When an architect across the street from one of our projects called us, she said “I’ve been noticing the attention to detail and how clean the job site is. I might need your help with something.”
She’d designed a custom accessory dwelling unit (ADU) —her first time using a new truss system—and had hired another builder to do it. But as the project neared the final stage of drywall, the results of a blower door test had a worse than expected result: 5.8 ACH, nearly double Boulder’s already generous maximum of 3 ACH.
That’s when she called us.
Her original goal was clear: build the most air tight structure possible. With the exterior already finished, we had to pivot the typical air-sealing strategy from outside-in to inside-out. Using an interior application of Intello membrane and careful seam taping (even behind electrical boxes) we dropped the airtightness score from 5.8 to 1.1 ACH.
That kind of improvement dramatically changes how a space feels (by 5x in fact). Especially in small-footprint living, every square inch matters. Drafty corners become dead zones. With high performance, the whole space becomes livable, balanced, and comfortable.
So we pulled it apart and did it right.
We rerouted air barriers, air-sealed wire penetrations by hand, and even found that properly sealing a single outlet box could improve airtightness by 0.1 ACH. Multiply that across a house and you start to see how quickly things add up.
While we typically focus on new Passive House builds, this project taught us something important: retrofitting, when done with care, can be transformative. For homeowners who can’t afford new construction—or simply want to honor what they already have—this project stands as a helpful example. Energy performance and comfort aren’t just for brand-new buildings. They’re achievable anywhere, with the right approach.
And for us, this ADU was a blueprint for what thoughtful retrofitting can look like. A reminder that high standards, collaboration, and trust are just as critical as air barriers and tape.
If you’re wondering whether your existing space could be something more (more comfortable, more resilient, more in line with your values) the answer might be yes. You just need the right team.
