Conceptual air sealing strategy at roof eave

Bird's-Eye View
Key Materials
Builder Tips

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 INSULATION

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

ATTIC INSULATION

How to choose attic insulation

There is no such thing as "bad" insulation, just bad ways to install insulation, and different shades of green in insulation types.

Insulation should slow heat loss
Insulation mainly buffers conductionMovement of heat through a material as kinetic energy is transferred from molecule to molecule; the handle of an iron skillet on the stove gets hot due to heat conduction. R-value is a measure of resistance to conductive heat flow., but air leakage is the critical factor. If the house is leaky, the insulation won't work. Some insulation types can seal air leaks and slow conduction.

Insulation affects drying potential
Another important consideration is vapor permeability of an insulation material. This directly affects the roof's ability to dry out when nit gets wet.

Green insulation has a small environmental footprint
Factors that can make your insulation choices greener:

Recycled content

Recyclability

Less energy required in manufacture

No ozone-depleting blowing agent

Low or no VOCsVolatile organic compound. An organic compound that evaporates readily into the atmosphere; as defined by the U.S. Environmental Protection Agency, VOCs are organic compounds that volatize and then become involved in photochemical smog production.