Double Stud Wall / Vaulted Ceiling

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.

Ventilation may reduce ice damming
Most building scientists agree that a ventilated air space between the insulation and the roof 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 lower the chance of ice damming in snowy climates. For more information on preventing ice dams, see Prevent Ice Dams With Air Sealing and Insulation.

Some builders choose to install ventilation channels above the roof sheathing; for more information on this option, see How to Build an Insulated Cathedral Ceiling.

Where Roofs Meet Walls is a Critical Connection

Corners and connections are where insulation and air barriers can have trouble. Compressed or insufficient insulation can cause cold spots, which lead to condensation, mold, and rot. Air leaks at this connection can cut the effectiveness of the insulation substantially. In cold climates, this is where ice dams begin.

To keep the air barrier continuous, span 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. over the framing connection and use adhesive or sealants at framing connections as shown.

Roofs and walls need to dry
Moisture from both outside and inside a house can thwart your best efforts at keeping the building dry. Moisture in roof and wall assemblies is inevitable, so it's a good idea to design them so that they can dry. Roofs and walls that can dry to either the outside or inside are good, but those that can dry both directions are even better.

  1. Designing to dry out means doing two things well:
  2. 1. Choosing materials carefully—each layer affects the vapor profileA vapor profile is an assessment of the relative vapor permeabilities of each individual component in a building assembly and a determination of the assembly's overall drying potential and drying direction based on vapor permeabilities of all of the components. The vapor profile addresses not only how the building's enclosure assembly protects itself from getting wet, but also how it dries when it gets wet. For a detailed treatment of this subject, see Building Science Corporation's article Understanding Vapor Barriers. of the assembly.
  3. 2. Planning the construction to be forgiving—flashing keeps water out, and ventilation removes water vapor.

Roofs leaks are harder to spot in cathedral ceilings
Sometimes a water stain inside the house isn't anywhere near where leaked in. It's easier to find roof leaks in a ventilated, unconditioned attic than in a house with an insulated cathedral ceiling.

The usual way to vent a roof is with continuous soffit vents acting as air inlets and a continuous ridge vent to act as an air outlet. In most cases, ventilation takes place beneath the roof deck. Install vent channels in each rafter bay before insulation is installed.

Learn more in the Green Building Encyclopedia

Enclosure overview
Exterior walls
Roofs: Attics, Structure, Claddings

DRAWING DETAIL

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