Karuna House Wall Assembly
Our goal with Karuna House's advanced envelope was to create an airtight, watertight, vapor permeable, and super-insulated wall assembly. The goal? Control energy flow through the building while ensuring durability for the life of the structure. To do this, the team designed a wall assembly that carefully manages heat, air and moisture.
Starting from the interior edge of the assembly and working our way out:
Interior walls of the green building are finished with lime plaster, 100% natural and VOC-free, applied on 5/8" drywall. Next comes the stud wall, built entirely from FSC certified wood, with high-density cellulose insulation (R-21) blown into the cavities between studs. The cellulose insulation consists of recycled newspaper and naturally buffers moisture, contributing to the wall assembly's durability. We used over 12 tons of it at Karuna House, easily dwarfing any other insulation used on the project, by weight.
Immediately outside the cellulose insulation layer comes the air barrier: a 1/2" layer of plywood sheathing covered with a continuous, vapor permeable liquid applied membrane (Prosoco CAT5 air and water barrier). The air barrier prevents air from flowing through the wall and ensures that the cellulose cavity insulation performs at its highest level, much like a Gore-Tex jacket enables a sweater underneath to warm your body, shielded from air penetration and the moisture it carries. The air barrier also keeps out wind-driven rain, protects sheathing and framing from moisture, and, because it is vapor permeable, allows the wall assembly to dry out from the inside in the unlikely event that moisture intrudes to the interior portion of the wall assembly.
The air barrier is wrapped by the home's winter overcoat, a 6"-thick exterior layer of foil-faced polyiso foam (R-40) that brings the insulative value of the overall wall assembly to nearly R-60. The three layers of 2-inch foam nest into a superstructure of Z-joists that stagger each layer's seams. Vapor permeable tape seals exterior seams. This continuous overcoat is key to the wall assembly's performance and durability. In addition to allowing very little heat loss, it also protects the structural air barrier by keeping it warm and dry.
Finally, the building is wrapped with a rain screen system built of FSC-certified cedar siding held one inch off the polyiso foam by FSC 1x4 battens. The airflow that this rain screen cavity facilitates across the face of the wall assembly provides constant drying and drainage, ensuring long term performance and durability.
Moisture management is the watchword when creating a high performance building envelope like the one at Karuna House. By controlling heat and air you control moisture (hence, the Heat Air Moisture, or HAM, focus of building performance experts), so we designed every layer in the wall assembly at Karuna to manage these three variables. But we also created an assembly with excellent drying potential so that if something unforeseen happens and moisture somehow enters the system, the wall assembly will be able to handle it. This ensures building envelope durability, the most fundamental definition of building sustainability.
Alternative Building Systems: Prosoco liquid applied membrane, including CAT5, polyiso insulation, high density cellulose insulation.
Building Envelope: Liquid applied Prosoco CAT5 air and water barrier.
Caulks and Sealants: Prosoco R-Guard. Tremco expanding foam tape.
Exterior Finishes: Thermo cromex and locally-sourced FSC cedar coated with Benjamin Moore arbor coat.
Insulation: Big Sky EPS geofoam, big Sky EPS on fdn walls, Hunter foil-faced polyiso exterior insulation, high density cellulose in walls and roof, paper-faced polyiso
Roof: Prosoco CAT5 air barrier, standard ppo roof membrane.
Windows/Skylights/Patio Doors: Optiwin triple-paned high performance windows and high performance doors.