How to Install Unfaced Fiberglass Insulation in Rafters

Set up your ladder or scaffolding so that you can comfortably reach the area requiring insulation.

Verify the width measurement of the insulation against the width between the rafters. If the insulation is wider than the space allowed, it will not provide the R-value requirements. The air trapped between the glass fibers during manufacture is what gives insulation its air value and compressing insulation to fit it in the space causes it to lose some of that R-value.

Install an unfaced fiberglass batt between the rafters. Start at one end of the cavity and work toward the other, gently tucking in the insulation as you go. Look at the insulation to verify that no corners or edges stick up along the rafter’s sides. The insulation’s appearance should be that of a tucked in bed. Smooth the insulation in the cavity with your gloved hand.

Repeat this procedure for the entire area.

Roll out sheets of the vapor barrier and attach to the rafter’s edges with a staple gun. The vapor barrier needs to cover the entire insulated area. While insulation resists heat flow, it doesn’t resist airflow. The vapor barrier retards the flow of air and moisture in both directions.