Why choose continuous insulation?
As many around the country look for economical ways to comply with stricter energy code requirements they are finding continuous foam insulated sheathing has some advantages.
Continuous foam sheathing eliminates thermal bridging from framing. About 25% of the wall surface is framing which offers little insulating value. Installing continuous foam sheathing can cut the heat loss though framing by half or more. In addition, it can reduce the heat loss through the insulated cavity by more than 20%. This makes it easier and simpler to reach higher wall U-factors.
Continuous foam sheathing also decreases the chance for moisture problems within the walls. It does this two ways: by increasing drying to the inside and moving the point where moisture condenses outside the wall cavity.
Continuous foam sheathing makes the wall cavity warmer. This extra warmth increases the vapor pressure which pushes moisture to escape. Although the sheathing will not allow moisture to flow to the outside, the increased pressure caused by the warmer air will force it to the inside and keep the wall dryer than if plywood or OSB sheathing is used.
Warm air can hold more moisture than cold air. As air cools it loses its ability to hold moisture until it reaches the dew point where moisture begins to condense. Foam sheathing can move that dew point into the sheathing itself where moisture is not able to enter, so no condensation will occur. This eliminates the ability of mold to grow.
Properly installed foam sheathing can also replace house wrap as the weather barrier.
Two reasons some are resistant to using continuous foam insulation is concern about wall bracing and how to fasten siding through the material. The solutions to these concerns are fairly simple.
Some areas of the country use let-in-bracing, a 1 x 4 wood board notched into the wall studs at an angle from 45 to 60 degrees reaching from top to bottom plates. Metal T bracing is also available that is placed in a saw kerf from plate to plate. Metal strapping extending up and over the top plate is also a code approved method. OSB panels that cover just enough of the wall to provide bracing can be covered with ½” foam while the space in between the panels contains 1” thick foam.
Changes to Table R703.3(1) now include fastening methods for various types of siding placed over insulated sheathing. The table specifies minimum nail diameter and length needed to securely fasten the siding, regardless of the type.