Have you ever come across the term “thermal bridging“? You may start hearing about it more now that the Massachusetts stretch energy code requires more analysis related to thermal bridging.
In short: thermal bridges are thermally-conductive points, lines, or areas in the building envelope which allow heat to pass through more easily.
Thermal bridges occur where materials that are better conductors of heat are allowed to form a ‘bridge’ between the inner and outer face of an assembly. These often occur at building parapets, floor slab perimeters, below-to-above-grade transitions, building corners, window perimeters, etc.
Understanding Thermal Bridging and MA Code Requirements
Thermal bridging can significantly reduce the building’s overall thermal efficiency. To get a better and more accurate understanding of the overall thermal performance of a building, we generally recommend taking into account the effect of thermal bridging conditions. This may be especially prudent when estimating heating and cooling loads when sizing HVAC equipment.
Moreover, the new 10th edition of the Massachusetts Stretch Energy Code mandates the following with respect to thermal bridging (225 CMR 23, Massachusetts Commercial Stretch Energy Code and Municipal Opt-in Specialized Code 2023):
- The effect of cladding attachment systems (i.e., subframing, brackets, anchors, etc.) on wall assembly thermal performance needs to be reflected in the field-of-wall U-factors. This is typically referred to as “derating the assembly”.
- The effect of linear thermal bridging needs to be included in the overall above-grade wall assembly performance.
- The thermal performance of opaque portions in glazing systems (i.e., spandrel panels) needs to be accurately represented.
The implications?
All commercial buildings will be required to do the above listed steps. This applies to all buildings except residential structures three stories or less, whether new construction, an alteration, or an addition.
Of course, this is only a requirement in jurisdictions where the Stretch Energy Code applies, though most Massachusetts communities have adopted the Stretch Energy Code.
Project-Specific Derating and Thermal Bridging Calculations
The Stretch Energy Code allows you to take into account thermal bridging using prescriptive derating factors and psi factors.
Derating factors take into account the effect of cladding attachment systems on the often not-so-continuous exterior insulation. Psi factors, on the other hand, can be used to take into account the effect of linear thermal bridging conditions (parapets, brick relieving angles, etc.).
Thermal model of thermal bridge condition at slab-on-grade perimeter detailing.
However, these prescriptive factors and psi values included in the Stretch energy Code are chosen to be conservative. As a result, following the prescriptive method may be more expensive by leading to an excessive increase in the amount of thermal insulation used for a project.
The stretch energy code also allows you to select pre-solved derated assembly values from the Building Envelope Thermal Bridging Guide (www.bchydro.com). Similar to the prescriptive approach, this often leads to an unnecessarily conservative outcome considering the emphasis of the Building Envelope Thermal Bridging Guide has thus far been on severe thermal bridging conditions.
CopelandBEC Thermal Bridging and Derating Services
CopelandBEC can save your project time and money by helping you to navigate the maze of the new MA Stretch Energy Code with respect to thermal bridging and derating requirements. Copeland BEC can simplify this process by providing the following services.
Identifying Thermal Bridging Conditions
Copeland BEC excels at identifying thermal bridging conditions within your building envelope and identifying which need to be taken into account to be in compliance with the MA Stretch Energy Code.
Derating & Calculating Enclosure Thermal Performance Decrease
We don’t just identify; we quantify by calculating (i.e. computer modeling) the performance decrease of assemblies due to thermal bridging conditions included in your design.
We can model virtually all linear and point based thermal bridges and perform wall assembly derating (C402.7.2) and take into account thermal bridging conditions (C402.7.3).
C402.1.5 Component Performance Alternative – “The Backstop”
Often referred to as “the backstop,” the Component Performance Alternative, may be required depending on the chosen stretch energy code compliance pathway. Currently, this entails meeting an area-weighted U-factor target for both the above-grade wall assemblies and glazing systems (after all thermal bridging conditions have been taken into account).
Other Thermal Bridging Calculations and Services
- Evaluating the thermal performance of curtain wall and glazing system spandrel areas (C402.7.4 Thermal Resistance of Spandrel Sections).
- Providing you with updated modeling results in near real-time.
- Helping improve building envelope detailing to eliminate or lessen the severity of thermal bridging conditions. Where required for code compliance, decreasing the severity of thermal bridging may allow the use of less thermal insulation, increasing material use efficiency.
- Providing tabulated summaries of the items mentioned above that can be incorporated into your Construction Documents used for permitting to show compliance with your chosen Stretch Energy Code compliance pathway.
Copeland BEC can stand as your partner in achieving thermal excellence. Let us be the bridge to your project’s success in meeting this new Stretch Energy Code.
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