Double Stud Wall / Slab on Grade w/Stem Wall

Double-stud walls create plenty of room for thick insulation

Double-stud walls use common materials and familiar assemblies to create a low-tech, energy-efficient wall with lots of room for thick insulation. This framing method virtually eliminates thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. through the studs and greatly reduces sound transmission through walls.

The basic strategy is simple: The exterior walls are built from two parallel stud walls with a gap between the rows for extra insulation. Many builders use two parallel 2x4 walls with a 5-inch gap between them to create a 12-inch-thick wall. Of course, the wall can be thicker or thinner as circumstances dictate.

The most commonly used insulation for this method of construction is dense-packed cellulose, although other types of insulation (including blown-in fiberglass, mineral wool batts, or open-cell spray polyurethane foam) can certainly be used.

For more information, see GBA Encyclopedia: Double-Stud Walls.

Foundations, Floors, and Walls Are Critical Green Connections

Air sealing is imperative. The connection between concrete foundations and wood framing is a place prone to air leaks and moisture problems. Wood is often warped, and concrete is rarely flat. There are at least three places for air to leak in and probably a lot more. Leaky connections can mean energy, moisture, comfort, and IAQIndoor air quality. Healthfulness of an interior environment; IAQ is affected by such factors as moisture and mold, emissions of volatile organic compounds from paints and finishes, formaldehyde emissions from cabinets, and ventilation effectiveness. problems. Extending the wall sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. past these connections is a good first step whenever practical. Caulks, adhesives, spray foam, and gaskets can seal them up tightly.

Keep water at bay

Unless you live in the desert, the ground is always wet; and that ground water is always pushing its way in. Footing drains can carry away bulk groundwater, but foundations also have to disrupt capillarity. Water in the soil will wick all the way up to the roof framing if you let it. CapillaryForces that lift water or pull it through porous materials, such as concrete. The tendency of a material to wick water due to the surface tension of the water molecules. breaks such as brush-on damp-proofing, sill sealer, and rigid insulation block this process. A foundation is a bad place to cut corners because problems are expensive and complicated to fix after a house is finished.

Every wall needs a water-resistive barrier, thick insulation, and one or more air barriers

In general, exterior above-grade walls should have continuous air, water, and thermal barriers.

Installing rigid insulation as the exterior sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. can accomplish all of these tasks, but you need to account for shear resistance (provide wall bracing) in other ways, and you must provide special details if you expect the foam sheathing to perform as a water-resistive barrier (WRB).

A double-stud wallConstruction system in which two layers of studs are used to provide a thicker-than-normal wall system so that a lot of insulation can be installed; the two walls are often separated by several inches to reduce thermal bridging through the studs and to provide additional space for insulation. insulated with dense-packed cellulose is an excellent thermal barrier; however, double-stud walls still need at least one additional air barrier to limit air leakage through the cellulose.

Check out the advanced framing section of Construction Detail Library for alternative techniques to achieve shear resistance, and check out Using Rigid Foam As a Water-Resistive Barrier for more information on walls without housewrap.

For more information on air barriers, see Questions and Answers About Air Barriers

Foundation walls cannot dry to the outside

Because foundation walls are mostly buried in the ground, it's important to keep them from absorbing moisture. Brush-on damp-proofing — or better, dimple mats — help keep basement walls dry. Basement walls should be backfilled with coarse granular material to interrupt capillarity and to help ground- and stormwater flow toward the footing drains.

Insulated slabs improve comfort. Install a continuous layer of rigid foam insulation under the slab, except in the hottest climates or in termite-infested areas where local codes may prohibit the practice. The insulation buffers the slab from outdoor temperature swings, keeps the slab warm and dry, and lowers energy bills.

Stem wall slabs are good on slightly sloped lots

Stem wall slabs eliminate the need for floor framing, and the concrete can be used as a finished floor. But you still have to dig below the frost line. After the foundation is filled with compacted gravel, an above-grade slab can be installed inside the stem-wall. In cold parts of the country, where the frost line is 4 ft. or more below grade, many builders conclude that if they have to excavate below the frost line, they may as well put in a basement.

Learn more in the Green Building Encyclopedia

Enclosure overview
Slabs
Footing Drains

DRAWING DETAIL

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