Story at a glance:
- A Phius-targeted home in McCall, Idaho, relies on dense-pack cellulose for performance, durability, and carbon storage.
- Building scientist Skylar Swinford estimates that 1,000 square feet of double-stud cellulose wall can buffer 80 to 100 more gallons of moisture than a wall insulated with a material that lacks cellulose’s buffering capacity.
- Moisture sensors built into the home’s north wall are now tracking how cellulose and hybrid foam-cellulose assemblies dry through their first mountain winter.
McCall, Idaho, sits at about 5,000 feet in the mountains of central Idaho. The area logs roughly 10,000 heating degree days a year (a measurement designed to quantify the demand for energy needed to heat a building) and, in some winters, more than 200 inches of snow. Summers are short enough that buildings often need heat just so crews can keep working. It’s the kind of place where a building envelope either performs or fails—and where every assumption about moisture gets tested.
Skylar Swinford has spent 15 years working as a passive house and building enclosure consultant and knows those aren’t abstract concerns. They’re the realities that shaped his own passive house in McCall, where the cold can push building assemblies to their limits.
Swinford had access to virtually every insulation material available. As the owner of EnergySeal, an insulation company founded by his parents, he knew his options well. When it came time to insulate his own home, he says there was only one option. “The gold standard is cellulose. I haven’t seen a cavity insulation system that performs as well as cellulose,” he says.
The decision wasn’t only about keeping the house warm. For Swinford, cellulose delivers on four fronts at once: it makes walls more forgiving of moisture, stores carbon, adds thermal mass, and quiets the house.
High-Performance in a Hard Climate
Swinford always planned to pursue Phius certification for his second home, just like his main living quarters in Boise. Phius establishes rigorous standards for energy performance, typically pointing to designs that use 80% less energy for heating and cooling. The goal in McCall was to prove that passive house performance is achievable even in an exceptionally cold climate, without losing sight of architectural design, budget, comfort, or health.
“We’re heavily utilizing Greenfiber cellulose insulation to meet not only our operational energy targets that are dictated by the passive house standard but also to reduce our upfront carbon emissions for the building as well,” Swinford says.
Certification involves more than hitting energy targets. Phius technical staff review every assembly in the building, verifying insulation ratios, vapor control, and each assembly’s ability to dry. Any wall or roof that falls outside established prescriptive paths requires hydrothermal modeling.
In McCall, that modeling supported a wall many designers would hesitate to build in a climate this cold: a double-stud assembly with rainscreen detailing, filled entirely with cellulose. “There may be some wetting in the wintertime, but the drying is far greater than the wetting,” Swinford says. “That’s really the balance. If the wetting is more than the drying, then we’re going to have moisture accumulation and potentially a wall failure.”
Built to Forgive
That balance is where cellulose earns its place. Real buildings are never built perfectly, and Swinford designs with that in mind. “We’re all people, and so we’re not perfect,” he says. “Even though we really strive hard to have perfect detailing, there will be mistakes. The really cool thing about cellulose is that it’s able to buffer moisture.”
The difference is significant. In a double-stud wall with 10 to 12 inches of cellulose, Swinford estimates that every 1,000 square feet of wall may be able to buffer 80 to 100 more gallons of moisture than the same wall built with an insulation that lacks cellulose’s natural buffering capacity.
From Paper to Passive House
Greenfiber is a North American manufacturer of cellulose insulation made primarily from post-consumer recycled paper—a lot of which comes from McCall’s local recycling facility. “A lot of that ends up back in our own insulation, which ends up in the walls of our homes. That’s pretty rare,” Swinford says. “Greenfiber supports our local recycling facility and community.”
Greenfiber cellulose insulation contains up to 85% post-consumer paper fiber, along with additives for fire resistance, according to Laura Woodford, vice president of marketing at Greenfiber. Rather than relying primarily on newly extracted or fossil-based raw materials, the product makes use of a resource that has already served another purpose. That circular approach helps address the environmental footprint of building materials before a building even opens.
“The cool thing about cellulose is that it’s made of recycled feedstock of paper products, and paper used to be a tree, and the tree became a tree by pulling carbon dioxide from the air,” Swinford says.
Installed in a durable building assembly, that carbon can remain stored in the insulation for decades. “About half of the tree itself is carbon,” he says. “So if you’re building a really durable building, the building should last 50 to 100 years. If you’re using cellulose in your walls and you care about the durability of the assemblies and the moisture detailing, if you put thought and effort into that all along, you end up locking that carbon in your walls for the life of the building and storing it.”
That storage adds up. Swinford’s R-40 to R-50 walls hold roughly 2,000 pounds of cellulose for every 1,000 square feet of wall. By his estimate, each pound of cellulose stores about 1.5 to 2 pounds of carbon dioxide—on the order of 3,000 to 4,000 pounds of CO₂ per 1,000 square feet of wall, held for as long as the building stands.
That’s why Swinford calls durability the unsung hero of a low-carbon building. “If we are having to redo wall assemblies, and we’re using products to try to reduce operational energy and carbon and upfront carbon, and they’re not actually performing as promised, then the carbon savings are only on paper,” he says. “What really matters is that it actually works.”
The House is the Battery
The home’s solar-connected battery system draws plenty of attention with its blinking lights, but Swinford says it isn’t the biggest battery on site. “The biggest battery in this house is actually the house itself,” he says.
An airtight, well-insulated building is far less sensitive to outdoor temperatures, and dense materials make it steadier still. Because the McCall house gains and loses heat so slowly, the mass of all that cellulose works harder, helping the home hold comfortable temperatures through cold nights and overcast days.
The Right Product in the Right Place
Airtightness starts with a continuous air control layer and careful air sealing at every transition. Dense-packed cellulose adds to it: its interlocking fibers fill cavities completely, limiting air movement within the assembly. Its density also makes it effective at absorbing sound. “You feel it the moment you walk into a home that has cellulose in the walls; the sound and acoustical quality right after it’s installed is remarkable,” Swinford says.
Airtight doesn’t mean stuffy. Swinford calls the home’s mechanical ventilation “the lungs of the home.” It constantly brings in fresh outdoor air, filters out particulates, pollen, and allergens, and delivers it to every bedroom. During wildfire season, when outdoor air can be worse than indoor air, an added MERV 16 and carbon filtration stage cleans incoming air before it reaches the energy recovery ventilator. An air-to-water heat pump provides heating, cooling, and domestic hot water without oversized mechanical equipment.
The roof uses a hybrid assembly: tapered rigid polyiso insulation above the deck, then a layer of closed-cell spray foam below it, kept as thin as possible while still providing condensation control and air sealing at the deck. High-density cellulose fills the rest of the cavity. Using each product for what it does best lets the team balance performance, moisture management, and material impacts.
Swinford says the cellulose in the roof offset the upfront carbon emissions of the foam. The cellulose also dampens sounds from rain, snow, hail, and other overhead noise.
“There are very few insulation materials that are commercially and readily available, especially from a regional manufacturer, that can store and lock carbon into the building,” Swinford says. “At the same time, it’s installed in a very high density, so if there are flaws at all in the building enclosure—we’re humans and we can’t do things perfect—it gives more resilience to the building assembly because it’s more airtight on its own because of its tight interlocking fibers, and because of the way it’s dense-packed in. It’s also able to buffer moisture if there are any flaws in the building enclosure; it makes the building more resilient if there are any small defects. It’s far more forgiving.”
Lessons for Every Build
Not every project has a passive house consultant, and Swinford is quick to point out that most homes are designed by builders and designers balancing cost at every step. The same building science still applies.
In a code-level home without a continuous air barrier in its sheathing, a layer of closed-cell spray foam on the inside of the exterior sheathing can air seal the wall and, in a cold climate, keep moisture from accumulating in the sheathing. Rather than filling the whole cavity with foam, builders can fill the rest with cellulose. “You get the condensation control and the moisture management benefits provided by the closed-cell foam, but also the acoustical benefits and the cost benefits and the carbon storage benefits of the cellulose,” Swinford says.
Spray foam can also tie together tricky transitions like attic walls, rim joists, and crawl spaces, so the cellulose can do what it does best. It’s a hybrid approach to the whole enclosure, “using the right product where it needs to be,” Swinford says.
Putting the Walls to the Test
Swinford isn’t relying on models alone. On the home’s north wall, the side that gets the least sun, he built a small test station with moisture sensors embedded in the exterior sheathing. It compares dry dense-pack cellulose, wet-spray cellulose, and hybrid assemblies that pair open- and closed-cell foam with cellulose, tracking how each one dries through its first mountain winter.
“It really helps us validate our assumptions with the hydrothermal modeling,” Swinford says, “and also gives us more confidence as designers to push the limits.
