Using Air Barriers in Masonry Walls

masonry walls

BY CLAYFORD T. GRIMM, P.E Using Air Barriers in Masonry Walls

ater in masonry relative humidity rises. When interior air Paul R. Achenbach provides an exam- walls creates nu- at 70¡ F and 50% relative humidity is ple of interstitial condensation in his pres- merous prolems: cooled to 50¡ F, its relative humidity in- entation “General Construction Principles,” freeze/thaw frac- creases to 100% and condensation occurs. Moisture Control in Buildings (ASTM In- tures, cracks form, In summer, the sun drives water vapor ternational, West Conshohocken, Pa., 1994, efflorescence ap- inward by diffusion or warm moist air is page 285). Wpears, wall stains, insulation failure, forced inward by convection towards an “Air-conditioned buildings constructed material swelling, wood warp, gypsum air-conditioned space, which diminishes of masonry block and finished with fur- decay, metal corrosion, paint peels, mold the air’s ability to hold water and increases ring strips, insulation, vapor retarder be- grows, mildew forms, and odors reek. its relative humidity. If air at 85¡ F and tween furring strips, and plasterboard in- Much thought is given to warding off 90% relative humidity is cooled just three terior frequently experience condensa- wind-driven rain by using wall drainage degrees to 82¡ F, its relative humidity in- tion on the back of the vapor retarder cavities, flashing, and weep holes. creases to 100% and condensation occurs. and softening of the plasterboard inte- However, little attention is given to in- Richard L. Quirouette in his paper “Dif- rior in humid and rainy climates. Even terstitial condensation that is often just as ference Between a Vapor Barrier and an though the masonry block is painted, ex- damaging. This form of condensation caused Air Barrier,” Building Practice Note, No. pansion and contraction can cause many $68 million in mold and mildew damage 54 (National Research Council of Canada, hairline cracks in the painted surface. in hotel-motel buildings in a single year. Ottawa, Ontario, 1985) provides a steady During heavy rains, water is absorbed For example, all the brick veneer was re- state example to demonstrate that air leak- into the cracks and is stored in the ma- moved from a hotel in Charleston, S.C. age by convection transfers more than 200 sonry block. Subsequent solar radiation due to mold growth caused by condensa- times the amount of water transferred by drives the water vapor inward to con- tion of infiltrating warm moist air. vapor diffusion. However, traditional cal- dense on the vapor retarder and to be culation methods cannot predict the amount transferred through the furring strips to Moisture movement of condensation because exterior and in- the plasterboard, causing it to soften and Wind, stack effect, and mechanical terior air and vapor pressures are con- discolor. Enough solar heat can be stored equipment create pressure differences that stantly changing. For a description of a in the masonry block during the day to readily force air through masonry walls. computer program that solves the prob- continue vapor transfer to the vapor re- In winter, warm moist air moves outward lem go to http://www.ornl.gov/ornl/btc/ tarder and the plasterboard throughout through the wall, becomes colder, and its moisture/index.html. the night. A surface coating on the ma-

Fig. 1 Fig. 2 Varies 43⁄8-inch (110-mm) Max 2-inch (50-mm) Min

Cold Air/Vapor Barrier Insulation in CMU SST or GV ST Moderate Connector Interior Finish Hot E. s

sonry block that retains sufficient elas- liquid applied rubber copolymers; “peal- simply require a continuous air barrier are ticity to avoid cracking under the tem- and-stick” self-adhesive rubberized as- not likely to achieve that effect in actual perature changes to which it is exposed phalt, EPDM (ethylene propylene diene construction. Because it may be impossi- would alleviate this problem. A saturated monomer), and elastomeric bitumen. ble to anticipate every airflow path, crafts- sheathing paper installed between the An air barrier system is a continuous men should be encouraged to understand masonry block and the insulation would and durable network of materials and joints the importance of the problem and to make also protect the insulation and the inte- providing air tightness with adequate strength onsite suggestions for practical sealing so- rior plasterboard.” and stiffness to prevent excessive deflec- lutions. If a vapor retarder is placed on or near tion under air pressure differences. An air Ensure that materials joined as part of the interior wall surface of an air-condi- barrier system requires continuity at four the air barrier system are compatible. As- tioned building in a hot humid climate and connections: phalt-based materials are incompatible with there is no air barrier to prevent inward air 1. Joints between sheets of the air bar- polyvinyl chloride, polyethylene, and poly- movement, condensate may form imme- rier membrane styrene. Synthetic rubber adhesives may diately behind the vapor retarder. Mold 2. Interfaces between walls and wall not be compatible with EPDM flashing. growth may flourish if wallboard behind openings Some adhesives are highly flammable and an interior vinyl wall covering becomes 3. Intersections between exterior walls give off noxious vapor. moist. The smell may make human occu- and interior walls and floors Follow instructions on the Material pancy intolerable and cause evacuation of 4. Cracks at wall penetrations. Safety Data Sheets. Manufacturers should the property. Air barriers must be continuous, durable, be consulted regarding compatibility of An air barrier should be placed on the impermeable, and structurally capable of their products with other materials. outside of the insulation if an interior vinyl sustaining the anticipated air pressure dif- wall covering is used in warm climates. ferential. Every effort should be made to ensure the tightest possible enclosure. How- Air barrier systems ever, it is practically impossible to com- Fig. 1. Recommended U.S. climatic regions Fig. 2. Air/vapor barrier in a masonry cavity wall The purpose of an air barrier is to pre- pletely seal the entire building envelope, in a hot climate vent the air from reaching a point in the although every effort should be made to Fig. 3. Air/vapor barrier in a masonry cavity wall wall cool enough to cause precipitation. do so. in a cold climate Air barrier membranes are available in Not all cracks can be sealed to ensure Fig. 4. Air/vapor barrier in a single wythe wall sheet form in a variety of materials: absolute air tightness. Specifications that in a hot climate

Fig. 3 Varies 43⁄8-inch Fig. 4 Varies (110-mm) 2-inch Max Cavity (50-mm) Min Air Barrier Insulation Air Space in CMU Air Barrier and Vapor Retarder Insulation

SST or GV ST Stucco on Connector Self-Furring Metal Lath Drainage Mat on Insulation masonry walls

Air barriers should be placed in cavity, masonry veneer, and single wythe masonry walls.

Placement reaching a point in the wall cold enough used to attach board insulation. A full bed Air barriers are placed in three primary to cause condensation. of adhesive should be fully troweled to types of masonry walls: cavity, masonry In veneer and single wythe walls in within 1 inch of each edge on clean and veneer, and single wythe. All these walls cold climates, an air barrier may be lo- dry boards using a 1⁄4-inch deep, notched have metal anchors, ties, and fasteners that cated on the interior face of the wall trowel. Other methods such as spot daubs cause thermal bridges. where it can serve also as a vapor re- or beads should not be used because they In winter, connectors provide cool in- tarder. This location facilitates inspec- allow air to circulate behind the insulation, terior surfaces on which exfiltrating warm tion. However, care must be taken to en- which reduces insulation effectiveness. After moist air can condense. In summer, infil- sure proper installation in plenum spaces, adhesive application, the board is positioned trating warm moist air can condense on at intersecting partitions, and behind cab- on the wall and pressed firmly over the en- metal ties cooled by interior air condi- inets, pipes, and equipment. tire surface. Joints between boards should tioning. For this reason, plus the perme- be staggered and tightly butted. ance of masonry to wind-driven rain, cor- Insulation When board insulation has a reflective rosion protection for metal connectors in Foamed-in-place plastic, plastic inserts, foil surface facing an air space in a well- masonry often necessitates the use of stain- and loose fill insulations are used to fill insulated wall, the foil may have very lit- less steel versions. cells in single wythe walls of hollow ma- tle effect on the wall’s thermal resistance. The table on page 38 provides recom- sonry units. Rigid board insulation is placed Furthermore, the corrosive effects of mor- mendations for the position of air/vapor in wall cavities and on interior wall sur- tar on aluminum are well known. For these barriers and insulation in masonry cavity, faces. A drainage mat on the cavity face reasons, reflective foil on board insulation veneer, and single wythe walls in hot and of board insulation drains mortar-clogged is of dubious value. cold climates. Those recommendations de- cavities. Batt insulation with a vapor retarder is pend on climatic regions (Fig. 1). Board insulation is usually 1- to 2- placed between studs in masonry veneer Cold climates have more than 4000 an- inches thick and is cut or kerfed to fit be- walls and may be placed between interior nual heating degree-days. Hot climates tween metal ties in cavity walls. Boards furring strips in single wythe walls. have less than 2200 annual heating degree- are attached to the air barrier on the exte- days. Moderate climates are between the rior face of the interior wythe in cold cli- Building code requirements two figures. mates. Insulation boards may be attached Section 1403.3 of the International Build- Air/vapor barriers are not usually nec- to a wall’s interior surface in single wythe ing Code [30] requires an interior vapor essary in moderate climates, with two ex- walls. The boards may be held in place by retarder in exterior walls, but specifically ceptions. They are recommended on the stainless steel screws with large plastic excludes masonry single wythe walls, com- warm side of the wall in air-conditioned washers. posite walls, and cavity walls designed in buildings within 50 miles of the Atlantic Masonry walls should be parged to pro- accordance with Chapter 21. However, that coast south of Cape Hatteras and in build- vide a smooth surface when adhesives are code requires a vapor retarder for veneer ings with refrigeration or other cold oc- cupancies. For any 24-hour day, the number of Fig. 5 Varies heating degree-days is the difference between 65¡ F and the mean outdoor tem- perature for that day. The number of an- Insulation nual heating degree-days is the sum of in CMU those values for a year. The number of annual heating degree-days can be found Stucco or for 284 United States cities in Local Cli- Paint Air/Vapor matological Data Ð Annual Summary Retarder with Comparative Data, National Cli- matic Data Center, Ashville, N.C. In cavity walls, location of the air barrier indicated in the table should prevent exfiltration or infiltration of moist air from

Fig. 5. Air/vapor barrier in a single wythe wall in a cold climate masonry walls

walls designed in accordance with Section 1505.5. Recommended Position of Occupants and buildings can be ad- Air/Vapor Barriers and Insulation* versely affected by the absence of a vapor retarder and an air barrier in building walls Wall Type Climatic Region in many parts of the United States, whether or not the exterior masonry wythe is in a Hot Cold cavity wall or a veneer wall. A vapor re- Cavity Place air/vapor barrier on exterior Place air/vapor barrier behind tarder in the interior of a veneer wall in a face of interior masonry wythe. board insulation on exterior hot-humid climate can be very damaging. Place insulation in the core/cells face of interior masonry wythe The National Building Code of Canada of hollow masonry units in the (Fig. 3). requires both a vapor retarder and an air interior wythe and/or between fur- barrier in all walls, except where an analy- ring on the interior face of the sis shows that the safety of the building interior masonry wythe (Fig. 2). and the health of its occupants are not ad- versely affected. Veneer Place air barrier on exterior face Place insulation between studs Clayford T. Grimm, P.E., is a consult- of sheathing. Place insulation and place air/vapor barrier at between studs and vapor retarder or near the interior surface ing architectural engineer and a mem- at or near the interior surface (Fig. 5). ber of MASONRY CONSTRUCTION maga- (Fig. 4). zine’s editorial advisory board. He can be reached at 1904 Wooten Drive, Austin, TX 78757-7702 or by e-mail at Single Place air/vapor barrier behind Insulate cores/cells of hollow [email protected]. Wythe stucco at the exterior wall face. masonry units and/or between Additional information on this sub- Insulate cores/cells of hollow furring on the interior surface. ject is found in “Masonry: Opportuni- masonry units and/or between Place air/vapor barrier on the ties for the 21st Century, ASTM STP furring on the interior surface interior surface (Fig. 7). 1432,” ASTM International, West Con- (Fig. 6). shohocken, Pa., 2003.

Fig. 6. Air/vapor barrier in a veneer wall in a hot * Buildings with low interior design temperatures, such as cold storage or ice rinks, should climate have the air barrier and vapor retarder on the exterior side of the insulation. Fig. 7. Air/vapor barrier in a veneer wall in a cold climate

Fig. 6 Fig. 7

Interior Finish Interior Finish Masonry Masonry Stud Stud Veneer Veneer Insulation Insulation Anchor Anchor Sheathing

Air/Vapor Barrier Sheathing Air/Vapor Barrier