The following Viewpoint is written by Stephen Holland, associate | enclosure at design firm LeMessurier.

Code revisions often have stricter requirements than their predecessors, but the 2023 Massachusetts Stretch Energy Code has taken a dramatic leap ahead. There are new requirements for building air tightness, glazing thermal performance, energy conservation, code compliance paths, and thermal bridging. Many of these topics aren’t even mentioned in past code editions and can be daunting for a first-time designer. Although two of the most nuanced aspects of the new code—navigating Code Compliance Paths and incorporating Thermal Bridging—can be confusing at first, both concepts are easily digestible after taking a step back to examine and understand each one carefully. Likewise, incorporating elements of the new code into the design can be seamless after some review. 

Useful Terminology to Understand the Code

Thermal bridge vs. thermal break: A thermal bridge is a pathway for heat to bypass insulation, typically a metal or concrete component. These are not desirable for design, but often necessary for structural reasons. A thermal break is when a new material is introduced to stop this heat flow. This can include hard plastic materials that can perform their structural purpose, but don’t allow as much heat transfer.  These are desirable to mitigate the effect of thermal bridges. 

Image courtesy LeMessurier

R-value vs. U-factor: R-value is the most common way to describe the amount of insulation in an assembly. U-factor is just the inverse of R-value (i.e. if an assembly has R-20 insulation, its U-factor would be 1/20= U-0.05). R-value represents the amount of insulation, and the U-factor represents how much heat flows through the assembly. Crucially though, the U-factor for an assembly considers the thermal performance of the insulation and all of its thermal bridges including studs, girts, fasteners, etc. That means that the U-factor is often much higher than the inverse of the insulation R-value. The new code specifies all performance criteria in U-factors only, and designers must show compliance in U-factors.

Image courtesy LeMessurier  Code Compliance Path

The largest source of distress with the code has seemingly been how to apply the varying requirements to different compliance paths. Based on the building’s size, use type, and owner’s preferences, every building meets the Stretch Code through a specific code compliance path. Each of these compliance paths will dictate different requirements. The five compliance paths include: Prescriptive, Targeted Performance, Relative Performance, Passive House, and HERS. In the table below, the corresponding code requirements for each path are shown.

One of the most important first steps is identifying the appropriate code compliance path for a given project. This will inform which requirements you will need to hit, and then you can start to plan accordingly.

Image courtesy LeMessurier 

Thermal Bridge Derating

Arguably, the most dramatic change in the code is the inclusion of thermal bridges in calculations. Until now, thermal compliance could be demonstrated by adding enough insulation thickness to exceed the required R-value. This process ignored the effect of non-insulation elements on heat transfer, which is often significant. After considering the effect of these thermal bridges, designers must now show that the assembly still meets its target. 

How can designers go about this? It’s useful to start by breaking the thermal bridges into 1) field thermal bridges such as studs, girts, fasteners, etc., and 2) linear thermal bridges such as parapets, window edges, and slab edges. After this, the code typically allows three ways to derate your assembly:

• Prescriptive Code Derating: The code will specify various penalties based on your assemblies. These tend to be the most aggressive penalties and should be avoided.
• Referenced Derating: The BC Hydro guide is a code-referenced source that specifies a number of common assemblies and lists their derated U-factors. This is often the easiest path forward for standard, non-custom designs. 
• Modeled Derating: Using 2D or 3D modeling software, designers can model an assembly's exact U-factor. This requires more work up front but can often provide the least-punitive derating. 

Many projects often require a combination of referenced derating and modeled derating to minimize the derating penalty. 

Additional Thoughts

We’ve only touched upon two of the most important criteria in the new code, but the code also specifies requirements for air tightness testing, glazing performance, energy consumption, and others. These requirements affect both new construction and renovations to existing buildings. Each project’s specific requirements will vary, and it's best to discuss with your building envelope consultant to understand which requirements apply. This new code has many facets, but it is possible to comply with the new code if you understand the requirements upfront. 

Holland joined LeMessurier's enclosure group in 2021. He has experience in the investigation, rehabilitation, and design of a variety of enclosure systems including roofing, curtain wall, windows, masonry, cladding, waterproofing, insulation, and below-grade systems. He serves as one of the chairs of the BSA's Building Enclosure Council and is an active member in the ACE Mentoring program.