The development of pressure equalized rain screen walls for masonry construction

By E. B. Gellert AIA posted 09-13-2019 11:35

  

The development of pressure equalized rain screen walls for masonry construction

By International Masonry Institute (IMI) and E. Bradford Gellert, AIA


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With the introduction of steel columns into “transitional masonry buildings” the structural responsibilities of the masonry lessened as the loads began to be carried by the steel. Image: Brad Gellert


The AIA Building Performance Knowledge Community Definitions Project has developed the following definitions for Rain Screen walls:

"A cladding system resists the five main forces of water penetration: gravity, kinetic movement, surface tension, capillary action, and pressure difference through compartmentalization. It is often misused to describe a cladding system that allows water penetration into a drained cavity."

Their definition for Back Ventilated Drained Cavity Wall Assembly (BVDC) describes how most masonry cavity walls are built today:

"A cavity wall where the drainage cavity is minimum 3/8" wide and with weeps and vents located at the bottom and top of the cavity intended to allow for air movement to facilitate drying."

This article provides an historical outline of the development of masonry wall types; it then discusses the development of rain screen masonry wall and its advantages.1

Cavity walls, following some of the principles of rain screen construction, were first introduced into masonry wall construction in 18942 although it took years for this type of wall to be widely adopted. Cavity walls are discussed as a building technique are discussed in Building Science literature in Principles Applied to a Masonry Wall 3. Prior to this period, masonry walls were barrier walls and structural walls that were thick enough that the wall mass would act as a barrier to keep the water from penetrating the thick facade into the building interior. Historic buildings from the pyramids to the Pantheon are constructed this way as were many structures still standing today.

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Mass Roman wall 300 BCE in Carsulae, Italy; Image: Brad Gellert


These mass masonry walls were constructed by skilled masonry crews and the mortars they used contained a high percentage of lime and very little cement which is a contrast to today’s building practices. This lime based mortar had the ability to allow a small amount of moisture to leave the wall system, however some of the moisture was absorbed by the brick that comprised these multi-wythe walls and dissipated within the structure, as in Fig. 1.

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Fig. 1 historic barrier (mass) wall; Image: International Masonry Institute


Mid-20th-century barrier (mass walls) were constructed with a masonry veneer with a support wall built parallel to the veneer with no cavity between the two materials. CMU was a common support wall material. Clay ‘Speed’ tile was also used earlier in the 20th century. These walls, as traditional barrier walls, used the mass of the wall to stop wall water from penetrating the interior.

With the introduction of steel columns into these “transitional masonry buildings” the structural responsibilities of the masonry lessened as the loads began to be carried by the steel. The masonry now was being relied upon to keep the steel dry, this steel was often fully encased by the masonry which was assumed to act as a damp-proofing to keep moisture from reaching the steel. However, depending on the detailing and movement, a good amount of the moisture could find its way to the steel members.

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Historic mass (barrier) masonry wall; image: Brad Gellert

By building the walls of different materials with different thermal and absorption properties, often with no expansion or control joints, these structures often developed cracks over time that allowed more water moisture into the building leading to further distress as cracks deepened with freeze thaw cycles and water found its way to steel lintels, columns, beams and reinforcing steel in concrete frames causing rust and displacement. Frame movement and creep contributed to this cracking.


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Fig. 2 Mid-20th century barrier (mass) wall; Image: International Masonry Institute


Cavity walls were developed which provide a space between the veneer and the support wall to allow a path for water, that inevitably found its way through the mortar joints, to be collected and diverted out of the cavity using flashing and weep holes to channel this water. Improvements in the weeping and flashing methods were developed over time. Traditional copper or stainless-steel flashing with welded seams and formed corners can still be viewed as the gold standard for masonry construction; it is viewed as too costly for many applications today.

The early form of these walls provided weeps, but not ventilation of the cavity. The traditional cotton weeps, and later PVC tubes would become clogged not allowing the cavity to drain and were not large enough for ventilation. Honeycomb vents, the full height of the course, allow for better drainage of the cavity. They are designed to minimize water penetration. They can also provide ventilation, if located not just at the bottom of the wall but strategically through the assembly. Mortar nets at the bottom of the cavity, also insure that the weeps do not become clogged with mortar droppings. With the energy crisis in the 1970’s, the energy performance of buildings and wall assemblies became much more important. Rigid insulation was added to the cavity. The dew point of the assembly changed.

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Fig. 3 ventilated cavity wall (right) and Fig 4. unventilated cavity wall (left); Image: International Masonry Institute

Back ventilated Cavity walls are the predominant form of masonry veneer construction today. By letting air in and out of the cavity, drying of the wall cavity is promoted. Use of cavity ventilation can reduce spalling, cracking and efflorescence. A water and air barrier is required to keep any moisture out of the support wall. By avoiding prolonged wetting of the veneer ties, corrosion is minimized. Below are recommended flashing locations in Ventilated Cavity Walls.

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Fig. 4 flashing locations; Image: International Masonry Institute


Masonry rain screen walls which resist the five main forces of water penetration: gravity, kinetic movement, surface tension, capillary action, and pressure difference through compartmentalization are the next design iteration in masonry veneer construction. Vertical cavity baffles, which break the cavity into compartments, limit moisture laden air from traveling horizontally; the compartments reduce pressure differentials in the cavities. BIA Tech Note 27 recommends that these compartments have closures no more than 4 feet. (1.2 m) apart at the sides and top of the building in a 20 foot (6 m) wide perimeter zone3. Without compartmentalization, pressure differentials can develop at building corners due to their different exposures that can cause suction to develop on one face drawing moisture into the cavity. Air pressure generated by winds, enter the cavity through the vents and equalizes the pressure in the cavity to the pressure outside of the cavity thus preventing wind driven rain from penetrating. BIA Tech Note 27 also provides guidance for spacing of these vents. For the pressure equalized rain screen wall (PERSW) to work, the air barrier portion of the assembly is critical; it must act as an air barrier to prevent pressure from leaking into the interior of the building.

The PERSW, with all the components it requires, not only is the most expensive of the rain screen wall systems but often is the most difficult to create. Just as the “mass” masonry walls of the past were constructed by skilled masonry crews, today’s journeyman modern masons have the training and skill required to not only assemble the exterior facade of these pressure equalized rain screen walls, but all the components to make the system air tight. Today’s masonry contractors can be a single source for the CMU back up walls, installing the air/vapor barrier, rigid insulation and exterior cladding.

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Fig. 6 Pressure-equalized rain screen masonry wall; Image: International Masonry Institute


The pressure-equalized rain screen masonry wall, meeting the AIA Definitions Project term, is designed to keep water out of the cavity thus minimizing damage to ties, insulation and the building interior.

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Air space compartmentalization; Image: Brad Gellert

This article has discussed the development of masonry construction through the ages and the relatively recent adaption of rain screen walls. The A/E and Construction industries are gradually incorporating rain screen walls that fully follow the forthcoming AIA Building Performance Knowledge Community Definitions Project parameters for a true rain screen wall. Further dissemination of knowledge to architects on when to utilize rain screen walls (factors include height/building pressurization, geometry, climate) and how to properly compartmentalize the walls and to masons and masonry contractors on how to implement these designs and to testing agencies on how to insure the designs are truly pressure equalized, is needed.


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Image: Image: International Masonry Institute


Citations

  1. For a more general history see previous Techniques article, by Michael J. Lough, AIA, The long view: History of rain water penetration, March 28, 2016.
  2. Neil B. Hutcheon, “Principles Applied to a Masonry Wall”, Canadian Building Digest 50, National Research Council of Canada, February 1964
  3. Pat Conway, AIA, Ventilated Cavity Walls, Interface, March 2016
  4. See BIA (Brick Institute of America) Technical Note 27, BRICK MASONRY RAIN SCREEN WALLS for additional information.
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