Two small buildings, a ton of ice in each, two or three weeks in the summer sun. One box is built to the local energy code, the other to the Passive House Standard. At the end we weigh the ice that’s left. The Ice Box Challenge makes visible what you feel in a Passive House building: steady comfort on very little energy.
On this page: How it works · The boxes · Results · Thank you · Host the next one
How it works
The two boxes go on show in a busy public place, usually for two weeks or more in the hottest part of the year. Each is loaded with about 2,000 lbs (900 kg) of block ice and closed. Scales under the ice track what’s left, and at the reveal the lids come off and the ice is weighed in front of a crowd.
Visitors guess how much ice will remain, and the closest guesses win prizes. Chapters and partners hold talks, socials, and tours around the boxes, and local officials often speak at the start and the reveal.
The idea began in Brussels in 2007 and has since run in cities including Vienna, Vancouver, New York, and Glasgow. The International Passive House Association (iPHA) keeps the worldwide record at iceboxchallenge.org.
The boxes
PHN’s two boxes were designed and built in Colorado in 2023, for climate zone 5. They have since traveled to Minnesota and Ohio. What differs between them is the enclosure:
| Code box | Passive House box | |
|---|---|---|
| Floor | R-30 (RSI 5.3) fiberglass batts | R-51 (RSI 9.0) dense-packed cellulose in 14 in (356 mm) I-joists |
| Walls | 2×6 studs with R-20 (RSI 3.5) fiberglass batts, plus R-5 (RSI 0.9) continuous insulation | R-52 (RSI 9.2) dense-packed cellulose in 14 in (356 mm) I-joists |
| Roof | R-60 (RSI 10.6) fiberglass batts | R-60 (RSI 10.6) dense-packed cellulose in 16 in (406 mm) I-joists |
| Windows | Amsco, U-0.26 (U 1.48 W/m²K), SHGC 0.36 | Alpen quad-pane, U-0.11 (U 0.62 W/m²K), SHGC 0.26 |
| Airtightness (design) | 3.0 ACH50 | 0.6 ACH50 |
Values are the builders’ design values; window values are for the center of the glass, and the airtightness figures are design targets. The code box’s continuous insulation is on the inside, so it can be taken out to match places with an older code.
Results
Ice left at each reveal. In 2024 about 1,500 to 1,600 lbs (680 to 730 kg) reached each box, since some melted while it was loaded in the sun.
| When | Where | Code box | Passive House box |
|---|---|---|---|
| July 1–15, 2023 | Fort Collins, Colorado, at the city’s Building Forward climate event | 714 lbs (324 kg), 34% of 2,093 lbs (949 kg) | 1,336 lbs (606 kg), 65% of 2,047 lbs (929 kg) |
| July 28–August 11, 2023 | Louisville, Colorado, at the Louisville Street Faire | 813 lbs (369 kg), 40% | 1,508.5 lbs (684 kg), 75% |
| September 9–23, 2023 | Denver, at Union Station | 817 lbs (371 kg), 40% of 2,027 lbs (919 kg) | 1,524 lbs (691 kg), 71% of 2,149 lbs (975 kg) |
| August 13–September 2, 2024 | Minnesota State Fair, with Passive House Minnesota | None: melted out a day before the reveal | 660 lbs (299 kg) |
| 17 days, to October 10, 2025 | Cincinnati, at HOMEARAMA, with AIA Cincinnati | None: ran out four days before the reveal | 345 lbs (156 kg) |
Students at Pratt Institute in New York City ran the first student-led Ice Box Challenge in the US in 2023, with boxes they built to New York City code and to Passive House. After a week, 900 lbs (408 kg) remained in the Passive House box and 737 lbs (334 kg) in the code box. Read about it.
Thank you
2023, Colorado: sponsored by Xcel Energy and 475 High Performance Building Supply, with the support of B.Public Prefab, Hammerwell, Geometry & Materials, Hord Coplan Macht, Alpen High Performance Products, Boss Architecture, Davis Partnership Architects, Greg D. Fisher Architect, BuildStrong Academy, and Emu Passive, and run by Passive House Rocky Mountains volunteers.
2024, Minnesota: hosted by Passive House Minnesota and sponsored by Xcel Energy, with the support of the Minnesota Renewable Energy Society and the Minnesota Pollution Control Agency.
2025, Cincinnati: hosted by AIA Cincinnati’s Committee on the Environment and its volunteers, and sponsored by Pepper Construction, Arts Rental, 475 High Performance Building Supply, AIA Ohio Foundation, Trilobite Design, CMTA, Keller Williams Seven Hills, Skanska, Sol design + consulting, Big Ice, Crapsey & Gilles Contractors, Champlin Architecture, MA Design, schaefer, Terracon Consultants, and Harlan Graphic Arts.
Host the next one
Our boxes are looking for their next stop. Chapters and AIA components have hosted them so far; a city, a school, or a museum would make a fine host too. What earlier hosts tell us comes down to a few things:
- Plan 12 to 18 months ahead. Good venues book early, and so do the officials you’ll want at the reveal.
- Go where the crowd already is. A state fair, a home show, or a street fair brings the foot traffic. Choose the event for the audience you want to reach.
- Run it two to three weeks in the hottest part of the year. The longer it runs, the starker the difference.
- Find sponsors for the costs. The host covers shipping the boxes, a forklift or crane for the lids, about two tons of block ice, signs, permits, and insurance.
- Line up volunteers to staff the boxes, log the scales, and run events.
We bring the boxes, our hosting guide and past budgets, help with signs, and publicity through our newsletter and social media. To talk about hosting, write to us at info@passivehousenetwork.org.
New to Passive House? Start with First Steps, or find a chapter near you.
Checked October 2026.
