It’s been 10 years since building scientist Joe Lstiburek published one of his most cited and talked-about articles. Called The Perfect Wall, it describes what Lstiburek, who has been investigating moisture problems in buildings for decades, considers the most durable wall system for a modern home. Variations of that wall have since become mainstream and may soon be required in many jurisdictions.

The gist of the article is that the best defense against mold and rot in walls is to place all four of the home’s control layers—air, water, vapor, and thermal—on the outside of the sheathing. That requires the use of an exterior insulation board.

Exterior insulation works in all climates and with any framing material—wood, steel, or masonry. Besides reducing the chance of moisture problems, it also eliminates thermal bridging, a process by which heat can escape from the house through any framing member that’s not interrupted by insulation.

Thermal bridging is a potential issue in all homes, which is one reason why builders are embracing it. In fact, the Home Innovation Research Labs’ 2019 Builder Practices Survey found that exterior foam insulation is now used in 11 percent of homes, and growing.

At least part of that growth is thanks to a particular framing type. “My sources tell me that sales of steel framing have increased 300% in the last couple of years,” says Lee Bybee of Ox Engineered Products, which makes exterior insulation and sheathing products. “Thermal bridging is more of a challenge with steel studs, so builders using them will add a layer of rigid foam to compensate.”

When it comes to moisture, one of the most common problems, after leaky cladding and poor flashing, is condensation in walls. Water vapor generated inside the home (by cooking, showering, and even breathing) can work its way into the wall cavity and condense on the back of cold sheathing in winter.

Addressing this problem is critical in a super-efficient home, which lacks the air leakage needed to dry out that condensation. And tougher energy codes are making super-efficient homes the norm. “It’s just a matter of time before every new home in North America will have to meet Net Zero Energy standards,” says Andy Oding, a building science trainer for the Energy and Environmental Building Alliance. He predicts that exterior insulation will be required in Zones 5 and higher by the end of the decade.

Those foam mandates will be less about efficiency than about durability. “Exterior insulation helps the building last longer by reducing the chance of condensation on the back of the sheathing,” says Oding.

While exterior foam can provide all four of the needed control layers, most builders still want a separate WRB. That’s why, even though polyisocyanurate (polyisi) board has been approved as a WRB by the International Building Code*, many products come with a foil facing. Some products offer even more. One example is OX-IS, a polyiso board with foil laminated to one face and sheathing to the other face. It’s fastened directly to the studs and, when properly taped and flashed, provides all four control layers as well as structural strength.

Bybee says that builders currently using 2×6 framing can, with a product such as OX-IS, switch to 2×4 framing and still get an R-21 wall. “Moving from 2×6 to 2×4 is a large savings in the lumber package,” he says. “In fact, we saw a lot of builders moving in this direction even before the big price spikes of the past six months.”

These builders realize that they can get a more efficient and moisture-resistant building envelope for a lower total cost. It’s the perfect response to higher lumber prices and tougher performance standards.