Chicagoland’s First Passive House

By Matt Baker

While it was popularized—and now thrives—in Germany and Scandinavia, the passive house concept actually started in the 1970’s, at the University of Illinois. The technology stagnated in the US after the energy crisis receded, though it took off in Europe.

But, as more developers and consumers in this country accept sustainable architecture, the search is always on for the next superlative in green building. Currently, a passive house truly is the greenest option for new construction. It will likely be some time before another technology dethrones it.

So when the Lema Family decided that they wanted to build a durable, super-efficient home, it wasn’t long before they decided to build a passive house in suburban River Forest, the first certified passive house in the Chicago area.

Before any passive house is built, before its drawings are finalized, it goes through rigorous energy modeling. The size of the shell, insulation R-values, window solar heat gain, thermal bridge coefficients and many other factors are input into proprietary software until an optimal layout results.

In the northern hemisphere, passive houses rely tremendously on their southern exposures; collect heat on the south side and lose little on the north. The energy modeling allows the architect to finagle the layout for optimum energy management. And as strictly engineered as passive houses are, every detail counts, even something as minute as bundling hot water pipes to scale down heat loss inside the home.

“The strategy is to minimize losses first and then maximize gains,” said Tom Bassett-Dilley, the Oak Park-based architect who designed the Lema’s home. Energy loss is controlled by insulation and airtightness; energy gain is maximized by the proper placement of high performance windows.

Another factor is the compactness of volume. “If you took these 3,600 square feet of treated floor area and spread it out as a ranch, you’d have huge heat loss,” said Bassett-Dilley. There are no hidden nooks and crannies in a passive house; the ratio of treated floor area to thermal envelope is very important. The south windows are no different than the north ones in terms of solar heat gain; they are simply larger and there are more of them.
The most distinctive feature of a passive house is the super insulation. Conventional wisdom says that there are diminishing returns when adding insulation to a build. The genius of the passive house concept, however, is that if you insulate to the point where you don’t need much energy to condition the space, the whole system becomes much more efficient.

The Lema house’s R-value rises from the ground up. An R-40 slab supports R-60 walls and an R-100 roof tops everything off. That’s more than double the recommended R-value for this climate. “You build to code, and it’s pathetic as far as energy goes,” said Bassett-Dilley. “And it’s obsolete. This would be pretty hard to get obsolete, because when you get to this level of insulation, there’s not much to be gained by doing more.”

Eight inches of foam separate the concrete slab from the earth. Atop that, Logix insulated concrete forms (ICFs) form the basic shell of the home. They are the bulk of its insulation too, with 6¼ inches of concrete sandwiched by two, 2¾ thick, foam forms. Appended to the outside are two inches of polyiso insulation and inside the concrete shell, 2×4 stud bays are filled with blown-in fiberglass insulation.

According to Brandon Weiss of Weiss Building and Development, who acted as general contractor on the project, recent studies have shown that blown fiberglass contains the highest level of recycled content of any insulation on the market, even cellulose. The product they used, Jet Stream by Knauf, claims a minimum post-consumer recycled content of 62%. Unlike blown cellulose, the fiberglass incorporates no formaldehyde, phenol or borates. Also, it can be blown in dry so there are no concerns of moisture content inside the wall.

Airtightness is another key to the success of any passive house. A drafty house is a house in need of constant conditioning. The latest Illinois energy conservation code, which will go into effect on January 1, 2013, requires new construction to test at five air changes per hour (ACH) or lower; a certified passive house, by contrast, needs to be at 0.6 ACH. The preliminary blower door test on the Lema home was nearly half that at 0.33 ACH.
The primary reason that this house can test so airtight is the ICF construction. There are no gaps or seams except for around windows and doors. Anywhere there may be air leakage is sealed with tape. Every window, every door, even the drainpipes that go down below the basement slab are taped off, a job that fell to Weiss. “I have an inventory of every hole in the house,” he said.

It’s common with passive houses to create an airtight shell and then fir in walls and ceiling. This not only provides room to run electric, plumbing and ventilation, but it prevents penetration of the air barrier. “We kind of treat it like a balloon,” said Eric Barton of Biltmore Insulated Concrete, the shell contractor on the house.

The roofline—where heat is most likely to escape—was the trickiest part of the project. Standard trusses were tied into the ICF walls, but unlike a traditional home, the bottoms of the trusses were skinned in half-inch plywood. The plywood seams, including the perimeter where they butt up against the ICFs, were all taped off creating an air-tight barrier. A drop ceiling grid was suspended from that, to which drywall could be hung. The cavity between the drywall and plywood also serves as a mechanical space.

While passive houses are often referred to as “furnace-free,” this can be a misnomer. Some buildings are erected in which the living area is never conditioned, but these tend to be in temperate climates with low humidity, like northern Europe.

But in a climate like Chicago’s, some HVAC is required. Two ductless mini-splits are more than sufficient to condition the whole house. The maximum design load for the home is 13,000 btu hours, or four kbtus per square foot. “That’s about a tenth of what a typical built to code house would be,” said Bassett-Dilley. In the summer, the Lemas can expect to use less than a ton of cooling, when a similarly-sized, conventionally-built home would use more like five tons in that time period.

Early designs of the house called for a pellet stove for extra insurance in the winter. The energy modeling was so granular that they even considered towel warmers in the bathroom, not so much as an amenity but as a heat source. With the mini splits in place, these ideas were easily abandoned.
The house is airtight, but it still needs ventilation and the true heart of any modern passive house is the energy recovery ventilation system (ERV). The system in the Lema house has an extremely efficient fan that recaptures about 85% of the outgoing heat.

Air continuously circulates through the home at a leisurely 80 cubic feet per minute, ensuring a healthy and comfortable living space without any heat transfer between the indoors and out. The ERV extracts air from the kitchen, bathrooms and laundry, and delivers fresh air to bedrooms and other living spaces. From a performance standpoint, an ERV will regulate the movement of air coming into the living space, rather than permitting a porous structure to allow air to come and go unregulated.

“It’s advantageous for the homeowner, too,” said Weiss. “There aren’t as many moving parts in their home.” Maintenance on the mechanical system is as simple as periodically changing the filters on the ERV unit.

“Most of everything we’ve used here has been third party certified for indoor air quality,” said Weiss. Indoor air quality is an important consideration in any construction project, but more so in a passive house where ventilation is kept constant but low. For this reason, most of the finishing products were Greenguard Children and Schools certified. “Passive house being so tight,” said Weiss, “if you’re sealing up and using formaldehyde in cabinets, for example, there’s nowhere for it go but stay in the house.”

The walls were clad in a sheetrock product that claims to capture and permanently remove VOCs. The filters are rated MERV-8 so that the homeowners will enjoy a high level of air quality once they move in.

Because passive houses ideally have as few penetrations of the thermal envelope as possible, they do demand different appliances. A standard range hood would be too drafty, so a self-contained, charcoal exhaust hood will manage the induction stovetop. A standard clothes dryer runs on gas and vents to the outside which would also introduce a possible spot for air infiltration. In condenser dryers, by contrast, the hot, damp air passes through a heat exchanger to condense the water out before being reintroduced to the drum.

The house is packed with plenty of other sustainable features as well. Most of the home’s ambient lighting is handled by high-efficiency T-5 fluorescent, packed into light coves directed at the ceiling. Before construction began, the site was home to a semi-demolished brick house. Most of that brick was salvaged and used in gabions surrounding the basement windows. The house is clad in siding manufactured from fast-growth trees. Unlike cement boards, these are lighter, more durable, have less embodied energy and don’t produce silica dust when cutting.

The house is also pre-wired for a five kW solar photovoltaic array, should the homeowner choose to install one. On the garage is a solar thermal installation to pre-heat the home’s domestic hot water. These are all extra steps, of course, and not standard to a passive house. “The theory of passive house isn’t gadgets,” said Barton. “Every passive house doesn’t have solar hot water. It doesn’t have to.”

Super insulating and taking all the other steps necessary to build a passive house does add about 10% to the upfront price. “But there’s not a big cost premium,” Bassett-Dilley said, “if you’re willing to consider the expense of your house in terms of mortgage and operation cost together.” Considering a passive house averages about 75% less to heat and cool than a traditional home, the extra cost disappears on a home with a 30-year mortgage.
In terms of design, passive house is very flexible. The only real limitations are on compactness and orientation. “It’s something that architects, if they’re really going to call themselves green, need to be honest about,” said Bassett-Dilley. “Once builders and architects are trained and consumers find out about it, the only reason you wouldn’t do it is if you’re building a disposable house.”

All images courtesy Tom Bassett-Dilley Architect, Ltd.

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