by Katie Daniel | September 29, 2016 10:00 am
by Dave Evers, PE, and Rich Grabmeier RRC, LEED AP
The roof assembly is an important aspect in achieving energy code compliance because it often accounts for a large portion of the building envelope. It becomes even more critical in single-story non-residential projects such as manufacturing plants, warehouses, distribution centers, retail stores, and offices.
The most recent code updates include changes that can have significant impact on how roofing decisions are made. While the 2015 International Energy Conservation Code (IECC) is mostly unchanged from the 2012 edition (and will carry through to 2018), the requirements in American Society of Heating, Refrigeration, and Air-conditioning Engineers (ASHRAE) 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings, have changed significantly with the 2013 edition. Today’s ASHRAE standards more closely align with the latest IECC, calling for much lower U-factors in all climate zones for both semi-heated and conditioned facilities. Additionally, the most recent version increased the U-factors assigned to many of the earlier prescriptive assemblies by 30 to 50 percent, so there are fewer prescriptive options that comply today than in previous years.
When making roofing decisions it is no longer about just meeting code, but rather meeting it in the most efficient way. Therefore, it is important to know the ins and outs of energy requirements and what options are available.
Codes, standards, and zones
IECC and ASHRAE 90.1 design standards are updated every three years, with new standards to be met. A thorough understanding of local code is imperative. Generally, builders on most projects will need to adhere to a particular edition of the IECC, which contains an alternative compliance option to use the ASHRAE standard (but one must adhere to the full code for the entire project). Some states have adopted their own codes or have made amendments to IECC. The particular standard year adopted by each state will also vary.
The Department of Energy (DOE) makes it easy to determine codes by state with its online map. Some states have not adopted an official code, so builders in this scenario need to work at the municipal level to determine which code and standards to follow on a given project.
Once the state code is determined, identifying climate zone is the next step. The eight climate zones in North America, listed within both the ASHRAE standard and IECC, are important because the performance requirements differ. Codes become more stringent as one travels north, into harsher winter climates. The type of roofing materials selected may need to meet a ‘cool roof’ standard, in addition to the insulating values. Cool roofing standards may require a certain verified maximum solar reflectance and emittance of a roofing product. This criteria varies by climate zone and roof pitch.
After dealing with code and climate zone, it is important to determine the energy demand for the building being constructed. Demand falls within three categories:
1. Low-energy buildings, as recognized by IECC and ASHRAE, have no air-conditioning, and a heating capacity of less than 19.3 watt/m2/C (3.4 Btu/hr/sf). For this type of building, no energy code compliance is required as the energy consumption is not deemed high enough.
2. Semi-heated buildings are not specifically covered in IECC, but the code allows a compliant option to use the ASHRAE 90.1 standard for the semi-heated end use. Semi-heated facilities are buildings having a heating capacity greater than 19.3 watt/m2/C, but less than the prescribed maximum heating capacities outlined for the various climate zones.
3. Fully conditioned buildings, recognized in both IECC and ASHRAE, includes any building with air-conditioning or a heating capacity greater than the semi-heated building category. The insulating values required for a semi-heated building are less than a fully conditioned building. This was deemed appropriate as the annual energy consumption is also typically lower than a fully conditioned and more highly insulated building.
The R-value refers to the resistance to heat flow in a single material or component. In practice, a high R-value means better insulating values can be achieved. However, R-values do not tell the whole story. U-factors, which measure the actual heat flow through an entire insulated assembly, not just a single material, provide a more detailed assessment of the actual thermal performance.
Mathematically, the U-factor is equal to the inverse of the R-value (U= 1/R).
In most assemblies, the R-value of the various components is not additive or accurately calculated; therefore, it will not predict the performance of the assembly. U-factor measurements, also known as the ‘effective R-value,’ can be more valuable because they measure the flow for multiple components, typical of most building assemblies, each with varying R-values. Focusing on U-factors when assessing roofing assemblies is important because U-values also account for additional components, such as insulation compression and thermal short circuiting that may occur in an actual assembly. U-factors provide a much more accurate prediction of performance and are necessary for sizing heating and air-conditioning systems.
The prescriptive assemblies listed in both IECC and ASHRAE 90.1 take this into account and list both the R-values used in the components and the assembly U-factor. For example, in ASHRAE 90.1-2013, a wall with metal studs 406 mm (16 in.) on center (oc) and filled with R-13 fiberglass insulation, has an assigned U-factor of 0.124 or an effective R-value of 8 (1/.124). The U-factor or lower effective R-value of 8 for the total assembly, when compared with the insulation’s R-value of 13, reflects the heat flow bypassing or thermally short circuiting the fiberglass through the metal studs.
It is important to note any portion of the building envelope can comply with the energy code by either selecting the prescriptive assembly listed in the code or standard, or by selecting any assembly with an equivalent or lower U-factor so long as you can provide a test report for the assembly.
To accurately test insulating values of an assembly, the building industry follows the procedure outlined in ASTM C1363, Standard Test Method for Thermal Performance of Building Materials and Envelope Assemblies by Means of a Hot-box Apparatus. A few manufacturers test their building products and systems to ensure they deliver the energy efficiency promised. Knowing the actual thermal performance of the building envelope helps ensure compliance to energy codes and can be used to more accurately size HVAC equipment. A hot-box apparatus is a state-of-the-art computerized data acquisition system collecting information from multiple sensors used to measure the surface and air temperature, humidity, airflow, and the total energy consumed during the test period. An assembly test specimen representing all the components of the desired building envelope construction is placed in the apparatus. Chambers are installed to both sides of the test specimen. The temperature is dropped to a specified level on one side of the specimen and the energy needed to achieve and maintain a specified different temperature on the opposite side is measured. A single test can take several days to complete. These results measure the actual heat flow through the assembly, expressed as the actual U-factor performance of the assembly in Kw-m2 C (Btu/hr-sf F).
ASHRAE versus IECC
Now that the fundamentals for codes and standards, energy demands, and specifications have been established, it is time to identify the best way to determine whether IECC or ASHRAE standard path is better to follow for a given project.
There are positives and negatives for each path and, in many states, one can select either IECC or ASHRAE to be the compliance standard. Up until 2009, the performance requirements for IECC and ASHRAE were nearly identical. Since then, they have remained similar, but now have slight variations and may result in different solutions for the building envelope components. It is often worthwhile to investigate both standards when designing one’s building. Generally, ASHRAE is the only compliance option for semi-heated buildings. When selecting this option, ASHRAE requirements for the entire building must be followed, including other areas such as lighting and mechanical systems.
Compliance paths: Prescriptive
Once the preferred code or standard is identified, the next challenge is selecting the correct compliance path through the applicable building energy code. This is not an inconsequential decision, as energy codes are truly flexible, offering multiple compliance paths to suit all types of designers. For roofing, selecting the appropriate path can lead to material and labor savings.
There are four basic paths to comply and they have the same process for either ASHRAE or IECC. Each has prescriptive, performance, component trade-off, or whole building analysis path options. A whole-building analysis typically utilizes a sophisticated software package including the building envelope and all other forms of energy consumption to determine compliance and often requires the assistance of a specialty consultant.
The prescriptive assembly path in IECC typically has just one assembly listed per envelope element type, per climate zone. The U-factor performance path allows builders to use any assembly with a U-factor at or below the stated U-factor for the particular climate zone or assembly, so long as they can provide a test or supportive data for the assembly. For the purpose of this article, the focus will be mainly on the prescriptive versus a tested U-factor assembly.
The prescriptive path in ASHRAE and IECC prescribes insulation requirements according to the building’s climate zone. While the prescriptive assemblies path was previously a common choice because of its ease and affordability, builders may find more cost savings today by opting for the associated U-factor performance path.
The prescriptive assemblies are often very generic in nature and may be conservative to the actual assembly. Over the years, the performance values assigned to these prescriptive assemblies have been revised and often reduced. Previous prescriptive assemblies that may have met a particular standard could fall short in meeting those of today. This exemplifies the importance of being aware of the U-factor path so builders can provide the most cost-effective solution for the building owner. Some manufacturers offer a variety of tested assemblies that will meet the performance U-factor at a more competitive cost than the prescriptive assembly.
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.
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 firstname.lastname@example.org.
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