Why drainage and ventilation are critical for adhered masonry walls

by Katie Daniel | May 26, 2017 10:11 am

[1]

By Art Fox
In masonry cavity walls—a design that has been in use for more than a century—the cavity provides a path for drainage and ventilation and acts as a capillary break. (The author would like to give special thanks to Steven 
Fechino [engineering and construction manager for Mortar Net Solutions], Jim Lucas of Lucas and Associates, and Scott Wylie of Wytech Building Envelope Solutions for their invaluable advice and expertise.) However, adhered masonry veneers like stucco have been installed for hundreds of years without drainage or ventilation. So why do we need to add drainage and ventilation planes to adhered masonry walls now? The short answer is American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings, and the Canadian equivalent, the National Energy Code for Buildings (NECB) 2011. (For more, read “The Sleeping Giant Awakes: NFPA 285.[2]”)

ASHRAE is an international organization that sets energy-use standards for commercial buildings. Its standards are frequently used as the basis for building codes for jurisdictions in Canada and the United States. Recent changes in building materials, as well as increasingly strict energy codes derived from ASHRAE 90.1, have made drainage and ventilation for adhered masonry walls just as important as they are for masonry cavity walls. This article looks at how modern adhered masonry veneer walls are different from those built before the 1950s, and why these differences are making drainage and ventilation essential for excellent, sustainable performance.

The physics of water
Water molecules are attracted to each other, which results in surface tension—the skin-like film on the water’s surface that makes water drops possible. However, there is a limit to how many molecules can stick together before their weight or air movement overcomes the force of surface tension and pulls them apart. Molecular attraction also causes capillary action, which, combined with the tendency of water to move from wetter to drier, explains why a molecule of water entering a small hole will draw other molecules with it.

Large clumps of water molecules form liquid drops, which quickly run off smooth surfaces like stone and stucco, but will not penetrate an unbroken wall. If water gets behind the veneer, it also will not penetrate small holes in the weather-resistive barrier (WRB), such as those caused by the WRB and lath fasteners, because the attractive force of the water molecules toward each other keeps the drops larger than the holes. The weight of water alone is not enough to force it through.

[3]

That being said, when pressure differentials caused by wind-humidity differences or heat become strong enough, they can push the water through even very small holes. In building science, this is called hydrostatic pressure. It is similar to pushing a partially filled water balloon through a garden hose—it will not go through the hose by gravity alone, but put enough pressure behind it and it will. Additionally, if there is no bond break between the scratch coat and WRB, high moisture content in masonry will move to lower concentrations in the substrate.

So-called ‘perched’ water can also wreak havoc on adhered masonry walls with trim elements such as lintels. (More information is available in “BSI-057: Hockey Pucks and Hydrostatic Pressure[4],” by Joseph Lstiburek.) When water becomes trapped behind such walls in the narrow space between the trim and the veneer, it cannot drain due to the clumping effect of water-molecule attraction. The water then soaks into the masonry, substrate, or both. When the sun’s heat causes the humidity in the space between the veneer and WRB to become higher than outside or inside the building, the solar energy drives the moisture both outward through the masonry and inward through the WRB, as well as through any holes in the WRB and sheathing.

How water gets into the wall
Every mason and masonry wall designer knows water gets into masonry walls either as liquid (such as rain or snow) or as vapour (the gaseous form of water). Water vapour does not cause any trouble until it condenses and becomes liquid water. Therefore, providing a drainage mechanism for liquid water is vital, but it is just as important to get vapour out of the wall system before it becomes liquid.

Water penetrates the wall in three different ways. It can:

• 
enter as liquid or vapour through tiny mortar cracks or through gaps around wall penetrations and at places where different materials meet;
• 
be drawn as liquid by capillary action through porous masonry; or
• 
move as vapour from a warm, humid side of the wall to a cooler, drier side of the wall, where it can condense into liquid if dewpoint conditions are met.

In cool weather, water vapour can move from the warmer, more-humid conditioned air inside the building to the exterior, and in warm weather, it can move from the more-humid exterior toward the cooler, drier interior. In either case, the vapour can condense at the back of the veneer.

[5]

Why old veneer installation methods 
do not work with modern walls
Traditional adhered veneer materials include stucco, manufactured and cast stone, and thin stone and brick. Manufactured and cast stone and thin brick were introduced during the 20^th century as alternatives to dimensional stone and full-sized brick. Traditional installation methods employ either wire mesh or expanded metal lath firmly attached to the substrate as the support for the veneer. Cementitious mortar fully encapsulating the wire or lath is then applied as an undercoat, or scratch coat, which provides a strong foundation for the veneer. Veneer materials—except stucco—are adhered to the scratch coat by applying a solid layer of mortar to the back of each veneer unit and pressing it tightly against the scratch coat for a few seconds. Stucco is applied continuously as one or two coats over the scratch coat.

When they first became popular, adhered masonry veneer walls did not have the sophisticated flashings, insulation, and WRBs used today, so they leaked a lot of air and would dry quickly if they got wet. With continuous insulation (CI) and WRBs being the best way to meet ASHRAE 90.1 and NECB standards, along with the use of more effective flashings and sealants around wall openings, modern adhered masonry walls leak very little air. This relative airtightness is a key difference between older adhered masonry walls and those of today, and is one reason drainage planes behind the veneer are so important in modern structures. Water that penetrates the wall stays trapped in the wall unless drainage and ventilation are added to get it out.

Weather-resistive barriers
Changes to WRBs make another difference between older and new building construction. WRBs are membranes applied continuously to a building envelope. They are rated in ‘perms,’ or permeability to water vapour. A WRB’s appropriate perm rating, and whether it is placed on the inside or outside of the insulation, is determined by the average heat and humidity of the climate in which the building is constructed. Ideally, a WRB allows enough water vapour to pass through it that the substrate dries without allowing liquid water through. Using two layers of building papers such as Grade D asphalt-impregnated building paper or Number 15 felt as a WRB is common when CI is not included.

[6]

The thinking behind using two layers of building papers is the assumption the outer layer will get wet when the scratch coat is applied. When the papers dry, they wrinkle, eliminating any mortar bonding by pulling away from the scratch coat and creating a sufficiently rough surface that water drains between the papers and the scratch coat. However, since the wrinkles are shaped randomly, they do not create continuous drainage channels running the height of the wall. This means they can obstruct water flow and create pockets where water can collect. Any buildup of water can lead to its lateral migration at the laps and allow a wider area of the substrate and scratch coat to get wet.

The absence of a mortar bond allows some drainage, but very little—if any—ventilation. Since older buildings with paper or felt as a WRB leaked a lot of air, they did not need to provide ventilation. Using plastic house wraps instead of building papers is now common, and some brands can bond strongly to mortar, which minimizes potential drainage. (For more information, see Lstiburek’s “BSI-029: Stucco 
Woes–The Perfect Storm.[7]”) Since today’s buildings do not leak much air, and modern papers and plastic house wraps allow minimal ventilation, there is very little drying behind the scratch coat with paper, felt, or house wraps.

Additionally, without a drainage and ventilation plane, building papers can absorb enough water to degrade and develop holes, which allows water to get to the substrate. They can also become saturated with water, creating an intensive vapour source inside the wall. If water stays in contact with the substrate, even if there is a WRB over it, it can migrate through fastener holes and other penetrations and be absorbed by interior materials. If even small amounts of moisture stay in contact with the substrate long enough, it can cause fastener corrosion, nail pullout, mould, and sheathing degradation.

Since CI is now commonly used in order to meet energy standards, manufacturers have developed rigid insulation with sealed seams, which serves as both CI and a WRB. These systems can be more efficient to install and more resistant to damage during installation than the WRBs mentioned above, but they still need drainage and ventilation. Also, when wire or lath is attached on top of CI, the fastening system must be designed to handle the eccentric loading placed on the fasteners, because if they bend or pull out, cracks in the veneer will develop very quickly. (The Foam Sheathing Coalition offers a “Guide to Attaching Exterior Wall Coverings Through Foam Sheathing to Wood or Steel Framing[8]” with tables for determining the right fastener sizes based on stud type, insulation thickness, and veneer weight.)

Since modern buildings do not self-ventilate by leaking air, and since all masonry building exterior walls leak moisture, using traditional installation methods with modern materials but without a drainage plane means most of the water penetrating the veneer must migrate into the substrate or out through the face of the veneer.

[9]

Sheathing
This article will now cover moisture behaviour in plywood and oriented strand board (OSB), but is not recommending one sheathing type over the other. Sheathing manufacturers are constantly improving their products, so it is best to check with them about their most recent products’ water absorption and drying characteristics before making a design decision.

Water in the substrate can potentially lead to mould growth, as well as structural, fastener, and veneer degradation. Water damage in this area can be made worse by using OSB sheathing instead of plywood. According to Joe Lstiburek of Building Science Corporation, use of OSB is important when it comes to water management behind adhered veneers because it reacts to moisture very differently than plywood. (For more information, see Lstiburek’s “BSI-029: Stucco 
Woes–The Perfect Storm.[7]”)

Plywood becomes more vapour-permeable as it gets wet, going from about 0.5 to 1.5 perms to more than 20 perms, which means its drying rate will increase as it gets wetter. However, OSB’s vapour permeability—and therefore its drying rate—stays low and relatively unchanged no matter how wet it is. Water in plywood also moves laterally much more easily than it does in OSB, so it will migrate out faster and have a significantly lower tendency to concentrate in one area. With OSB, moisture will concentrate at the OSB/building paper interfaces, which can cause localized moisture stresses and damage such as softening, swelling, delamination, and fastener pullout. Moisture is most likely to collect around wall openings. With stucco veneers, control joints—especially horizontal joints—can also collect and hold water, meaning cracks most often appear around windows, doors, and control joints first. (From “The Performance of Weather-resistant Barriers in Stucco Assemblies” by Karim Allana of Allana Buick & Bers Inc., presented at October 2016’s RCI Symposium on Building Envelope Technology.)

[10]

Water moves through the pores of masonry from wetter to drier areas. If enough water stays in contact with the masonry long enough, it saturates the masonry by distributing itself throughout via capillary action. While the rate at which a masonry wall dries depends on temperature, humidity, wind, altitude, and sun exposure, a good rule is it takes about 30 days for water to move 25 mm (1 in.) in porous masonry. Since a scratch coat and the mortar used to hold the veneer are, together, normally close to 25 mm thick, adhered masonry walls without drainage and ventilation that are exposed to wetting events more than once every 30 days are unlikely to fully dry once the masonry becomes saturated.

Continuous insulation
Continuous insulation creates a high level of thermal resistance so heat will not easily move through the wall—a good thing for occupant comfort, but not so good for drying. In poorly insulated buildings, temperature differentials between the interior and exterior walls create a heatflow across the wall that warms moisture within it, in turn making the moisture move through the wall as vapour.

Combining this high vapour movement with lots of air leaks means the wall and veneer dry efficiently, because the vapour is carried away by the air movement. More-efficient insulation means less heatflow, and therefore less vapour movement. Compounded with the much-lower airflow inside today’s relatively airtight walls, this makes for much less drying.

Claddings in cold climates operate at colder temperatures in a CI-insulated building, so if the cladding temperature is at or below the dewpoint, any water vapour that touches it will condense into liquid water more readily than if the cladding is close to the same temperature as an uninsulated substrate. Colder claddings also go through more freeze/thaw cycles and will freeze harder than warmer claddings, so water in the wall has the potential to cause more damage. Finally, OSB and plywood sheathings may increase in moisture content during heating periods as insulation efficiency rises, because conditioned warm air inside the building carries moisture that gets absorbed by the sheathing but cannot move past the insulation. Modern insulation approaches mean more moisture in the sheathing, more moisture in the masonry, and more trouble if it does not get out.

[11]

Drainage and ventilation
The best way to let both the masonry and the substrate dry rapidly between wetting events is to install a mesh drainage plane at least 7 mm (¼ in.) thick between the WRB and scratch coat. Canadian codes require a minimum 10-mm (³/8-in.) thick drainage plane. (For more information, see Lstiburek’s “BSI-029: Stucco 
Woes–The Perfect Storm.[7]”)

It may help to think of the drainage plane as serving the same function as the cavity in a masonry cavity wall—it allows water and air to move freely behind the veneer for both drainage and ventilation, and acts as a bond break to prevent mortar bridging so water cannot travel from the masonry to the substrate. It also helps eliminate localized water stresses on the sheathing and other components, because it allows moisture to migrate rapidly away from areas like wall openings and control joints. Further, a drainage plane allows water vapour to rapidly migrate from behind the veneer so hydrostatic pressure will not force it through small holes into the substrate, and allows masonry to dry from the back, so even if wetting events are frequent, the masonry can dry more quickly and avoid saturation.

A polyester drainage mesh with a factory-adhered water- and vapour- permeable fabric on one side is currently available in rolls of various nominal thicknesses. A new system that has a 90 per cent open-weave mesh factory-assembled to a galvanized, G-60, 1-kg (2½-lb) expanded metal lath is also available, with variable thickness and weight.

Conclusion
A drainage plane between the veneer and substrate is not yet required by most North American building codes for adhered masonry. However, changes to building materials and construction methods are needed to meet more stringent energy codes and increase building sustainability, meaning the old ways simply do not work anymore. Further, modern building science has proven drainage planes can significantly reduce or eliminate moisture problems in adhered masonry veneer walls.

Twenty-five years ago, mortar-dropping collectors in masonry cavity walls were not part of code either, but as their value was demonstrated over and over, codes caught up, and now no mason or masonry designer would consider building a cavity wall without them. Drainage planes for masonry veneers are currently at the same stage as mortar-dropping collectors were then—quality builders are using them because they reduce problems and callbacks, and because they make the buildings that implement them a source of pride for both the builder and the designer. When codes catch up, everyone will benefit.

FURTHER READING

For more information on the topics broached in this article, professionals can consult: Michael Anschel’s “Building Things Right: Rainscreen Siding Systems,” published by Professional Remodeler in February 2016 and available at www.proremodeler.com/building-things-right-rainscreen-siding-systems[12]; John Straube and Jonathan Smegal’s research report “Modeled and Measured Drainage, Storage, and Drying Behind Cladding Systems,” available at buildingscience.com/documents/reports/rr-0905-modeled-measured-drainage-thermal-x/view[13]; Norbert V. Krogstad’s “Condensation and Rigid Insulation Placement,” available at masonryconstruction.com[14]; Joseph Lstiburek’s “BSI-038: Mind the Gap, Eh!” and “BSD-105: Understanding Drainage Planes,” available to read online by visiting buildingscience.com[15]; and Steven Fechino’s “Adding Drainage to Stone Veneers and Adhered Masonry,” from Masonry Magazine—access online by visiting www.masonrymagazine.com/archive/magazine/features-2/adding-drainage-to-stone-veneers-and-adhered-masonry[16].

[17]Art Fox has been the head of marketing and communications at Mortar Net Solutions since 2012. He was also the chief operating officer of the company when it was initially formed 25 years ago. Fox has been involved in the building trades since he was a contractor specializing in new home and light commercial construction in New Mexico in the 1970s. 
He can be reached via e-mail at afox@mortarnet.com[18].

Source URL: https://www.constructioncanada.net/why-drainage-and-ventilation-are-critical-for-adhered-masonry-walls/

---