Energy codes

Kofi Mensah, the mechanical engineer who worked with the students on the community hub's ventilation, opens the tenth week with an infrared camera. He points it out of the classroom window at the school's 1960s wing across the courtyard, and on the screen the building glows: bright yellow lines at every floor slab where the concrete runs through the wall, orange squares at every window, a hot streak up the side of the stair where warm air leaks out. 'That is money and carbon going into the sky,' he says. 'In January, in this climate, a building like that burns through its heating budget faster than any building I design today. The reason is not that its engineers were careless. It is that when it was built, there was no energy code at all.'

Why buildings have energy codes

Buildings use a large share of the energy consumed in both countries, for heating, cooling, ventilation, hot water, lighting and equipment, and they produce a large share of greenhouse gas emissions, directly by burning fuel in furnaces and boilers and indirectly through the electricity they use. Depending on how the share is counted, buildings account for somewhere between a fifth and two fifths of energy use and emissions. Most of the buildings that will stand in 2050 already exist, but the new ones built now will set the pattern for decades, and a building's envelope, once built, is rarely improved.

Energy codes are newer than fire codes, and their purpose is different. Fire and structural requirements protect people from harm; energy requirements protect a shared resource and the climate. The first energy codes appeared after the oil crises of the 1970s and set modest limits on insulation and window area. They have tightened steadily, edition by edition, and both countries now treat them as one of their main tools for reducing emissions. Mensah tells the students that the energy code is the part of the code most likely to change during their careers, and the part where the gap between the minimum and what good design can achieve is largest.

In the United States the model energy code is the International Energy Conservation Code, which has separate provisions for residential buildings, meaning detached houses, townhouses and Group R buildings of three stories or less, and commercial buildings, meaning everything else. The commercial provisions allow ASHRAE Standard 90.1 as an alternative. States adopt energy codes on their own schedules, often lagging the model codes. Minnesota's commercial energy code, chapter 1323 of the Minnesota Rules, has since January 2024 been based on ASHRAE 90.1-2019; its residential energy code remains based on the 2012 IECC. Because Greenway Homes is an R-2 building of four stories, it is a commercial building for energy purposes and must comply with the commercial code, not the residential one.

Climate zones

Energy requirements depend on the climate. The IECC and ASHRAE 90.1 divide the United States into zones numbered from 0, the hottest, to 8, the coldest, with letters for moisture: A for moist, B for dry and C for marine. Minneapolis, with about 7,400 heating degree-days measured from 65 °F, is in climate zone 6A; the students calculated 7,396 degree-days from monthly temperatures in Design Studio III. The colder the zone, the more insulation the code requires and the more demanding its limits on windows and air leakage.

Three ways to comply

Energy codes offer several routes to compliance, and choosing among them is an important design decision. The simplest is the prescriptive path: the code sets a maximum heat transmission for each kind of envelope element, such as walls, roofs, floors and windows, a maximum window area as a share of the wall, minimum efficiencies for heating and cooling equipment, maximum lighting power and requirements for controls. A building that meets every prescriptive requirement complies. The second is a trade-off path, in which the designer may exceed the limit for one element if another is better than required, so that the envelope as a whole performs at least as well as a prescriptive envelope. The third is the performance path, in which the designer models the energy use of the proposed building with software and compares it with the modeled energy use of a reference building of the same shape built to the prescriptive requirements; the proposed building complies if it uses no more energy, or, in the tiered codes, a set amount less.

Under ASHRAE 90.1-2019, the prescriptive envelope requirements set maximum U-factors, or minimum insulation R-values, for each element by climate zone, measured through the whole assembly. In zone 6 the limits for a nonresidential building are roughly U-0.051 for wood-framed walls, which is R-13 cavity insulation plus R-7.5 continuous insulation, U-0.032 for roofs with insulation above the deck, and about U-0.34 to 0.42 for windows depending on whether they open; the exact values are set in the standard's tables, with separate tables for residential spaces. The prescriptive path also limits the window area to 40% of the gross wall area. Larger window areas require the trade-off or performance path.

Kostyshyn adds a word from the examiner's side of the desk. A prescriptive compliance can be checked in an afternoon. A performance compliance requires a qualified modeler, a report and a review that can take weeks, and the model must match the drawings, the specifications and finally the building as built. She does not discourage performance compliance, which is often the only way to design an ambitious building, but she asks the students to decide which path they are on at schematic design, not at permit.

Checking the Greenway envelope

The students check the north bar of Greenway Homes against the prescriptive path. The north face has a window in every room; the south face, along the glazed corridor, is mostly glass. The script calculates the area-weighted heat transmission of each face and of the whole bar, compares the window share with the prescriptive limit, and estimates the heating energy lost through the envelope in a year.

# Prescriptive check of the Greenway north bar envelope
hdd = 7400                              # heating degree-days (base 65 F)
faces = {  # face: (gross wall area ft2, window area ft2)
    "north": (5170, 1810), "south": (5170, 2840), "east": (1300, 195), "west": (1300, 195),
}
U_wall, U_win, U_roof = 0.04, 0.24, 0.025     # proposed assembly values (Btu/h ft2 F)
roof = 5290
wall_total = sum(w for w, g in faces.values()); glass = sum(g for w, g in faces.values())
ratio = glass / wall_total
print(f"window share of wall: {ratio:.0%} (prescriptive limit 40%)")
for name, (w, g) in faces.items():
    print(f"  {name:<5} {g / w:4.0%} glazed")
UA = U_wall * (wall_total - glass) + U_win * glass + U_roof * roof
print(f"envelope UA {UA:,.0f} Btu/h F")
annual = UA * hdd * 24 / 1e6
print(f"heat lost through the envelope in a year: about {annual:,.0f} million Btu")
# what the prescriptive limits would give with the allowed window share
lim = 0.4
UA_ref = 0.051 * wall_total * (1 - lim) + 0.38 * wall_total * lim + 0.032 * roof
print(f"reference envelope UA at the limit {UA_ref:,.0f} Btu/h F -> proposed is {UA / UA_ref:.0%} of the reference")

window share of wall: 39% (prescriptive limit 40%)
  north  35% glazed
  south  55% glazed
  east   15% glazed
  west   15% glazed
envelope UA 1,658 Btu/h F
heat lost through the envelope in a year: about 294 million Btu
reference envelope UA at the limit 2,532 Btu/h F -> proposed is 65% of the reference

The window share of the whole bar is about 39%, just under the 40% prescriptive limit of ASHRAE 90.1, so the prescriptive path is available for the window area. Because Wren specified walls and roofs much better than the code requires, with triple-glazed windows, the proposed envelope loses only about 65% of the heat a reference envelope at the prescriptive limits would lose. Mensah warns that the glazed corridor will gain heat in summer and that 90.1 also limits solar heat gain through windows; Wren adds operable vents at the top of the corridor and a deep overhang, and the team decides to model the building anyway to show the design's advantage for the utility's incentive program.

Air, thermal bridges and systems

Insulation is only part of envelope performance. Air leaking through gaps in the envelope carries heat out in winter and moisture into the walls, where it can condense and cause rot and mold. Energy codes now require a continuous air barrier around the whole building, with every joint, penetration and transition sealed, and some require the whole building to be tested for air leakage once it is built. Thermal bridges, places where a conductive material such as concrete or steel runs through the insulation, can lose far more heat than their small area suggests; the yellow lines on Mensah's infrared image of the old school were slab edges and balcony connections. Recent codes count thermal bridges in the overall U-value of an assembly, so a wall with good insulation but many steel connections may not comply.

The code also sets requirements for mechanical and electrical systems: minimum efficiencies for furnaces, boilers, heat pumps and chillers; heat recovery from exhaust air in many buildings; insulation of pipes and ducts; controls that turn off lights, fans and heating when spaces are empty; and limits on the lighting power per unit floor area for each kind of space. Many owners now go further than the code by choosing all-electric buildings with heat pumps, which, on a grid powered largely by hydroelectricity, as Minnesota's is increasingly by wind, solar and nuclear power, cut operating emissions to a small fraction of those of a gas-heated building.

Beyond the code

Many clients and cities now ask for performance well beyond the energy code. Passive House, a German standard from the early 1990s, sets strict limits on heating demand, airtightness and total energy, and produces buildings that need very little heat even in a prairie winter. Zero energy certifications and the energy credits of LEED set targets for energy and carbon. Rating systems such as LEED, whose fifth version appeared in 2025, reward energy performance alongside water, materials, health and site. And a growing number of governments require the embodied carbon of materials, the emissions from making and transporting them, to be counted, which the students studied in Building Materials and Assemblies. Mensah's advice is to treat the code's energy requirements as the floor of an energy strategy, not its ceiling.

Exercise 10: energy codes

• Identify the energy code, edition, climate zone and compliance path for your Design Studio IV project.
• Calculate the window share of each facade and of the whole building, and compare it with the prescriptive limit.
• Calculate the overall U-value of your main wall assembly, including the framing, and compare it with the code.
• Identify three thermal bridges in your project and propose a detail that reduces each.

Self-check

• Do I know which energy code applies, and why?
• Have I chosen a compliance path at schematic design?
• Have I counted framing and thermal bridges, not just insulation?