Eyebrow roofs, porch roofs, and other projections of the wall or roofline often are built before a house is sheathed or are built around partial sheathing because the section of wall will be covered up later. This inadvertently lets air leak out of the building through the exterior walls.

Unless these areas are carefully sealed and insulated, the structure will
have one of two problems, and probably both, according to the Journal of Light Construction. In the simplest cases, heated air will escape from the house completely, with cold makeup air finding its way in somewhere else.

But also, convection currents within the wall cavities can cause condensation which leads to mildew, mold, and rot.

Therefore, it is critical to air-seal a frame house while you’re building it, rather than coming back later to plug the leaks. A continuous plane of wall sheathing behind a porch (or eyebrow) roof can stop air and improve a home’s energy efficiency and comfort. Here's how it's done:

1. Sheathe the wall before you frame the eyebrow roof, attaching the small rafters to a ledger board nailed over the sheathing (see illustration).

2. Frame the roof as a stand-alone assembly. Don't frame the eyebrow roof by nailing small rafter-tail pieces onto the sides of second-floor wall studs. This creates a lot of small, irregular voids making it difficult to seal and insulate the area.

3. Seal all of the joints in a complicated assembly, like a rim joist, to further boost energy efficiency.

OTHER CRITICAL JUNCTURES
In addition to eyebrow roofs, four other framing details cause most of the energy problems in wood-frame houses. They are:

—floor-kneewall transitions
—cantilevered floors
—balloon-framed gable ends
—balloon-framed shed dormers

Taking the time to seal and insulate each of these spots will pay off in lower energy bills and improved comfort for your customers.

Related article:
Product Review: Sheathing (EcoBuilding Pulse)
Plywood vs. OSB: Which is the greener sheathing choice?

Air Sealing: The Story and a Half (JLC—paid subscription article)
For increased energy-efficiency in a wood-frame house, plug air leaks at overhangs, knee walls, and gable ends.