Cold Walls: Testing Insulation Retrofits for Wesconsin Winters

The question

Much of Porter Falls’ housing — whole streets of Founders Triangle and Cedar Bluff — was built between the 1940s and the 1970s with little or no wall insulation. Filling those walls is one of the cheapest ways to cut a heating bill, and the county weatherization program does dozens of houses a year. But adding insulation changes how a wall handles moisture. The outer sheathing gets colder, and in a long Wesconsin winter, warm indoor air that leaks into the wall can condense there.

Dr. Gustav Okafor, assistant professor of construction management, whose work covers building information modeling and mechanical and electrical systems, wanted measured answers for the people actually doing the retrofits: which approaches keep walls dry, and how much does air sealing matter?

The approach

The centerpiece is the test hut, a 16-by-24-foot wood-frame building at the University Farm, built by students in the summer of 2024. Its north and south walls each hold four removable 4-by-8-foot test panels, so eight wall assemblies can be compared side by side through the same winter. Inside, the hut is kept at 68 °F and a humidity typical of a lived-in house.

• Each panel has moisture pins in the sheathing, plus temperature and humidity sensors at three depths, logged every 15 minutes.
• Assemblies include an uninsulated original-style wall, dense-packed cellulose with and without interior air sealing, and cellulose with 1.5 inches of rigid foam added on the outside.
• In the field, students run blower-door tests before and after on houses retrofitted by the county program, with owners’ permission.

Panel construction and load checks run through the Structures & Materials Testing Laboratory; energy modeling is done with the Sustainability Research Institute.

Students on the team

• Jacob Rindfleisch, senior, construction management — student lead; designed the panel frames and swap-out procedure.
• Amira Nasser, junior, architectural technology — sensor installation and the monthly data review.
• Tyler Engebretsen, junior, construction management — blower-door testing in the field.
• Sofia Lund, sophomore, mechanical engineering — energy modeling.
• Ethan Vue, first-year, construction management — logger maintenance and, in January, snow removal from the hut’s door.

Most of the team met the material first in CNST 3700 Mechanical and Electrical Systems and ARCH 2500 Building Construction II. Panels are swapped each October, before the heating season, which means a long Saturday of unscrewing, lifting and resealing with the whole team and a borrowed University Farm forklift. Students also open one panel every spring to look at the sheathing directly, because a moisture reading means more once you have seen and smelled the wood it came from. Those photographs will go into the field guide alongside the data.

What they found so far

After two winters of data:

• North-facing sheathing behind dense-packed cellulose, with no interior air sealing, stayed above 20 percent moisture content — the level at which wood decay becomes a concern — for 47 days in the 2025–26 winter.
• The same assembly with careful interior air sealing exceeded 20 percent for 9 days. With rigid foam on the outside, sheathing never went above 16 percent.
• South-facing panels dried out quickly in every case; the risk is concentrated on north walls and in late winter.
• On 12 field houses, air leakage fell by a median of 31 percent after retrofit. The houses with the biggest improvement were the ones where crews sealed the attic floor and rim joists first.

“Insulating a wall is still a good idea. The data say do the air sealing first, pay attention to the north side, and if the siding is coming off anyway, put foam on the outside.”

— Dr. Gustav Okafor, assistant professor of construction management

What’s next and who’s involved

Winter three tests two lower-cost interior vapor-control products and a wall with mineral-wool board instead of foam. The team is also writing a four-page field guide for the county weatherization crews and local contractors, with photos of what each retrofit should look like before the wall is closed up.

Partners: the Porter County Housing Authority, which runs the weatherization program and arranges field-house access, and Ironwood Construction Group, which donated framing labor and materials for the hut. Funding is a three-year, $274,000 award from the Wesconsin Energy Office. Construction students who want in should talk to Okafor during the term before they want to start; the data review runs year-round.