Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
From the above questions, and relying on advice from experienced modellers (for which I’m very grateful), I ended up sending 4 related interpretations/questions to ASHRAE back in 2024. Questions were reformulated so each answer could be a simple "YES" or "NO". The entire process, including meetings and email threads, took 12 months. ASHRAE published Interpretation IC 90.1-2022-6 OF in April 2025, see here. I’m very grateful to ASHRAE and active committee members for their time, effort and patience. Good news is that there is some much needed clarity for energy modellers. Bad news is that there is still a fair amount of uncertainty for modellers regarding requirement 5.5.5.5. Here’s my overview - hope this helps.
The initial Q1, Q2 & Q3 above were attempts to better circumscribe the scope of 5.5.5.5. In a nutshell, do linear thermal bridges NOT listed under 5.5.5.1 through 5.5.5.4 systematically fall under the scope of 5.5.5.5.? ASHRAE’s answer to Interpretation #1 is "YES" - all other linear thermal bridging fall under 5.5.5.5.’s umbrella. This includes corners, perimeters around unfenestrated doors, demising wall edges, etc. These are lumped in with point conductances like columns, anchors for signage or fixed shading, etc. There are exceptions, such as individual thermal bridges having material thermal conductivities less than 0.433 W/m·K (e.g. wood, acrylic, fibreglass, rubber). Let's call this exemption A. For non-exempted (conductive) materials like concrete, steel or aluminium, individual point thermal bridges (e.g. columns, anchors) may still be exempted if their cross-sectional area is under the following thresholds:
Let's call this exemption B. If exempted (via A or B), modellers need not consider such benign thermal bridges in their models. To be exempted, e.g.:
Such diameters would not be enough for most structural applications, e.g. ground-floor colonnade, like the Seagram Building on NY’s Park Avenue (left), the N2 building in London (bottom-right) or the Palazzo Ducale (top-right) - fairly common archetype.
Stronger (non-exempted) columns would therefore need to be accounted for using Equation 5.5.5.5 (see above Q5), as confirmed by ASHRAE’s answer to Interpretation #2. Consider the following (compliant) hypothetical building, as an Equation 5.5.5.5 case study:
The above-grade area of the building envelope in Equation 5.5.5.5 would be 9,500 m2, giving a whole-building threshold of 14.3 (i.e. left-side of Equation 5.5.5.5). I consider either side of Equation 5.5.5.5 as indices (no units).
Switching over to the right side of Equation 5.5.5.5. For corners, assuming a continuous 1.5mm thick galvanized steel edge (Z-bar for cladding/insulation), giving a total of 0.0075 m2 per 5m edge. 4x corners per floor, times 3x floors, times 50 W/m.K (k) = 4.5. Also factoring 18x reinforced concrete, ground-floor columns (400mm in diameter), which gives a total area of 4.5 m2. Times 2 W/m.K (concrete + rebar) = 9.0. Adding together corners + columns, the right side of Equation 5.5.5.5 becomes 13.5, which is 5% below the admissible target for the entire building.
So in the previous example, linear and point thermal bridges falling under 5.5.5.5 are considered, as a whole, compliant. Yet what is an energy modeller, applying ECB, supposed to do? Let's start with the Proposed Design (PD) model. Section 5 (Building Envelope) in Table 12.5.1 states:
a. All components of the building envelope in the proposed design shall be modelled as shown on architectural drawings or as built for existing building envelopes [...].
Exception: The following building elements are permitted to differ from architectural drawings. 1. Each linear thermal bridge and point thermal bridge as identified in Section 5.5.5 shall be modelled [through] (b) an adjustment of the clear-field U-factor in accordance with Section A10.2.
ECB doesn't state you can ignore compliant linear and point thermal bridges, and only model unmitigated linear and point thermal bridges. One needs to model all of them, unless exempted (see previous discussion on exemptions A & B). And what about the Budget Building Design (BBD) model? The corresponding requirement states:
Where linear thermal bridges and point thermal bridges, as identified in Sections 5.5.5.1 through 5.5.5.5, are included in the proposed design, they shall be modelled by adjusting the U-factor of the parent assembly in accordance with the default values in Section A10. If the proposed design does not have linear thermal bridges and point thermal bridges, as identified in Sections 5.5.5.1 through 5.5.5.5, they shall not be modelled in the budget building design.
So whatever one ends up modelling in the PD, there has to be a corresponding (compliant) object in the BBD. Fair enough. Yet it is unclear how an energy modeller converts the output of Equation 5.5.5.5 (here, 13.5) into something useful for energy modelling. One can't easily convert this number to a (more practical) PSI or CHI factor. And the comments provided in Interpretation IC 90.1-2022-6 OF aren't that helpful either. Here's what I suggest:
For the PD model, I'd specify in TBD a corner PSI factor (e.g. 0.200 W/m.K), and a column CHI factor (e.g. 0.48 W/K per column) - see Customizing. As thermal bridges falling under 5.5.5.5 in this case are compliant, I'd simply copy/paste the same inputs in the TBD file for the BBD model - no energy penalty or credit between PD vs BBD. It may even be legitimate to increase the corner PSI and column CHI factors in the BBD by 5%, as Equation 5.5.5.5 results are below the threshold by 5%. I think this is something one can reasonably argue with an AHJ.
What happens if this hypothetical building has 21 columns, instead of 18? Based on Equation 5.5.5.5, one would then exceed the admissible threshold by around 5%. In such a case, I'd instead lower corner PSI and column CHI factors in the BBD by 5%. This would generate a 5% penalty for the PD. Again, I think this is practical for energy modellers. And fair, in the spirit of the Comment #3:
In Section 12 the equation is used to establish an “amount of thermal bridging” that must be considered in the proposed design (if the limit of Eq. 5.5.5.5 is exceeded).
For the moment, I have no better solution to suggest. I’m certainly open to questions, comments & suggestions from modellers. I can only hope some future revision of the 90.1 User Manual provides further clarification (see Comment #4).
FYI, I decided at the time to set aside Q6 (on F-factors vs more explicit solutions like KIVA), and Q4 on related PSI factors. I may repost a more concise, more focused UMH question on this.
Thank you for your time.
