Welcome to week 3 of PBA's blog series on the coming updates to the Michigan Energy Code! We're covering essential updates in the new Michigan Commercial Energy Code (based on ASHRAE 90.1-2019) before the adoption date on April 22, 2025. Click here if you've missed our previous posts!
Section 5 of the Energy Code deals with Envelopes - the architectural assemblies that keep the conditioned air inside and keep the rest of the world outside. Envelopes are far more than just considering the amount of insulation installed - they control winter drafts, they manage the migration of humidity in-and-out of a building, and need to look good while doing it!
In his article "The Perfect Wall" (link), legendary building scientist Dr. Joseph Lstiburek describes the four major roles of building envelopes:
Two of these functions - thermal energy management and control of air leakage - are affected by requirements of ASHRAE 90.1. In this week's post, we discuss updates to the thermal energy management requirements for 90.1-2019. In next week's post, we'll discuss the updates to air leakage management, which are among the most significant changes in this new Energy Code.
Thermal energy management through an envelope is all about picking materials with good insulating properties, and making sure there's adequate thickness of those materials around the entire building. The performance of insulation materials is commonly measured in R-value 
, where a higher R-value is better. Confusingly, the performance of envelope assemblies is commonly measured in U-factor 
, which is the inverse of the R-value. Lower U-factor is better.
Having more insulation isn't the only factor though. Remember the line about having insulation around the entire building? Great insulation can easily be hamstrung by thermal bridges - places where the insulating material is thinner because of building structure. The classic example is wall studs - fill a steel-framed wall full of R-13 insulation, and the resulting wall assembly is only R-8 due to the metal studs providing an express route for heat to leak through. To address this, Standard 90.1 generally requires a degree of continuous insulation - a layer of insulation on the outside of the building, outside of the walls, regardless of the thickness of insulation in the wall. By putting the insulation outside of the structure, thermal bridging is adequately addressed.
ASHRAE 90.1 lists maximum requirements for U-factors. As long as you provide as low or lower of a U-factor as required (aka, as high or higher R-value), you comply with the Energy Code. ASHRAE determines the U-factors for the Code by weighing performance and cost effectiveness - adding more insulation will always reduce building energy performance, but at some point it becomes an issue of diminishing returns.
And now here's the part you're waiting for! What are those requirements? Within southeast Michigan (Climate Zone 5A)…
But wait - the maximum window-to-wall ratio remains 40%? I feel like I've seen buildings with more glass than that out in the world. Ah, yes. There are exceptions - I say "requirements," but you can get out of those requirements if you run an energy model to show that you're just as good. We'll talk more about that later in the blog series.
All this good insulation is great for energy conservation, but a drafty building can cause a building to lose more energy than poor insulation. Next week, we'll talk about air leakage rules in 90.1-2019, which are significantly more stringent than before.
