The Passive House Build, Part Two: Air-Sealed Mudsill Assembly
You only have one chance to get this critical detail perfect.
Part Two of the Passive House Build is devoted to “Air-Sealed Mudsill Assembly.” Architect, Steve Baczek writes that the mudsill is one of the most critical components of a successful Passive House. Airtightness is essential to a Passive House. Minor irregularities of the stemwall or shrinkage of the sill plate once the water and preservative treatment dry alone will make a mudsill prone to gaps, which would pose a major problem to the Passive House design. The article gives a detailed process of assembling an airtight, mudsill sandwich comprised of poly, termite shield, EPDM, Pressure-treated 2×6, and, interestingly enough, acoustical sealant.
The mudsill is one of the most critical components of a successful Passive House. It involves a connection between dissimilar materials, and making such a connection airtight is a challenge. Even the best stem wall will have some imperfections. Also, the mudsill typically will be wet from its preservative treatment and from the lumberyard, and it will shrink as it dries. This means that there likely will be gaps between the wood and the concrete. Traditionally, this part of the building is sealed with a foam gasket. In a Passive House, however, even a minor gap is a major problem, so our assembly is a bit more complex. This part of the build typically is done on the carpenters’ first day, so it’s often their first hands-on involvement with the extreme airtightness requirement of this kind of house. In most cases, the carpenters never will have built even close to a Passive-House-level of airtightness, so establishing a good mental standard for the job starts here.
The mudsill is a one-shot deal. This project relies on several blower-door tests to evaluate air leakage, but the first test won’t happen until the walls and roof are in place and sheathed. By…