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.
PASSIVE HOUSE WALL SYSTEMS: AT A GLANCE
Thanks to a nifty BIM design tool from DuPont, designers can map out and compare different cladding, insulation and weather barrier options. By selecting values such as the wall type, framing type, exterior veneer and insulation, the software applies an EnergyPlus application to calculate both spot and total R-value and Perms.
Pictured here is a fiber cement, stucco and brick wall, each offering an R-value of between 31 and 32 to meet passive house design goals. For the fiber cement wall, the EnergyPlus table is included.
www2.dupont.com/Tyvek_Weatherization/en_US/applications/archi-tects/BIM/BIM_Studio.html
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