Ever since iron-framed construction allowed architects to build taller buildings without load-bearing walls, the exterior building envelope evolved to principally offer protection from the elements and comfort for building occupants. Unfortunately, these thinner walls proved no match for moisture and were not thermally efficient.
As time and technology marched on, we began to see cavity wall construction, where an exterior masonry rainscreen wall provided the primary defense against moisture intrusion. Because masonry is a porous material, however, an air cavity and a masonry block inner wall were added to prevent moisture from entering the inner space of a building. This type of wall assembly was further refined when metal studs and sheathing replaced the masonry block inner wall. The use of metal and glass for rainscreens increased because they were not only lightweight, but nonporous. As construction methods continued to progress, building wraps, air, water and vapor barriers and batt insulation all added to improved performance of the inner barrier wall—or so we thought.
Because of the number and types of products required to arrive at this solution, there also are a number of drawbacks associated with it. Issues include complex material sequencing and orchestration of installation, increased opportunities for improper installation and decreased energy efficiency due to discontinuous insulation. These factors ultimately lead to poor building performance, which is the very issue this solution attempted to address.
The Single Component Solution
In recent years, a single-component back-up wall system has been used, replacing the four separate components from the standard multi-component assembly. In this insulated composite back-up panel—or ICBP—the air, vapor and moisture barrier is created by an interior steel liner. This replaces exterior gypsum and a separate vapor barrier. Batt insulation between or outboard of the studs is replaced by rigid foamed-in-place insulation within the panel. Finally, the outer steel face creates a rigid metal drain plane and replaces the normal building wrap rain barrier.
The use of ICBPs provides a barrier from the penetration of liquid water, is airtight, minimizes drafts and air/water vapor intrusion and provides ample insulation to ensure energy efficiency. Although the system itself is not new, its application strictly as a one-piece back-up wall is new and has the potential to revolutionize building design.
Common Rainscreen Wall Assemblies
Modern rainscreen wall design consists of two walls separated by an air cavity. The outer, visible portion of the wall is considered the rainscreen. The purpose of a rainscreen is to deflect the majority of rainwater away from the building and, therefore, it can be constructed of a variety of materials: non-porous materials like metal or polymer-based claddings, or highly porous materials like stone or brick.
The drain plane is the outer plane of the inner wall. Similar to the cavity walls of old, where a tar coating was used, this surface is made of a material that’s impervious to water. Water hitting this surface flows downward to exit points in the rainscreen, such as weep holes.
The final major feature of modern rainscreen design is the inner wall. This wall is referred to as the barrier wall or the back-up wall. This inner wall must not only provide protection from liquid water, but it also must be an air barrier for both energy efficiency and air/water vapor protection since on a molecular level water vapor is smaller than liquid water and air can carry vapor with it. The inner wall also must provide thermal insulation.
Multiple Components, Multiple Issues
The most commonly used method of insulation is putting batting between the steel studs. Fire-resistant exterior gypsum sheathing closes in the building and provides a flat, solid surface to attach the building wrap. This building wrap creates the air/water barrier. It’s important to note that this building wrap often is specified as vapor-permeable; should moisture develop inside the back-up wall, the vapor has an opportunity to migrate back through this barrier to the outside. To ensure vapor doesn’t enter the interior of the building, an impermeable vapor barrier is applied to the outer side of the interior gypsum wall.
However, because the insulation in the multi-component system is interrupted by the steel studs, a material with high thermal conductivity, this creates discontinuous insulation. This discontinuous insulation results in a lower R-value for the wall. If R-21 batt insulation is installed between 6-inch metal studs, located at 16 inches on center, ASHRAE 90.1 from the Atlanta-based American Society of Heating, Refrigerating and Air-Conditioning Engineers Inc. assigns this assembly a thermal correction factor of 0.35 to calculate its true R-value as 7.4—a 65 percent reduction. What’s more, for multistory buildings, this insulation method also becomes discontinuous at the point where the floor slab extends through the wall stud.
An improvement on this system, precipitated by the 2001 Massachusetts Energy Code, was an assembly that featured continuous insulation, air and water/vapor barriers by locating the insulation outboard of the studs. Relocating the insulation outboard of the studs accomplishes several objectives: with the insulation no longer between the studs, the issue of thermal conductivity goes away. Because the studs now are on the warm side of the wall, the stud system no longer is in danger of corrosion due to water vapor.
Although the outboard-of-the-studs assembly is an improvement, there still are numerous drawbacks to this system. For example, the exterior sheathing and building wrap must be installed flawlessly, with all seams and fastener penetrations meticulously sealed, or air and moisture penetration is likely to occur. This moisture also can cause rust and corrosion of the steel framing. Perhaps even more damaging, it can lead to the buildup of toxic mold, which can cause building occupants to become sick.
Build a Better Wall
A promising new single-component back-up wall system is gaining acceptance in the building industry. The single component back-up wall panel is an insulated metal composite, similar to the wall panels that are widely used today in commercial construction to create single layer exterior metal walls. However, this panel system has been modified for performance, affordability and constructability as a back-up wall system.
In terms of energy efficiency, the 2-inch-thick panels provide an R-14 insulation value and 3-inch-thick panels provide a value of R-21. With these numbers, the back-up wall panels can easily meet and exceed standards of ASHRAE/IESNA 90.1—2007.
Because of the nature of its use, modifications have been made to allow greater affordability. For example, the metal skins that encase the rigid foam-insulated core of these back-up systems can be lighter in gauge and the insulation itself can be less dense than what’s found in traditional insulated metal composite panels. Because these panels will never directly face sunlight or weather head-on, a more affordable coating system can be used. In addition, because of the ICBPs’ lightweight nature, structural requirements for the building sometimes can be reduced, further saving materials and cost.
The back-up panels come in standard sizes with basic configurations of 32- and 36-inch widths and lengths up to 20 feet. They are installed horizontally on steel frame construction with studs spaced every 16 or 24 inches, on center. Concealed multi-function clips are used to attach the panels to the studs. It’s important to remember, however, that this penetration through the panel occurs at pressure-equalized panel horizontal joinery, which reduces the possibility of water penetration.
The cut ends of panel lengths are aligned to terminate centered over a stud with a 4-inch-wide, 22-gauge steel plate attached. A 3/8-inch diameter bead of non-curing butyl sealant is applied here. A marriage bead of sealant ties the shop-applied horizontal joint sealant to the field-applied vertical sealant. For added protection, durable weather-resistant tape is applied at the vertical joints and at corners.
The resulting installation creates a wall with continuous seals around the full-perimeter of each panel. Upon completion of the installation, a simple test can be performed to ensure the back-up wall system has been installed properly. For this test, water is sprayed at 30 to 35 psi along 5 feet of joinery for five minutes. A person on the inside of the wall verifies no water penetrates the joints. Because it’s done with one component and one installer versus four or five, time and effort saved equates to real cost savings.
The largest benefit of an insulated composite back-up panel versus a multiple component system is its ability to create a superior air, thermal and moisture barrier. This is accomplished by doing things like creating full perimeter seals, installing outboard insulation, using only materials that are not affected by moisture and integrating rainscreen attachments whenever possible with the panel’s clip attachment system. This helps minimize penetrations through the moisture and air barriers, thereby reducing the potential for leaks.
The single-component design takes the place of four separate building components, speeding installation time and reducing costs and the chance for errors. The same system is suited for every climate. And the system is compliant with the thermal requirements of ASHRAE/IESNA 90.1—2007. Finally, the system has undergone complete structural, air, water, thermal and fire testing and meets all building codes and all major fire codes.
With ICBPs, we’ve come a long way from the thick stone walls of our ancestors, and even a long way from the multi-component cavity walls of just a few years ago. As the construction and building industry moves into the future and continues to innovate, ICBPs, with their numerous and varied features and benefits, look to be a large part of that future.
Bill Hornfeck is a product manager at Centria, Moon Township, Pa. He can be reached at email@example.com or (800) 759-7474.