Posted by: jeb1 | September 23, 2010

Making It Tight, Building It Right

An airtight home can cut a homeowner’s heating and cooling bills, make a house more comfortable, and reduce a family’s carbon footprint—if that buttoned-up envelope is part of an energy-efficient construction strategy that treats the building as a system whose parts depend on each other.

Here are seven ways you can avoid careening off course in a quest to make a home airtight and still run right.

1. Don’t break the barrier. Drywall is a fine air barrier over large, flat surfaces like walls, floors, and a standard ceiling. But when the architecture involves dropped ceilings or soffits, cantilevers, or a bonus room, the drywall juts out of alignment with the top plane of the wall or floor. That leaves a gaping space for hot, dirty attic air to creep into the walls.

Brian Coble, director of high-performance homes for Advanced Energy, a North Carolina nonprofit devoted to energy-efficient home building, calls these unprotected spaces “areas of high complexity” and advises builders to seal them off with sprayed-in foam. The same goes for trusses and knee walls that don’t have top or bottom plates—and therefore don’t boast the blocking components of dimensional lumber. The builder, notes Coble, should ensure that the house has a complete air barrier around the living space.

2. Ventilate right. Kitchen exhaust vents and bathroom fans—even powerful models—can’t cut it when it comes to exchanging bad air for good on a regular basis in a tightly sealed residence. That kind of spot ventilation—vented to the outdoors and never into the attic—is important for releasing cooking odors, steamy humidity, and cigarette smoke from indoor air, but a whole-house mechanical ventilator like a heat- or energy-recovery ventilator is the best device to prevent an airborne buildup of mildew, mold, and chemicals like volatile organic compounds and formaldehyde. Building scientists recommend that homes be ventilated to meet ASHRAE Standard 62.2 through a combination of spot and whole-house devices. 

3. The devil is in the ductwork. A surefire way to know if a house has a leaky duct system, quips Coble, is if it has any duct system. Indeed, the DOE estimates that 20% of a home’s air is lost through the ductwork—even in brand-new buildings. It’s such a waste that utility companies are offering to pay homeowners to have theirs sealed. And duct tape isn’t sturdy enough to keep dust- and allergen-filled air from blowing through seams, cracks, and joints in the duct system, says Coble, who recommends brushing on bucket mastic instead.

4. Insulation isn’t enough. Foam insulation doubles as an air barrier, but fiberglass and cellulose insulation—useful for slowing down heat transfer—allow air to leak through them and should be used in conjunction with, but—not instead of,— an air barrier. Plus, advises Todd Russo, owner of REEis, a home performance and energy auditing specialist in Scottsdale, Ariz., improperly installed insulation doesn’t do its job. He points to voids in coverage, misalignments, and gaps between the insulation and the air barrier as common and costly gaffes, and notes that a 5% failure in the insulation can cut its effectiveness in half. “Insulation is our No. 1 construction defect, and it’s not always intentional,” says Russo, who advises builders to spend more time training installers. 

5. Ease the pressure. It used to be that every room had its own air supply and return, so if the homeowner closed a door, the air remained comfortable. Most dwellings today, though, are built with a central return, usually in a hallway. So closing a door cuts the room’s supply off from the return, and causes the space to become pressurized—which can force the supply to replenish the room with stale, dirty air rather than clean air from a controlled source.

A semi-quick fix: Balance the pressure by installing a dedicated return in every room with a door; a transfer grill above the doorway; or a jump-over duct that allows the air to flow over the door and into the hall even when the door is closed.

6. Sweat the small stuff. It seems like a no-brainer to seal penetrations where nails, plumbing pipes, or cables have pierced the roof and walls, but Russo says those small holes are often left open, simply because it’s nobody’s job to fill them.

“The framer frames, the stucco guy puts the stucco wrap around it, the plumber runs the pipes, and nobody goes around to see what needs sealing,” he says. Seattle builder Jon Alexander, owner of Sunshine Construction, agrees. “Most builders do very little sealing,” he says. 

7. Test and retest. Alexander does a blower-door test after framing but before insulating, and another one at the end of the job. As the door-mounted fan pulls air out of the house, it lowers the indoor air pressure so much that higher-pressure outdoor air whooshes in though unsealed cracks. A smoke pencil reveals where the leaks are.

“Sometimes we see a really big leak that we didn’t know about, like a hole in the ceiling or in a closet,” notes Alexander.

Russo says it costs a few hundred dollars to hire an energy consultant to do the test and advise you about ways to make the building more efficient by considering how insulation, the size of the HVAC system, and even the shades on windows work together. “You can build the tightest home on the planet, but if you do a handful of other things wrong, the performance won’t be good,” he says. –Sharon O’Malley is a contributing editor to Building Products magazine and ebuild.com

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