Proper Test Methods to Determine Thermal Resistance

Posted on November 30, 2017 by bzabcik

For a given assembly, if the right information is not specified in conjunction with the desired R-value, the designer will likely not achieve the results he or she expects. This can lead to code compliance issues as well as poor performance of the finished building. Therefore, a more thorough approach must be considered to ensure the specified assembly will be building energy efficiency code compliant. Where to begin? When looking at proper test methods to determine thermal resistance of metal panels, the place to start is ASHRAE 90.1 Chapter 5 (Building Envelope) and Appendix A.

Thermal Resistance
ASHRAE 90.1 Section 5 specifies requirements for the building envelope.

Code Compliance for Thermal Resistance

The most widely accepted energy efficiency standard for commercial construction in North America is ASHRAE Standard 90.1. This standard provides both a prescriptive and a performance path to be chosen at the designer’s option. The prescriptive path is most commonly used. It also provides the baseline performance level that is used to determine compliance for the performance path, so understanding this set of requirements is critical. Within the prescriptive path, two possible methods of compliance are available to determine the minimum thermal performance of opaque areas on the building envelope. Section 5.5.3 is the pertinent passage and it reads:

  1. Minimum rated R-values of insulation for the thermal resistance of the added insulation in framing cavities and continuous insulation only. Specifications listed in Normative Appendix A for each class of construction shall be used to determine compliance.
  2. Maximum U-factor, C-factor, or F-factor for the entire assembly. The values for typical construction assemblies listed in Normative Appendix A shall be used to determine compliance.

Exceptions: For assemblies significantly different than those in Appendix A, calculations shall be performed in accordance with the procedures required in Appendix A.

What does this mean? Basically, there are standard types of construction that ASHRAE recognizes and if you have a wall that fits the description in Appendix A, you don’t have to test or do anything special to determine its thermal resistance. Appendix A provides tables based on calculation methods that have been derived on the basis of previous tests and general experience. What is perhaps less obvious is that if your assembly is adequately described by one of the standard assemblies in the Appendix, you may NOT use a tested or modeled value in place of the values in the table, even if that value has better performance! (i.e., lower U-factor) This is explained in Section A1.2.

The reason the code is set up this way is to prevent people from building unrepresentative assemblies that achieve high performance in the lab but are likely not built to the same specifications in the actual building.

Conversely, if the assembly you want to use is NOT adequately described in Appendix A, the appendix goes on to specify which methods are acceptable to determine the U-factor based on the assembly to which it is most similar. This is covered in Section A9. Two and three-dimensional finite element models are always acceptable and in some cases, simplified calculation alternatives are also available. Note that hot box testing is not always allowed.

Conclusion

To summarize, whether using a prescriptive or a performance path, the first and last stop when determining thermal resistance for metal panels is ASHRAE Standard 90.1 Chapter 5 and Appendix A. Designers would be well advised to familiarize themselves with the Standard and the specific set of requirements for their particular scenario in order to utilize proper testing methods for high-performance results.

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