How to build a Passive House off-grid, and without foam
The Passive House Institute is known to have one of the strictest energy-efficient building standards in the world. What does it take to build your own Passive House, and how can you do it with healthy, high-performance materials? This is the second of a three-part series on the challenge of building a certified Passive House without foam or other harmful materials. The challenges are numerous—from sourcing materials to making it as airtight as possible, and keeping an unseasoned team (and myself) on task. For healthy materials, simple substitutes for traditional products are not only typically easy but also cost effective. Take a look at what it takes to build a Passive House in the Colorado Rockies.
Carefully keeping the surrounding Ponderosa Pines safe, which provide critical shading in the summer, we dug out a foundation stem wall and crawl space, a design that is critical to the foamless flooring insulation system. Not only does this minimize the concrete use but 20 percent of the cement was replaced with fly ash.
To do air sealing right you almost have to find religion and be obsessed with making sure that every place where two building elements come together is properly sealed, well before they’re covered. At the foundation, we applied a plywood “seat” for the I-joist to sit on and a similar detail happened at the wall-to-ceiling connection. Because the walls will be very well insulated, we used vapor open materials for air tightness so moisture does not build up in the wall over time. The air barrier is closer to the living space so water will not condense on it if it gets really cold outside. The ceiling and North wall were wrapped with Intello—a “smart” membrane that changes its vapor profile depending on the relative humidity.
Why is air tightness such a big deal? Passive House requires a leakage test of .60 ACH at 50 Pascals or less because leakage is a fundamental way a building loses energy, and allows mold-making, rot-inducing moisture into a wall. The building’s airtight layer is done so we can test it before we install windows, insulation, and all the other stuff that can cover a potential problem. We hit a respectable .45 ACH at 50 Pascals, or roughly the equivalent of 8 square inches of total opening in the entire envelope.
Next, it was time to make the insulation layer. The primary insulation material is Applegate cellulose, sandwiched with Roxul mineral wool batt on the inside and Drainboard on the exterior. We started with Larsen Trusses—basically a ladder like frame made from 2×3 supported by plywood scraps and wrapped in weed fabric to hold the insulation. The Larson Trusses were screwed to the exterior wall to make large bays, and a 2.3/8″ layer of drainboard was then attached to that. After not finding a competent contractor to insulate the bays with cellulose, I purchased a machine and got dirty, learning the ins and outs of properly filling a 24″ wall cavity so the insulation won’t settle over time. Overall, we installed 1100 25-lb bags of the stuff, which is just about an entire semi-load. The best part is the insulation is all recycled from nearby Denver and produced only 100 miles south. The mineral wool board, on the other hand, had to be specially made and shipped from a factory in Canada.
The Intus windows come from Lithuania, in their own container. While less than ideal, the cost and performance are untouched by any American window manufacturer. They swing inwards and can be placed so the exterior frame can be over-insulated to reduce heat loss through the frame. The PVC windows also are the largest compromise by far from my unhealthy materials list because it is highly toxic to manufacture and very hard to recycle.
Another vital Passive House technology is the Heat Recovery Ventilator. This technology uses two fans: one to extract bad air, and one to provide fresh air. A heat exchanger keeps the energy in the building, and if hooked up to an earth tube the house can be naturally cooled in the summer. The unit I selected was the first in the US from a Czech Republic manufacture called Air Pohoda. It uses an a stingy 32 watts in regular mode (important for being off grid) and is over 90 percent effective at reclaiming waste heat.
In the meantime, the energy model seemed to go haywire when new climate data was entered. I find some interesting issues when I went sleuthing for what happened. Finishes such as siding, drywall, finish plumbing, and electrical all have to be installed—the punch list never seems to end. In the last installment of this series, I’ll discover if it’s possible to live in a house in Colorado in wintertime with no working heat, and after doing the Passive House Planning Package software modeling for myself I get a huge surprise. We do the final blower door test with fingers crossed, and I decide to submit to the Passive House Academy for the German certification and forgo the Passive House Institute US certification. Ironically, I was the first to report on the US-German split in 2011, and that news became a very personal journey.
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