Passive House: Construction Details

Our previous post introduced a Passive House project that we designed and carried out in Maine. This post will describe the building materials and methods used.
We didn't need or want a basement, so one of the first decisions was to sit the house on a slab on grade. Our local builder, Jesper Kruse, had experience with an approach called shallow frost-protected foundations, so he ordered a system of custom molded foam blocks that fit together like Legos. The foam provides an insulation value of r-50 beneath the slab, which is 5 or 10 times more insulation than normally installed beneath slabs. Without a well insulated slab, the house would not perform well, nor would it meet Passive House standards. And since the concrete also serves as the finished floor material for the first floor, it was important to make it comfortable under foot. A heavy duty vapor barrier was placed on the foam prior to pouring the concrete.
Wall and roof construction:
There are a number of providers of factory-built wall and roof panels that meet the Passive House standard and we explored some of these options. In the end, though, we decided to site build the house, using a very robust system developed in recent years that involves two cavities in the walls and roof. The wall construction started conventionally with 2x6 studs faced by Zip exterior sheathing. Had we stopped there, the walls would have had an r-value of about r-20, barely enough to meet building code and nowhere near enough for our purposes. So 11" i-joists were bolted vertically to the exterior of the sheathing, as seen in the photos below, resulting in a wall thickness of almost 18" and allowing an r-value of close to r-60.

The extra-thick walls allowed us to meet our insulation goal, but what about airtightness? To address that, the crew installed two membranes imported from Europe. The exterior membrane (applied to the outside face of the i-joists) is a water and air resistant barrier called Solitex Mento. The interior product is a smart vapor barrier called Intello. The seams of both products were carefully sealed with high-tech and extremely sticky tapes, also imported. In the course of the work, the membranes were carefully sealed to similar products installed beneath the slab and in the roof structure, so that the final result in a continuous air barrier, from slab through the roof. These products, by the way, are vapor open which allows any moisture in the wall cavity to diffuse instead of getting trapped and creating problems. The Zip sheathing - taped at the seams - provides another layer of defense against air infiltration.

Windows and Doors:
To achieve Passive House performance we ordered triple glazed doors and windows made in Europe. The triple glazing provides a high r-value compared to most domestic windows and the construction is carefully detailed to minimize thermal bridging and air leakage.
To realize our goal of having a very low maintenance home, we ordered the windows and doors with aluminum cladding. The roof is standing seam metal, which will need no maintenance.
The siding - rough-sawn western red cedar shiplap boards that - should only require a coat of stain every 8-10 years or so. One of the construction details common to high performance homes is a rain screen, which is basically an airspace created behind the siding material that allows for the siding to dry in both directions, and allows any water that gets behind the siding to drain away without causing harm to the building. Our rain screen was formed by installing wood strapping to the exterior, both vertically and horizontally, against the Solitex product. This airspace should help the cedar siding and trim to last for many years.

Mechanical systems 

The house is heated and cooled with a Mitsubishi heat pump system. One outside condenser connects to four indoor wall mounted units. We’ve tied this system to the internet so that we can turn the heating and cooling on or off, up or down, remotely with an iPhone app. Multi-zoned heat pump systems can be finicky but so far this system has served us well. Due to the low heating load, we have the system programmed to only run for a few hours in the morning and, if it is particularly cold outside, for a few more hours in the evening. 

The indoor air is constantly being refreshed with outside air via a Zendher Heat Recovery Ventilator (HRV), pictured below. Stale air is pulled from the kitchen, laundry room and bathrooms where it enters a heat exchanger in the HRV, giving up most of its heat to the incoming fresh air. The fresh air is distributed to the bedrooms and living rooms. The HRV also filters the air. Occupants of passive houses frequently comment on the comfortable indoor air and our house is no exception.