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The Super Shell

Passive resistance works.

By Barbara Horwitz-Bennett

ALTHOUGH GERMANY IS LIGHT-YEARS AHEAD IN BUILDING PASSIVE houses where the home’s air infiltration rate is practically airtight, the U.S. is finally starting to catch on. While no match for Europe’s some 20,000 passive homes, a couple dozen construction wonders have been built on U.S. shores, with lots more on the boards.

In simplified terms, the passive house standard states that air infiltration cannot exceed 0.6 air changes per hour at 50 pascals, and annual heating and cooling use cannot exceed 4,755 Btu per square foot. By contrast, Energy Star requires less than 5 or 6 air changes per hour, depending on climate zone, and an average home built to current code standards typically consumes more than 10 times as many Btu.

In other words, a passive house can actually operate on an astonishing 90% less energy. Although this sounds pie-in-the-sky, it’s actually not. Granted, a genuine passive house requires an airtight envelope, a significant amount of insulation, high performance fenestration, passive heating and energy-recovery ventilation—but it can be done.

Addressing each of these key points, passive house and green building experts share some best practices for achieving this lofty goal with its associated energy savings.

Insulation Insights
Generally speaking, in order to build the super shell, significant amounts of insulation must be used—sometimes as much as 16” of rigid foam at the foundation. However, the exact thickness is dependent on climate, so energy modeling can be a good way to determine this.
At the same time, placement can be a tricky issue as the direction of moisture dissipation, which creates condensation, also varies based on climate.

“The insulation should be located on the exterior of the structure, or in-line with it, but the approach we found that works the best in the widest range of conditions is to have vapor permeable wall construction both inside and out,” explains Miloš Jovanović, co-founder of Root Design Build in Portland, Oregon.

Starting with the below-grade walls, an insulation level of R-20 is a good rule of thumb, and can easily be achieved with products like rigid foam insulation on the exterior wall. This type of insulation can also be effective in protecting the concrete foundation walls from water and the effects of freeze/thaw.
For above-grade walls, Mark Wahl, co-owner, Cobblestone Homes, Saginaw, Mich., says, “we typically go with 19.2” or 24“ on-center 2’ x 4’ and 2’ x 6’ stud framing combined with closed-cell spray foam cavity insulation and insulated sheathing to provide the necessary thermal breaks. As an example, the whole-wall R-value of our last zero [energy] home was about R-34.”

Dow’s STYROFOAM SIS Brand structural insulated sheathing is actually a neat solution because it offers structural, water resistance and thermal insulation all in one product.

“We were an early adopter of this Dow product and have been very happy with it this far,” reports Raymond Pruban, chief manager, Amaris Custom Homes, Minneapolis. “This product is an exterior sheathing. Because it wraps 100% of the exterior structure, there is no thermal bridging, and it provides a minimum of R-5.5 at the stud locations.”

This is actually key as the studs and framing typically make up about 25% of the wall area, according to Dale Winger, marketing manager, Dow Building Solutions, Saginaw, Mich.
At the same time, even though a number of sheathing products have been tested for strength, Pruban prefers to use metal strapping from the footings to the roof, along with metal lateral bracing for reinforcement.

Another issue is protecting the above-grade insulation from ultraviolet rays and physical abuse. For this, “various materials can be used such as fiberglass reinforced panels, stucco, coatings, pressure-treated plywood, aluminum or vinyl coil stock and pre-coated insulated panels,” says Winger.

Moving on to the floor, the codes require around R-30, while Energy Star mandates R-38.
“In the practical world, what works best is to use a minimum of a 10” joist depth for floor framing and to fill the joist bay completely with insulation,” advises Tom Reid, owner, Green Home Construction, Hood River, Ore.    

For example, Dow’s STYROFOAM’s Tongue and Groove insulation can be used under the basement slab, and if the boards are taped at the seams, then a vapor barrier isn’t required.

When it comes to wall insulation, a number of options are available, such as insulated concrete forms (ICF) and structural insulated panels (SIP)—making other cladding systems thicker with foam by using a double stud configuration or an alternate stud wall.

While Reid is in favor of all these options, he points out, “the double stud is a simple solution and allows for lots of flexibility and maximum potential for insulation.”

“We’ve used SIP panels which have benefits in easy, fast erection as well as in the large scale—which helps in preventing the air infiltration,” says Jovanović. “We have also used high-density blow in cellulose, which does have a lesser R-value than the rigid expanded polystyrene board of the SIP panel, but is more pervious and has less embodied energy.”

Lastly, to combat rising heat, the ceiling and attic require a high level of insulation. Because space can sometimes be an issue, Reid recommends raised-heel trusses, so that the full depth of the attic insulation can extend all the way to the exterior wall, where the truss lands on top of the wall.
Another approach, as was used in the InVision Zero Home, a demonstration net-zero house in Midland, Mich., built by Cobblestone Homes and Dow, is to layer spray polyurethane foam in the wood framing cavity, for both insulation and air sealing, with spray cellulose insulation for a continuous layer.

Air Infiltration Advice
Air infiltration is the leading cause of energy loss in homes, according to the U.S. Department of Energy, accounting for between 25% and 40% of lost energy.

Consequently, carefully detailing the building envelope is of utmost importance. In fact, building science experts who understand the amazingly complex nature of vapor pressure, moisture diffusion and the like, recommend what they call a “belt and suspenders” approach, which means two lines of defense against air leakage.

For example, Amaris Custom Homes makes a point of taping the seams on the exterior wall’s structural insulated sheathing, and gluing all the wood-to-wood connections. Then for the second layer of protection, interior cavities are sprayed with closed-cell foam.

“We also use closed-cell insulation in the attic ceiling to a depth of around 2”, and then use fiberglass from there,” says Pruban. “We make sure the foam wraps from the ceiling deck, up to the wind washing and up to the roof air shoots to seal any side air leakage. We then extend the SIS up to the roof air shoots on the exterior for our wind washing. Once sprayed from the attic, there is one continuous closed-cell foam barrier from ceiling deck to roof air shoots.”

For the walls, Pruban’s team also sprays the interior rim joist and lower level interior walls without any break. Meanwhile, the spaces in between the fenestration and the rough openings are filled with low expanding foam both before and after the wood window and door jambs are installed.

Although Pruban appreciates spray foam’s ease of installation, he does try to use it judiciously and detail as much as possible with locally made products.

“Finally, we use acoustical sealant extensively to seal any and every wood-to-wood connection on the interior connection points. This approach, along with good windows and good exterior caulking has yielded us an air leakage rate of around .70 ach50 on our last home,” says Pruban.

Taking a harder line on spray foam, Jovanović tries to stay away from it altogether due to its negative environmental impact and high price tag. Instead, he places large panels of sheeting in the middle of the wall, away from punctures, and treats the seams with mastic and adhesive tape.

Acknowledging that connecting the different materials at the building’s transitions is a big challenge, Jovanović explains, “the only way to ensure airtightness is to design continuous elements that have easily accessible edges that can be adhered together. This is often in conflict with the insulation and thermal bridge-free requirement, so balance must be achieved. Avoiding clips and protruding elements at this location is crucial, as it is difficult to air seal these conditions.”

Taking a “sealing as they go” approach, Reid’s carpenters have foam guns in their job boxes to spray areas during assembly which may be hard to reach at a later time. Similarly, at every step, joints are sealed with a double-layer sill sealer or flexible sealant.

Another key to Green Home Construction’s approach is multiple pressure testing—once with a bare frame; a second time after air sealing and insulation; and a final test at the end of the job.

As for other products, Reid likes VaproShield’s weather resistive barrier and roof and wall sheathing from Zip System.

Builders can also look to DuPont’s suite of Tyvek weatherization systems which include home wraps, flashing systems, tapes and sealants.

Combating Thermal Bridges
As noted, thermal bridges can significantly compromise insulation value and must be dealt with. One strategy, says Jovanović, is to split the wall into two layers—structural and insulating. “This allows us to have an uninterrupted insulation and minimize thermal loses,” he explains.

Taking a different approach, Reid is a big fan of the double stud wall. “This eliminates most thermal bridging and provides for a much larger cavity to fill with insulation.”

At the same time, this method can be more costly, so a more affordable alternative might be to run 2’ x 2’s horizontally across the interior face of the studs at 24” on center, leaving a smaller cavity.
However, in cases where the studs aren’t taken care of by the cavity insulation, then products like rigid foam insulation or polyisocyanurate rigid insulation can be used to cover the studs to eliminate thermal bridges.

Let the Sun Shine In
While a big part of the passive house equation is to fully seal and insulate the shell, the other key is to maximize free sources of heating and cooling, namely solar heating and natural ventilation.

While a fully passive solar-heated home requires a long and narrow floor plate, this type of architectural look doesn’t always blend into a typical suburban setting, so Pruban’s group tends do as much as they can to maximize solar heat gain while not going that “full nine yards.” Still, this includes a south-facing lot, colored concrete floors for increased thermal mass and a higher U-value for the windows.

Another important component of passive heating is to design shading for the summer months, as passive solar can easily lead to overheating.

“Many strategies can be used for summer shading, but an exterior shade/blind is the most effective,” says Jovanović. “A great way to provide shading to the south-facing windows is to plant a deciduous plant on a trellis, which loses it leaves just in time for the heating season.”

Sharing another best practice, Pruban has found that blocking the cold, north winds from the home’s primary living spaces to be an effective technique. Essentially, the idea is to site spaces where comfort and temperature isn’t as crucial on the north side—such as the laundry room, storage area and closets—so that these rooms can act as a buffer, thereby reducing heating needs in the winter.
Touching briefly on heat/energy recovery ventilation, Cobblestone’s Wahl states, “balanced ventilation systems that incorporate heat recovery are a must in a tight, energy-efficient home” (For more on HRVs, see “Mechanical Ventilation Update,” on the right sidebar).

Adding It All Up
With such a high level of sophistication and detail required to design a successfully operating passive house, builders are encouraged to use other readily available tools, such as energy modeling software to sort through different approaches,

“The energy modeling is essential in determining which strategies are the most effective for each climate, site and building layout,” asserts Jovanović.

Similarly, Wahl relates, “we do a lot of ‘what if’ scenarios involving changing window performance, insulation systems, HVAC efficiencies, lighting and appliance consumption. It’s essential.”  
One good program for building a super shell is the Passive House Planning Package from the Passive House Institute, known for its accurate level of information. In addition, THERM from Lawrence Berkeley National Laboratory calculates thermal bridges, and Oak Ridge National Laboratory’s WUFI software provides a detailed moisture analysis of the enclosure.

Ultimately, such software will provide lots of good ideas, calculations and values— but it takes a committed builder to truly follow through to the level of detail necessary to build a super shell.

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