Conceptual air sealing strategy at upper floor band joist

The simplest way to ensure that all elements of your air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. system connect up is to take a footing-to-ridge cross section drawing of your building and connect the air barrier from the footing to the ridge without lifting your pencil or cursor. Typically, the tough spots will be transitions from one assembly to another (for example foundation to above-grade wall) and penetrations like windows and dormers. These details are a part of that process, keeping the pencil or cursor flowing from footing to ridge (or top floor ceiling).

Exterior rigid foam on walls - minimum foam R-value

Using exterior rigid insulation to 'warm' the wall cavity

To keep the wall cavity (and everything to its interior) warm enough to prevent any sustained condensation (building scientists call this the condensing surface temperature), you have to work out the relationship between the R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of the exterior rigid insulation and the R-value of the cavity insulation. This is not tricky, but is likely to require guidance, which you can get here:

Building Science Corp

A general rule of thumb: with 2 by 4 or 2 by 6 walls and cavity insulation around R-3 per inch, use 1/4-inch of XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation. exterior rigid insulation per 1,000 heating degree days. For detailed climate-specific recommendations on exterior foam thickness, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

The relationship between the exterior rigid and cavity insulation is based on representative interior wintertime relative humidities; care must be taken if interior relative humidities run outside of this range (25% - 35%). Higher interior relative humidities may indicate a more aggressive interior vapor retarder.

Video: Basics of Rigid Foam Sheathing

Omitting an interior vapor retarder

Vapor retarder requirements for interior finish

Most of GBAGreenBuildingAdvisor.com's above-grade wall details don't include an interior vapor retarder. This approach works if you keep the framing cavity warm with exterior rigid insulation of the appropriate type and thickness. Your wall assembly specification should consider the climate and the average indoor relative humidity during the coldest months. For more information on this issue, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

GBA defers to Building Science Corporation's approach to designing wall assemblies based on the indoor climate, the outdoor climate, and the specified wall materials. Not all building codes have adopted the changes that allow builders to follow BSC advice, however.

To learn more about the BSC approach and the "dew point test," see:
Building Science.

For more information on the performance properties of common building materials, see:
Building Science Corp.: Building Materials Property Table

Protecting rigid foam on walls from insects

Protecting bottom edge of exterior rigid insulation from insects

The exposed bottom edge of exterior rigid insulation in above-grade walls may need protection from insects (carpenter ants, termites, boring bees, etc.) A metal shield (flashing) pinned between the mud sill and the foundation that extends out to the rigid insulation is one technique to shield the bottom edge; another is to wrap the bottom edge with screening, which can protect the vented air space as well. Stainless-steel mesh works well but is expensive.