Insulation should fit snugly within the framing cavity without gaps or voids between the edges of the batt and the framing members. When inset stapling batts, a small void adjacent to the stud where the flange is stapled flush with the front edge of the stud is permissible (see Figure 3A on page 3.) Even the smallest openings between framing members should be insulated. Recommendations applicable to insulating walls are discussed in Installation Techniques (See page 3.) 2x6 Walls
For optimum thermal performance walls with 2x6 framing (or larger) should be filled with insulation. Installing 3-1/2 inch thick insulation in a 5-1/2 inch wall cavity is considered substandard construction practice. The recommended practice is to completely fill the wall cavity.
Insulation installed in knee walls (see figure 2, item 3, on page 3) should be secured to prevent it from falling out of the cavity and covered with an air barrier material on the unconditioned side of the wall. The air barrier should be a durable material which is permanently attached to the wall framing. Examples of acceptable air barriers include, but are not limited to housewrap attached with plastic cap nails, thin plywood or OSB attached with standard nails and extruded polystyrene attached with roofing nails or comparable fasteners. Confirm the air barrier materials exposed on the unconditioned side of the wall comply with any applicable fire safety requirements in the building codes. When unfaced insulation is used a proper vapor retarder must be installed on the appropriate side of the insulation in accordance with the vapor code requirements in the building code. Refer to the vapor retarder section on page 13 of this document for further information.
Insulating walls of existing homes is a difficult task. Either insulating from the outside or inside requires removal of the existing materials. A more common method is to blow in loose-fill insulation. Manufacturers produce fiber glass and rock wool materials specifically designed to be blown into existing wall cavities through small access holes.
Interior walls may be insulated to provide sound control. Either faced or unfaced insulation may be used, but faced insulation is generally easier as the tabs can be used to hold the insulation in place. Denser batts do not require stapling and are held in place by friction.