the case of metal furring, wetting results in corrosion (Figure H). If there are wood roof or floor joists or trusses that bear within the masonry walls and the ends of these members are not treated, decay can occur resulting in potential future structural problems. This phenomenon is significantly worse as the rate of water penetration and absorption increases. Walls exposed to significant winddriven rain typically have more problems than walls that are partially shielded by roof overhangs since they are exposed to greater volumes of water. Brick units with high coldwater absorption (11-13%, as allowed by ASTM C216) will absorb significantly more moisture than units with low cold-water absorption (8% or less). Poor tooling or fill in the joints also increases the rate of water penetration. Water readily penetrates through cracks or surface voids in the mortar joints. Typically, the volume of water penetration through the surface of the masonry is more related to the frequency of these voids, separations and cracks in the surface than it is to the absorption of the masonry units. Mortar joints that are acid-etched as a result of excessive cleaning will allow much greater water penetration than joints that are not etched. Flashing problems, especially the masonry copings, sills and caps at ledges, allow significant water to penetrate the masonry along the top. In multi-story buildings, water pene tra - tion can be increased both by the exposure to the heavier winds near the upper levels as well as by the rundown water from above on lower floors. All of these factors increase the rate of water penetration on the exterior surface of the masonry walls, which can result in greater storage of moisture within the masonry. In many cases, features of the wall system allow water to penetrate the backup wythe or the interior surface of the masonry directly. In multiwythe walls, especially multi-wythe walls with a narrow air space less than 1", mortar bridges can form during construction (Figure I). Mortar bridges allow water that penetrates the exterior wythe to bridge across the air space and wet the CMU backup wall. This allows water to be in direct contact with the wood furring as the backup units absorb moisture. Another common problem occurs on masonry walls that extend above the roof to form parapets. Often, the masonry on these parapet walls is exposed on the roof side (Figure J). Masonry exposed on the roof side of the parapet can absorb water that can wet the backup wythe if there is no through-wall flashing. Typically, through-wall flashing cannot be provided at these locations because it creates a bond break at the base of the parapet, which prevents the parapet from functioning as a cantilever. Moisture problems are significantly worse in these masonry wall systems if there is excessive air infiltration. Air infiltration will carry moist air from the masonry air space to the interior at gaps in the wall system. Gaps exist around windows and through voids in the masonry backup wall. Air infiltration is especially a problem in buildings where the mechanical system creates negative pressurization. Negative pressurization occurs when the air pressure on the inside of the building is less than that on the exterior of the building. Negative pressurization often occurs in taller buildings at the upper floors during warm weather due to stack effect. Stack effect occurs due to the difference in density between the warm exterior air and cool, air-conditioned interior air. This difference causes positive pressurization at the lower floors of the building and negative pressurization at the upper floors. Moisture problems are also significantly worse in situations where evaporation to the interior is restricted. The polyethylene sheeting will effectively prevent moisture from evaporating to the interior spaces. This allows moisture to be held within the walls. Moisture problems are typically significantly worse when vinyl wall-coverings are used on the interior surface of the gypsum sheathing. Not only does the vinyl wall-covering act as a vapor retarder, preventing drying to the interior, but the moisture that condenses on the exterior surface of the vinyl wall covering will facilitate mold because the wall-covering paste and the paper of the gypsum wallboard are food sources. Preventing Moisture Problems in New Construction These problems can be prevented in new construction by modifying the wall system design and employing quality assurance on the masonry workmanship. The wall system can be designed as a masonry veneer wall, an insulated masonry cavity wall, or, in the case of older buildings, as an uninsulated masonry mass wall. Veneer walls have a masonry veneer, an air space and water resistive barrier/air barrier on the exterior surface of the backup wall. Water that penetrates the exterior wythe is prevented from wetting the backup due to the presence of the air space and water resistive barrier. In insulated masonry cavity walls, rigid insulation will not only help to prevent water from bridging across the wall system, but causes the dewpoint to occur within the rigid insulation, where it will not result in condensation. Many older buildings used multi-wythe wall systems without air spaces or water resistive barriers. However, these wall systems are effective because they were not insulated on the interior face. Instead, plaster was applied directly to the inside face of the walls. This allows moisture within the masonry walls to evaporate to the interior and exterior. Older mass masonry walls that have been retrofitted with insulation on the interior to increase thermal performance often develop the moisture problems described above. Regardless of the wall system used, good masonry workmanship helps to prevent problems. Mortar joints must be properly filled and tooled. Expansion joints must be provided at recom mend ed spacing to avoid cracking problems. Proper cleaning techniques must be used to prevent mortar joints from being etched. Flashing must be properly installed, especially below masonry copings, sills and caps at horizontal ledges in the masonry wall. The masonry on the roof face of the wall must either be covered or flashing must be provided to prevent water migration to the masonry below (note that the structural continuity must be maintained). Insulating the interior face of well-constructed multi-wythe or single-wythe masonry walls can be effective if moisture migration during warm weather is either blocked or drying to the interior is facilitated. Moisture migration can be blocked by using closed-cell polyurethane spray foam insulation bonded to the interior face of the masonry (Figure K). The spray foam insulation functions as an air and vapor barrier. Also, because it is bonded to the interior face of the masonry, it eliminates the air space onto which condensation could otherwise occur. To work effectively, spray foam insulation must be installed to form a continuous layer across floor lines and extending behind electrical outlets. Studs containing electrical conduit would be positioned to the interior of the insulation. It is important to note that because this system prevents drying to the interior, it can allow walls to remain wet for longer periods of time. As a result, potential for efflorescence is increased. Moisture migration during warm weather can be facilitated if galvanized steel furring is attached to the interior face of the masonry, unfaced insulation is installed between furring strips and the interior face is covered with a smart vapor retarder. Smart vapor retarders have variable vapor permeance depending on the relative humidity of the air adjacent to the warm side of the membrane. When relative humidity is high, these function as vapor permeable membranes allowing moisture to pass through. When relative humidity is low, generally 30% or lower, these membranes function as a vapor retarder preventing moisture migration. Therefore, during warm weather when the air within the wall system is often warm and humid, moisture is allowed to dry to the interior preventing moisture accumulation. During cold weather when the interior relative humidity is 30% or less, the membrane functions as a vapor retarder preventing interior air from entering the wall system and condensing within the masonry. For this system to function properly, the interior surface must be finished with a vapor permeable material such as a latex paint and vapor permeable wall coverings such as paper. It is also recommended that paperless or highly mold-resistant gypsum wallboard be used. In all cases, it is important that the masonry wall system be detailed and constructed in a way that it does not permit excessive water penetration. Repairing Existing Masonry Wall Systems Repair of wall systems that have moisture problems can be difficult. The method of repair typically depends on the severity of the problem. In masonry wall systems with minor problems that have metal furring and minimal interior damage, the problems can sometimes be addressed by performing limited repairs to reduce water penetration and storage within the masonry and to use the mechanical system to facilitate drying. Water penetration through the exterior face of the masonry can often be greatly reduced by: performing partial or complete repointing of the mortar joints; applying clear water repellents following repointing; repairing failed sealant joints; performing flashing repairs, especially at copings, caps and sills; and preventing water penetration on the roof side of parapet. Mechanical system can be used to facilitate drying by increasing pressurization of interior air relative to the exterior during warm weather. In this way, cool conditioned air, which has relatively low moisture content, will flow through the wall system at any discontinuities in the vapor retarder. This migration of cool, dry interior air through the wall system will tend to dry the masonry over time. However, this is only effective in cases where there is minimal damage and not a large amount of water already stored in the wall system. Also, pressurization should be reversed or balanced neutral during cold weather to prevent moisture from interior air from condensing within the wall. Any existing mold growth within the wall system should be removed as required under direction of a certified industrial hygienist. Impervious interior finishes such as vinyl wall coverings must be avoided. In wall systems where there is widespread advanced interior moisture damage including wet insulation, significant mold growth and decayed wood furring or corroded metal furring, the interior side of the masonry wall system must be completely repaired. This will typically involve removing all interior finishes, insulation and furring. Seals will need to be constructed between the masonry wall and window perimeters and at any other penetrations in the masonry backup in order to eliminate airflow paths. A layer of continuous closed-cell polyurethane spray foam insulation can be added on the interior surface of the masonry. This insulation layer must be installed without interruption on the exterior side of electrical outlets and studs. Spray foam must extend across floor lines in order to maintain continuity. This method is not appropriate if untreated wood floor or roof joists bear within the masonry backup wall since moisture held within masonry can promote decay of untreated wood. In this system, no vapor retarder would be used on the interior side of the insulation because the closed-cell polyurethane spray foam insulation functions as the air barrier, vapor retarder and insulation all in one material. Paperless or highly mold-resistant gypsum wallboard should be used on the interior as a precaution. The interior surface must then be either painted with a latex paint or with paper wall coverings that are water vapor permeable. Repairs should also be per formed to the masonry as described above in order to limit water penetration and storage. In cases with excessive water penetration and storage, and where moisture problems are detected before significant damage occurs, an alternate approach would be to perform the majority of repairs on the exterior. In this approach, the entire exterior masonry wythe is removed to permit installation of an air/moisture/ vapor barrier on the exterior face of the masonry backup. This membrane should extend across floor lines to prevent moisture from absorbing into the backup. The exterior surface of this membrane is then covered with rigid insulation, which will be positioned within the cavity space. Brick veneer would then be added to the exterior side of the insulation. Drainage mat can be used to facilitate drainage in narrow air spaces between the exterior face insulation and the interior face of the masonry veneer. The interior finishes, interior insulation and vapor retarder should be removed in regularly spaced sections, likely totaling at least 50% of the interior surface depending on the configuration. This will allow moisture within the areas where the vapor retarder and insulation are not removed to dry towards the areas where the insulation and vapor retarder were removed. As with any masonry wall system insulated on the interior face, it is important that the interior finishes are vapor permeable and that paperless or otherwise highly mold resistant gypsum wallboard is used.

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