Passive House design methods typically rest on well-established building science principles. A building assembly will ordinarily include an air pressure control layer, a vapor diffusion control layer, and an insulated plane. Purists try to define each of those boundary materials individually—for example, designers will prefer not to expect insulation to resist airflow or vapor transmission, but only to do its job of limiting the flow of heat. Ideally, an assembly should be vapor open to both the interior and the exterior of the building, and any vapor barrier should be located at a central plane where the material will stay warm enough that vapor will not condense. Here's a look at a few of the details being developed for Passive House construction by New York-based 475 High Performance Building Supply. These details rely on advanced materials imported from Europe by 475.
Drawing courtesy 475 High Performance Building MaterialsThis perspective drawing illustrates a cellulose insulation retrofit of the triple-wythe brick face typical in turn-of-the-century brick row houses in many cities along the U.S. Atlantic seaboard, including Boston, New York, Baltimore, and Washington, D.C. Retrofitting these structures to Passive House levels can be accomplished by insulating with cellulose against the brick. An intelligent membrane between the insulation and the indoor space allows moisture to escape the assembly to the inside during warm summer months, but prevents the passage of interior moisture toward the cold brick during winter months. Not shown is a key element: a mortar parge and fluid-applied sealant membrane on the interior face of the brick that controls air leakage and bulk water intrusion.
Drawing courtesy 475 High Performance Building MaterialsIn this section of the window opening shown in the previous perspective-view slide, the air pressure control layer—also a vapor control element in this assembly—is indicated by the red line. For this assembly, the smart vapor barrier membrane on the inside face of the stud wall has to be integrated into the window using adhesive tapes. Insulation levels have to be calculated to keep the interior face of the brick warm enough during winter to avoid vapor condensation at the interior face of the brick—a complex situation that Passive House consultants typically assess on a climate-specific basis using the software tool WUFI.
Drawing courtesy 475 High Performance Building MaterialsThis drawing illustrates a complex situation: the intersection among a super-insulated wall system that uses a wood I-joist buildout, an insulated floor system, and the uninsulated concrete foundation (in this case, a crawlspace). The air-pressure control layer in this example is structural sheathing on the floor and stud wall, sealed at the joints with tape. On the outboard face of the insulated wall, a vapor-open air barrier and drainage plane membrane contains and protects the insulation. On the underside of the crawlspace, membrane choice depends on the humidity conditions: 475 recommends a vapor-open fabric if the crawlspace is dry, but a vapor barrier membrane if the crawlspace tends to experience high humidity. Tapes and caulks are applied as needed to maintain continuity of the air control, vapor control, and bulk water management layers.
Drawing courtesy 475 High Performance Building MaterialsThis drawing illustrates three steps in the construction of the intersection where a second-floor joist system rests on a first-story wall, and then supports the second-story wall. The inside faces of the stud walls receive a smart vapor barrier membrane, while the joist assembly end is wrapped with a tough vapor-open air barrier material. Rigid fibrous insulation (such as rock wool) on the outside face of the wall is protected with a vapor-open drainage plane fabric.
Drawing courtesy 475 High Performance Building MaterialsThis drawing shows the plan view of a wall corner in a deep energy retrofit. To build this, siding is stripped off the wall, a vapor-control and air barrier membrane is applied, followed by a build-out of wood I-joists. A vapor-open membrane is applied over the wood I-joists and the large cavities between the joists are blown with dense-pack cellulose. This assembly is expensive because of the hand labor involved, but it is a simple and effective way to turn an old stud wall into an advanced, robust air-tight super-insulated assembly with good moisture control. One advantage: The airtight membrane shown in red, applied over the existing sheathing, can protect interior assemblies and finishes from the weather during construction of the outboard I-joist assembly.
Drawing courtesy 475 High Performance Building MaterialsThis drawing illustrates the intersection of a double-stud wall supporting an insulated cathedral roof. In this example, a deeper insulated cavity in the rafter assembly is achieved by dropping the ceiling using wood gussets to support ceiling nailers. At the upper surface of the roof, a vapor-open drainage plane material, topped by 2x nailers applied flatwise, allows the insulated roof cavities to breathe into an air space under the roof sheathing. On the underside of the roof system, as well as on the inside face of the walls, a smart vapor barrier membrane promotes summertime drying of the assembly to the inside, while limiting wintertime wetting of the wall and roof sheathing by minimizing vapor diffusion into the wall and roof of interior moisture during winter.
