There are many metal roof and wall products available to us today. When designed properly, these metal products provide attractive options that can service a building for many decades. Success of the design is dependent upon the entire metal panel assembly whether your project is new construction or retrofit. Metal panel assemblies specifically require the design of “control layers” to provide a properly performing building envelope. By most definitions, there are four main control layers.
1. Water control layer
2. Air control layer
3. Thermal control layer
4. Vapor control layer
A given product may provide more than one of these control layers. While this can cause some confusion, there is a great deal of information available on the subject. Publications from ASHRAE, the International Code Council, the National Institute of Building Sciences (Whole Building Design Guide) and the U.S. Department of Energy all provide guidance on the subject. Consulting with a building science expert and manufacturers are always recommended when preparing for a project.
Defining the Control Layers
Insulation provides our thermal control layer reducing heat loss or gain through the building envelope. The air and vapor control layers are often one in the same. Depending upon the climate, the use of an air barrier, vapor barrier or vapor retarder may be necessary.
“The function of a vapor barrier is simply the control of water vapor diffusion to reduce the occurrence of intensity of condensation. Air barrier systems control air flow and thereby control convective vapor transport. The control of air flow provides other benefits such as increased comfort, reduced energy consumption, control of odor, and sound transmission.” (Straube, “The Influence of Low-Permeance Vapor Barriers on Roof and Wall Performance,” Building Science Press 2011)
Control Layers in Metal Wall Panel Assemblies
As an example, here are two metal wall panel assembly types and how they handle the control layers:
1. Structural Insulated Panels or SIPs
2. Rainscreen assemblies
Structural insulated panel systems are those where the metal skin and insulation are laminated together to create a rigid panel. The benefit of these panels is that they combine control layers together. Most notably, they combine the water and the thermal control layers together.
The unitized nature of these panels means that the use of SIPs may result in labor savings, especially for high-rise construction where the installation cost becomes a larger part of the budget. However, the performance of these panels is dependent upon the seal in the panel joints and flashing at their terminations. Water control and thermal continuity is dependent upon these details being installed properly.
Flashing SIPs around windows, doors and other penetrations depend further upon the designer and control of the installation process. These details must be designed and installed properly to realize the benefit of this panel system. Lastly, panel finishes and profiles are limited, reducing options for the designer and owner when it comes to the aesthetic of the project.
Rainscreen assemblies are multi-component systems where the metal cladding and insulation are installed as separate layers. The metal cladding and insulation is separated by a drainage and ventilation cavity similar to that behind a brick veneer.
Rainscreen systems provide some significant benefits. Most significant is control of the flashings and transitions. With the insulation and metal cladding separated, it becomes easier to provide continuous R-value (as required by 2009 IECC) and tie-in the water, vapor and air control layers at detail transitions. Further, with products like closed cell spray foam insulation, the water, vapor, air and thermal control layers may be applied in one product. The designer and owner are then open to a multitude of options in metal cladding finishes and profiles.
While rainscreens provide many benefits to the design of a project, they can be more expensive to install than SIP on projects where there is limited work area and access. Examples include high rises or congested industrial job sites. It should always be remembered however, that while some products provide an initial cost savings, repair or replacement of a failed installation is far more expensive than investing in the proper design and products.
Additional Thoughts
Success is not dependent upon the SIP or metal cladding alone and no one product can provide the whole answer. Detail flashings and transitions between building elements are as important as the roof or wall assembly themselves. When it comes to preventing water intrusion and air leakage, these tie-ins are often more important because they require the greatest amount of attention by the designer and installers.
Look to partners who can provide the essential services to ensure success on your project. Essential services include:
1. Building code compliance review.
2. Project budgeting based upon life cycle cost analysis and energy savings evaluation.
3. Shop drawings detailing the entire assembly.
4. Regular job inspections during construction.
John L. Pierson, PE, is the engineering services manager for The Garland Co. Inc., Cleveland, a manufacturer and distributor of high-performance roofing and wall systems for commercial and institutional markets. He frequently delivers seminars and AIA-approved classes on installation techniques and building envelope technology for the Garland Speakers Bureau. Prior to his work with Garland, he was employed in the construction industry as a field engineer and consultant. Visit
www.garlandco.com to learn more.
