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Rain Screens: Tilt-Up Façade Options for Achieving Barrier Conditions

Español | Translation Sponsored by TCA

by James Baty, FACI, FTCA

What Is a Barrier Condition?

Over the past few decades, building teams have increased their attention to the performance of the building envelope. This attention has prompted design professionals to seek critical solution information from manufacturers, contractors, and from the industries themselves, to learn about acceptable barriers performance in different kinds of weather conditions.

The most well-known barrier condition required by building envelopes is that of humidity. The control of humidity affects both the interior climate comfort and the efficiency of the mechanical system. As those familiar with the Psychometric Chart know, humidity is a function of water vapor volume and air temperature. The higher the air temperature, the greater the volume of water it can hold. As the amount of available water volume carried by the air increases, the humidity increases – and thus, the pressures change on the building envelope as water vapor pressure is created from warm-humid to cold-dry, drawn by the mechanical systems that are attempting to cool or condition the interior climate accordingly. Humidity, however, was only the start of the increased attention to building envelope performance.

During the 1980s and 90s, investigations into dilapidated and failing building envelopes (constructed largely with masonry systems and having furred drywall interiors) revealed that moisture driven by wind could make its way through porous and unmaintained envelopes. Where not directed away from the interior, the moisture could exacerbate conditions for mold and mildew and foster a myriad of other maintenance problems.

Beginning with the 2009 International Energy Conservation Code (IECC), building envelopes for new construction in a broadening cross-section of the North American climate regions became subject to more stringent insulation requirements. The thermal barrier for temperature control once again became a focus, and for the first time, continuous insulation became characteristic of the requirements.

Finally, in 2012, the IECC added more stringent language to the design and construction of building envelopes in a new requirement for continuous air barriers. Building envelopes throughout North America were to be evaluated for air movement control, making sure buildings meet a maximum overall air infiltration component, to be measured by a Blohridor test where required (if envelope componentry could not validate such control).

The sum of these measures places significantly higher responsibility on the architect and the mechanical engineer when they attempt a creative configuration of the building envelope, especially when they introduce new solutions to building programs in which they are otherwise experienced.

Building Envelopes – A Whole Sum of Parts

This phrase has always been debated, but truthfully, is the whole greater than the sum of its parts? In the case of building envelopes, this is most certainly not true. The problem most building envelopes face is that the constructed whole has inherent weaknesses. The parts assembled – in other words, the sum – result in gaps or have ineffective or inefficient components.  

In “The Perfect Wall: Ultra efficient, to ensure energy will last for our grandchildren” (ASHRAE Journal, May 2008), author and building science guru, Dr. Joseph Lstiburek, addressed the growing problem of building envelopes that use inferior materials to make modern aesthetic statements. “As we change our building technology to account for the new energy cost realities,” Dr. Lstiburek wrote, “we are in for a world of hurt in terms of corrosion, decay, mold and other moisture-induced deterioration.”

Figure 1: “The Perfect Wall” or “Face-drained System” – In concept, the perfect wall has the rainwater control layer, the air control layer, the vapor control layer and the thermal control layer on the exterior side of the structure.  The insulated tilt-up sandwich panel places all four systems integral in one unit that is easily maintained at each vertical joint.

This changing technology he referred to in 2008 is the advent of engineered wood products, cavity systems, and complex building envelope assemblies that capitalize on cellulous fiber products, all absorbing or wicking moisture through wall assemblies. This, however, is not much different than the masonry wall assemblies of the 80s and 90s, where people observed moisture issues that led to entire structures being torn down or their façades being removed, in lieu of high-performance envelopes with a new requirement, the “rain screen.”

What Dr. Lstiburek referred to as “the perfect wall” is one where truly the whole is no less than, and is likely greater than, the sum of its parts. Such a wall is to be measured for its ability to provide rain control, air control, vapor control and thermal control. In his report, such a wall is one that has an exterior cladding layer (aesthetic and protection), a control layer (thermal and moisture) and an interior structural layer. Dr. Lstiburek wrote, “The best place for the control layers is to locate them on the outside of the structure in order to protect the structure (Figure 1). When we built out of rocks the rocks didn’t need much protection. When we build out of steel and wood we need to protect the steel and wood. And since most of the bad stuff comes from outside the best place to control the bad stuff is on the outside of the structure before it gets to the structure.”

In this new age, Dr. Lstiburek has found inherent weakness in a lot of building technologies, the multi-component façades constructed from a wide variety of porous, soft and modular materials. Though these materials are brought together and detailed to make a harmonic building envelope, they remain just that, a constructed assembly. The process of building individual layers, installed by multiple trades, sequenced in layers one on top of another, and then fastened for durability – this process leaves many routes for envelope performance failure. What is slightly baffling is that what was available to Dr. Lstiburek then remains available to the market now… and has even continued to evolve.

Perfect Wall to Perfect Barrier

Dr. Lstiburek’s “perfect wall” method gets results and meets barrier profile requirements. It is paralleled by the description of the “perfect barrier,” described by Dr. John Straube of RDG Building Science Inc, the author of “Maintenance and Inspection Manual for Precast Concrete Building Enclosures” (Canadian Precast/Prestressed Concrete Institute [CPCI], June 2016).  

In this CPCI manual, Dr. Straube establishes that modern precast wall systems offer the best protection and completely fulfill the purpose of a rain screen.  Modern precast wall systems do this without using extensive collection and removal systems found in lesser multi-component building envelopes. Building envelopes, Dr. Straube explains, serve three main physical functions for environmental separation. These functions are support, control, and finish. Support is, basically, structural, and finish is the aesthetic. Control, however, is often the most complicated, as it must exist between support and finish – and, in many cases, must exist in spite of support and finish. In fact, control as a physical function was a problem identified in the 1980s and 90s, as control was sacrificed in favor of combining support and finish within a more efficient economic framework.

“For physical performance,” writes Dr. Straube, “the most common required enclosure control functions include resistance to: rain penetration, air flow, heat transfer, condensation, fire and smoke propagation, sound and light transmission (including view, solar heat, and daylight, insect infestation, particulate penetration, and human access).” This list is comprehensive and demonstrates the complexity often overlooked when a building system is chosen for the exterior aesthetic – when the interior space configuration and aesthetic only get worked out much later. A building’s durability, as Dr. Straube further states, is greatly impacted by its ability to resist the elemental pressures of rain, air, heat and vapor. Hence, the rain screen.

As Dr. Straube continues explaining in the manual, a good response to the complex control problem seen in durable façades is often found in precast building envelopes. As found in Building Code Requirements for Structural Concrete (ACI 318), precast refers to both in-plant and on-site (including tilt-up) wall elements that are either non-prestressed or prestressed. Dr. Straube theorizes that, for precast wall assemblies, the control function is delivered in one of two ways: face-sealed façades or drained façades. “A face-sealed façade is a type of ‘perfect barrier’ approach to rain and air control,” he writes. (figure 2) “Control of rain penetration occurs at the exterior face of the system using the concrete panels and sealant joints.” And then, “to accommodate the joints between panels, the concept of a drained joint, or two-stage seal has been promoted.” (figure 3) That is (particularly in retrofit, maintenance, and even older widely used configurations), the joints between the panels are looked at separately, incorporating a drained rain screen approach.

Therefore, to Dr. Straube, the designer is given options with precast (tilt-up) wall systems to achieve an effective, durable approach to controlling environmental impacts that otherwise require rain screen systems in multi-layer assemblies.

Figure 2 – Three-dimensional representation of a Face-Sealed Joint, common to the tilt-up industry, with a supportive concealed joint back-up system, oriented vertically between tilt-up panels.  Note the integration of the “Perfect Barrier” components of the monolithic sandwich panel construction.

Optional definitions to include in the piece from Straube’s manual:

Barrier System or Perfect Barrier – The general term used to describe a rain control approach that relies on the perfection of a single plane of material(s) to resist rain-water penetration. Two sub-types, face-sealed and concealed barrier, are commonly used.  See Figure 2.

Drained Screen – A building enclosure rain-control strategy that accepts that some water will penetrate the outer surface (the cladding, which “screens” rain) and directs this water back to the exterior by gravity drainage over a drainage plane, through a drainage gap, and exiting via flashing and weep holes. Also called rain screen.  See Figure 3.

Insulated Tilt-Up Building Envelopes Offer Truest Rain Screen

In Dr. Straube’s manual and in Dr. Lstiburek’s article, the building envelope’s performance is most directly addressing conditions for offices, mixed-use builldings, or multi-family residential living. It is in these structures where the configurations of building envelope assemblies are often the most complex and the control layer is interrupted, lost or overlooked. This is where tilt-up separates from the general discussion. In tilt-up structures, no matter the building program, the basic concept is a continual panel from the footing through to the roof, or in mid-rise structures, limited in horizontal joints to one, such as the 4/2 configuration for a six-story office building having a four-story panel supporting a two-story panel (figure 4). Otherwise, tilt-up panels have now exceeded 110 feet in height and continue to be constrained in their height and width purely by the size of crane available (or economically chosen) for the project.

Figure 3 – Three-dimensional representation of a Two-Stage Drained Joint, made possible by pressing the backer rod deeper into the panel joint prior to caulking, caulking the lower joint system first back to the depth of the recess and then completing the caulking on the upper section.  Detail is less common but possible in tilt-up projects.

In tilt-up concrete panels, the other major point of separation from the general discussion of precast is the delivery of insulated sandwich wall panel designs. Tilt-up has decades of experience with building envelopes providing 100% separation between the two concrete layers by a layer of high-performance extruded polystyrene, (figure 5) expanded polystyrene, or, in some building programs, a polyisocyanurate insulation. These rigid insulation board assemblies offer the most complete solution to the 2012-and-beyond IECC requirements for continuous insulation in any climate region (earlier IECCs had requirements for portions of climates). Combined with the monolithic continuity of reinforced concrete shell layers with a minimum exterior thickness of 2.5 inches and a structural or interior layer of a minimum 4.5 inches, the insulated tilt-up sandwich panel combines all of the required elements of the control layer into one, face-sealed (Straube) system. It is then the determination of the design team in concert with the construction team to determine the best solution for the panel joints based on the material specification. Using traditional polyurethane joint systems to seal the exterior and interior faces of the panel joints, the exterior achieves a face-sealed parameter and the interior is further protected by a back-up system.

However, it remains a worthwhile question to ask about the maintenance of the joint and the life cycle of the joint sealants when determining whether the face-sealed joint detail (figure 2) should be used, or a configuration of the drained joint detail (figure 3). Admittedly, there are very few examples of tilt-up buildings today incorporating the drained joint detail, but as the number of building programs continues to rise, design teams looking for maintenance-free or maintenance-proof building envelopes may turn to this solution.

Figure 4

Market Challenge or Ignorance

In September of 2009, Pro-Demnity Insurance Company, the largest provider of E&O insurance for architects in Canada, issued an exclusion statement to policy holders stating that support, coverage and defense on policy claims for issues of breeches of the control layer of buildings would not be provided. This statement, however, did acknowledge that if the building envelope contained an active rain screen drainage layer, was constructed of solid concrete with no surface material that would adversely affect drying, or had precast panel systems incorporating a two-stage drained join, the exclusion would not apply, and therefore, such claims would be supported.

More recently, however, this same issue has been raised through Pro-Demnity and other insurance providers, seemingly ignoring or failing to recognize the proven performance and language of the research and exclusion documents that provide such clear direction. Despite the rich history of the perfect barrier solution for building envelopes provided by precast (plant-cast and tilt-up) industries, certain markets continue to be challenged by the black-and-white interpretation of code requirements for a “rain screen.”

Examples with concrete as the premier control system for building envelopes, incorporated with support and finish systems, can be found in numerous additional resources. The International Energy Conservation Code (IECC) 2015 initiates that in order to meet the requirement for continuous air-barrier, concrete is an acceptable prescriptive selection in accordance with C402. Materials, item 13 (International Code Council,, 2015). On their website, the Building Science Corp published a document describing moisture control in walls. Regarding precast wall assemblies, this report states: “The vapor barrier in this assembly is the precast concrete itself. Therefore, this wall assembly has all of the thermal insulation installed to the interior of the vapor barrier.” And: “In this wall assembly the precast concrete is also the drainage plane and air barrier” (BSD-012: “Moisture Control for New Residential Buildings” by Joseph Lstiburek,

Tilt-up buildings will continue to perform to a higher level than any other building envelope assemblies available to architects and construction teams due to this combination of materials and systems. If you are faced with decisions related to exclusions based on rain screen prescriptive requirements, contact the TCA for more information for assistance through our advocacy services to members.

Figure 5 – Seen here, the insulated tilt-up sandwich panel provides all three cladding control functions of a “Perfect Barrier” in a singular construction.  The embedded brick or other architectural cladding solution is part of a monolithic concrete skin of a minimum of 2.5 in. thick.  The function is principally to act as an ultraviolet screen and an impenetrable weather barrier, along with the aesthetics.  The vapor and thermal control layers are integrated into one continuous, while the structure exists inside, yet fully connected and additional in its control.
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TILT-UP TODAY, a publication of the Tilt-Up Concrete Association, is THE source for Tilt-Up industry news, market intelligence, business strategies, technical solutions, product information, and other resources for professionals in the Tilt-Up industry. A subscription to TILT-UP TODAY is included in a TCA membership. Subscriptions for potential TCA members are also available. If you would like to receive a complimentary subscription to the publication, please contact the TCA.