Basements: For the roughly half of all new homes built on a basement or crawlspace, effective insulation and air-sealing application comes down to two choices and also depends somewhat on the end use of the space.

For a conditioned, full-height basement, builders can insulate either side of the perimeter walls with rigid foam insulation panels and spray foam or elastic sealant applied to the joints and the band joist area. “The difference in thermal performance is incremental,” says Parsons. “But if you insulate and seal the exterior, you get the benefit of water protection [combined with a waterproofing membrane applied to the panels] and a drainage plane,” which helps mitigate water intrusion and related latent defects and potential indoor air quality issues.

Builders can also insulate the concrete slab, either prior to the pour (achieving similar waterproofing benefits as the sidewalls) or on top of it to create what’s called a “floating” floor of a vapor retarder, R-10 foam panels (taped at all joints), and a T&G wood subfloor ready for the owner’s choice of finish. “If you can plan and prep for it, insulating and sealing from the outside is faster and easier,” Parsons says.

For occasional-use basements and crawls, filling the floor joists with a batt, blow-in-membrane, or flash-and-batt application and air-sealing the band joist offer more affordable yet very effective thermal values to seal and enclose these areas away from the conditioned living spaces above and perhaps create a semi-conditioned area for duct runs.

Regardless of where and how a builder seals and insulates, Easley advocates adding two in-house inspections—one just before the drywall goes up and another before the exterior cladding is applied—to make sure the building envelope is thoroughly sealed against air and moisture infiltration. “A building inspector may not look that closely,” he says. “But a builder should.”

A Systems Approach

The ability to right-size mechanical equipment, incorporating advanced framing techniques that allow larger framing cavities, and the need to provide controlled fresh-air ventilation into an airtight and well-insulated house speak to how delivering an energy-efficient house requires far more than optimizing the insulation package.

“It’s not just a laundry list of products to achieve certification from a green building program,” says Easley. “It’s how they work together to optimize the performance and the economics of energy-efficient housing.”

Rich Binsacca is a contributing editor for EcoHome.