Compliance paths: the envelope trade-off path
The next path option is the envelope trade-off path. Envelope trade-offs are tightly defined exchanges allowing over-performance in one building component to compensate for underperformance in another. This path is an important resource for projects that include many doors, windows, and skylights, as it allows the designer to optimize the cost of all the components in the entire building envelope while achieving the required overall total performance.
In terms of roofing, this path recognizes the significance roofing insulation plays in determining the overall envelope performance. A modest variation in roofing insulation can have a big impact on the requirements for other building components.
The tool most often used and most widely accepted for component trade-off analysis is called COMcheck, which is a free program developed by the DOE. With this software, builders can mix and match building components (e.g. roofing, doors, windows, and walls) to achieve compliance. It includes options for IECC, ASHRAE (including semi-heated buildings), and selected state amendments. While there is an option to simply input prescriptive values to pass, the full value of the tool is utilized by testing out various trade-offs or taking the benefit of using lower U-factors to offset other components of the building to make the overall envelope more effective. It should be noted COMcheck is not accepted in every state. Therefore, design/construction professionals need to determine if it can be used in their location. However, the local municipality may accept COMcheck even if the state level does not.
COMcheck establishes a budget or target performance based on code requirements for a particular building’s envelope. It blends all the components of roof systems, walls, foundations, and skylights, and allows trade-offs between various components and assemblies as long as the total building envelope is compliant. COMcheck also provides a printed compliance report and checklist for the local building code official to use in permitting a design. Those who are smart about the trade-offs can achieve both material and labor savings by simply following the prescriptive path for each component.
There are different ways material and labor savings can be achieved with COMcheck. For instance, one can install more insulation in the roof to ‘make up’ for putting in more window area than the code allows. Further, one could trade decreased wall efficiency (i.e. lower R-value) for increased window efficiency (i.e. lower U-factor). Another option could involve increasing the roof insulation in order to reduce or eliminate slab-edge insulation.
Looking ahead
With a solid understanding of the changing codes and the options available, design/construction professionals are best-equipped to identify roofing assemblies (and make other building envelope decisions) that not only achieve code compliance, but also do so in a cost-effective manner.
Dave Evers, PE, is the retired vice president of research and development at Butler Manufacturing. He is tasked with new product development, product and materials testing, and operations at the Butler Research and Development Center in Grandview, Missouri. Since joining the company more than 40 years ago, Evers has held positions in structural design, sales, product development, and materials testing. He earned his undergraduate degree from the Missouri University of Science and Technology.
Rich Grabmeier, RRC, LEED AP, is a business development manager focusing on innovation with Butler Manufacturing. He has more than 30 years of experience in the building industry, with a focus on product and process trends and improvements. He graduated from the University of Texas at Austin with a bachelor’s of science degree in architectural engineering. Grabmeier can be reached at editorial@butlermfg.com.
