In the world of high-performance home construction, there's an ongoing discussion of how best to achieve ultra-low energy costs for homeowners. Airtight building shells, with heavily insulated foundations, walls, and roofs, are usually the starting formula. But on-site power production can also play an important role. And these days, there's a debate about how to strike the balance: When budgets are limited, at what point do you call a halt to the insulation, and shift the focus to the rooftop photovoltaics?

In the town of Harwich, Massachusetts, Cape Cod builder and remodeler Mike Horgan is placing his chips on PV. He's building a new addition onto a small house. The shell will be airtight (using advanced membranes, tapes, and gaskets), but it won't be superinsulated. Even so, Horgan is shooting for net annual zero energy cost for the owner — because the roof of the new addition, as well as the existing house roof, will be almost covered with photovoltaic panels.

If the house hits the net-zero goal, it will be partly the result of the occupant's lifestyle, says Horgan. In winter, most of the existing house will be shut down, and the owner plans to live mainly in the new high-performance addition. In summer, when there's no heating cost (and most Cape Cod residents don't air-condition), the owner will open up the rest of the house.

The addition will be conventionally framed with 2x6 stud walls. But the air-tight envelope details will be unusual, with a continuous vapor-permeable air barrier membrane extending up the walls and onto the roof plane, allowing both the walls and the cathedralized roof to dry to the exterior. JLC's Coastal Connection is following the job. This week, we take a look as framing contractor Mike Hill frames the addition's lowest level, a walk-out basement room. This phase of the work is relatively conventional and straightforward. But there are a few new twists — for example, the flexible EPDM gasket, supplied by Conservation Technology, that Horgan is using instead of conventional sill sealer. For a closer look, see the slideshow.