Energy modeling on small projects / Passive design approach vs calculating EUI

Simona Fischer

Director of Sustainable Practice and Associate

MSR Design

LEEDuser Basic Member

0 thumbs up

March 1, 2024

I'm working with the AIA Minneapolis chapter on guidance for small firms related to how and when to energy model. A lot of small firms would like to sign the AIA 2030 Commitment (we had 100 in a room in 2022 in a session led by Nadav and Gwen Fuertes). But we have learned that a) they don't have a good way to energy model small commercial to residential scale projects and report EUI, and b) if they were to invest precious time in learning how to model small-building EUI, would the EUI metric even be meaningful in driving their design? Probably not. From Passive House training we learn that getting the envelope U-values and airtightness right and sizing equipment appropriately are the three most important things to do in impacting small buildings. But if you want to report, you need EUI for the DDx. Do you have thoughts on a good energy analysis workflow for small projects, to not add a bunch of extra modeling work?

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Kim Shinn

Executive Principal and Senior Wizard

TLC Engineering Solutions

LEEDuser Premium Member

62 thumbs up

Fri, 03/01/2024 - 20:10

Small (less than 10,000 sf) buildings' energy use is usually dominated by envelope performance - they just innately have high skin to volume ratios, which falls in line with PH. So EUI might not be the best information to use to drive down energy use. The DDx allows you to use an energy code to estimate % reduction against baseline. Area-weighted U-value (UA) could be a useful metric, as energy use in small buildings is more or less directly proportional to the envelope's U-value. So, let's do an example. Two-story office building in St Paul, MN - 80 ft long, 30 ft wide, 20 ft tall, wood frame construction. 40% WWRmax (1600 sf of fenestration), 2400 sf opaque wall area, 2400 sf roof area. Energy Code is 90.1-2019, which makes window Umax = 0.42, wall Umax = 0.027, roof Umax = 0.021. The Code max UA is (1600 x 0.42) + (2400 x 0.027) + (2400 x 0.021) = 787. DDx tells us that the CBECS baseline EUI is 70, and the 90.1-2019 code equivalence is 52% reduction on that baseline, so the code EUI is 34. The design uses a 20% WWR, window U = 0.4, wall U = 0.02, and roof U = 0.018. So the design UA is (800 x 0.4) + (3200 x 0.02) + (2400 x 0.018) = 427. The ratio of 427/787 = 0.54. Multiplty that by the Code EUI and your design EUI is probably in the vicinity of 18. You could also apply a similar improvement to infiltration. Assume that inflitration is about 20% of the energy loss (so, about 7 of the 34 units of the code EUI). The code allowable infiltration is 0.40 cfm/sf of envelope area. If you design for 0.36 cfm/sf (a 10% reduction) you can apply that to the factored design EUI and report 17. There are a lot of cases where you might have buildings that are dominated by internal loads or ventilation, for instance, where this wouldn't be a very good approximation, but I'm willing to let that slide. Let's not let perfect be the enemy of the good.