Are you in Massachusetts and your project needs a TEDI model to prove compliance with the Stretch Energy Code? Do you have all the questions and none of the answers? You’ve come to the right place.
Here you will learn:
- What is TEDI and how does design impact it?
- What type of energy model do I need to prove compliance in MA?
- How can one meet the very stringent thresholds set forth by MA?
TEDI (Thermal Demand Performance Index) is not a new concept. It is one of the metrics used as part of the Passive House certification process and for compliance with local energy codes in cities such as Toronto and Vancouver. And now, since the adoption of the 2023 edition of the Massachusetts State Stretch Energy Code (a.k.a. the 10th Edition MA Stretch Code), a new compliance path (Targeted Performance Path) requires meeting very stringent heating and cooling TEDI thresholds.
Note that there are many other requirements in the new MA Stretch Code that we’re not covering here, such as the very stringent thermal derating requirements for envelope performance. But you can always reach out to us if you have questions on those topics too.
What is TEDI?
Thermal Energy Demand Intensity (TEDI) refers to the annual heating or cooling energy per unit area that a building demands from the HVAC system to keep the indoors at the desired temperature setpoints and to provide ventilation.
In other words, imagine an office building in the middle of the winter. Heat is being lost through the envelope and gained through solar, lighting, occupants and equipment. To keep occupants at a comfortable setpoint, say 72F, you need to provide a certain amount of heat to that space. That amount of heat is the heating thermal demand. Now, you could meet that demand by installing an electric heater, by using a ground source heat pump, or simply by burning wood. Regardless of what method you choose to use, the annual heating demand, (i.e., the heating TEDI) will not change.
What’s the TEDI path, and which projects must follow it?
The new Targeted Performance Path (or TEDI path) sets forth heating and cooling TEDI thresholds, obtained through energy modeling, which may not be exceeded. These thresholds vary depending on the building typology and square footage and are, except for residential buildings, really challenging to meet. (To give you a sense, they are more stringent than Passive House requirements.)
The TEDI path applies to the following building typologies, as long as these buildings are new construction, exceed 20,000 sf, and have average ventilation at full occupancy of 0.5 cfm/sf or less:
- New dormitory
- Fire station
- Police station
- Post office
- Town hall
Nevertheless, any building, new or existing, of any size can choose to follow this path if desired.
How to quantify TEDI?
To quantify TEDI we need to compute the total energy that is added to or removed from the building for heating or cooling and ventilation. In practice, this is done using an energy model, as heating and cooling TEDIs are two of the typical outputs of most energy modeling programs. Moreover, the energy model can be used to explore how different design changes impact the TEDIs of the building.
If you like equations, here’s what the two TEDIs look like:
For the new MA Stretch Code compliance purposes, building conditioned area also includes “indirectly unconditioned spaces,” such as storage closets and mechanical rooms not insulated from other conditioned areas. These spaces need to be accounted for when computing the TEDI.
Energy Demand (TEDI) is not Energy Use (EUI)
TEDI is measured in units of kBtu/sf-yr, the same units of another well-known building metric, the Energy Use Intensity, EUI. Although they share the same units, TEDI and EUI are different metrics. Heating/cooling EUI measures the total annual energy that is needed to power the plant conditioning a building, while TEDI measures the demand that this plant is meeting. EUI directly depends on the efficiency of the systems serving the building, while TEDI does not change whether this demand is satisfied by a highly efficient system or not.
In other words, if you have a project that has a TEDI target, improving the efficiency of the boiler and/or chiller serving it, will not make a difference in the TEDI. Of course, this does not mean that the efficiency of the system does not matter. If the project is additionally targeting low carbon emissions and a certain certification, a highly efficient HVAC system is key to achieve these goals.
How does your design impact TEDI?
If the efficiency of the HVAC system does not impact the TEDIs of a building, then what building design elements do? Big picture: you want to minimize the building’s heat losses in the winter and heat gains in the summer, just as you would do in a Passive House building. Set up yourself for success by optimizing these elements:
- Building massing: prioritize north-south orientations and minimize façade area (minimize façade innies and outies)
- Window-to-wall ratio: aim for 30% or less (strategically placed to maximize daylight and views)
- Envelope R-values: meeting minimum code requirements (after thermal derating) should suffice. Beware: this is not as easy as it might seem at first
- Window U-value: aim for assembly U-0.25 or less
- Solar heat gain coefficient (SHGC): lower SHGCs will increase the heating TEDI and decrease the cooling TEDI – tweak accordingly
- Internal thermal mass: any will help
- Infiltration rates: aim for 0.25 cfm/sf at 75 Pa or less
- Exterior shading: reduce summer solar gains, while harnessing them in winter
- Ventilation rates: consider minimizing unnecessary conditioning of outdoor air by using demand control ventilation
- Ventilation heat recovery effectiveness: aim for 75% or higher
- HVAC economizers: optimize the use of air-side and/or water-side economizers through controls
The following elements will not impact your MA Stretch Code TEDI. Some, however, will be critical to meet other energy and carbon goals.
- Heating and cooling setpoints: these are predefined in the Stretch Code
- Internal gains and schedules: also predefined in the Stretch Code
- Exterior lighting and equipment: this also includes equipment that is used to provide heating and/or cooling, such as patio heaters
- HVAC system, fuel source(s) and efficiencies
- Domestic hot water and pool heating systems
Engage your modeler as early as possible
In our experience, progressing the design without understanding what the project’s TEDI is from SD, has led to substantial late design changes. This is because the MA Stretch Code TEDI limits are rather stringent and, unlike previous codes, cannot be met by just selecting a highly efficient HVAC system or complying with the prescriptive requirements of the code. Instead, the limits are directly tied to the overall design of the building, its massing and WWR, its façade thermal performance, and require careful consideration of many design elements. We recommend that the design team engage a TEDI modeler early in the design process to establish performance targets and make design decisions that will live with the project throughout its life.
Keep in mind that, because of its assumptions (e.g. prescribed occupancy and schedules) the energy use predicted by a TEDI model will not be representative of the predicted energy use for your building. This means that in order to quantify EUI, perform a life cycle cost analysis or estimate LEED points, your consultant will need to create a separate energy model.
If you have any more questions or need environmental design consulting, do not hesitate to reach out to AIRLIT studio.