How To Install Fiberglass Insulation

Fiberglass insulation is a popular choice because it’s affordable and effective. This guide will walk you through the process of installing fiberglass insulation, helping you create a cozier and more energy-efficient space. Plus, This Old House host Kevin O’Connor explains the key steps and techniques for a successful installation in the above video.

Preparing for Fiberglass Insulation Installation

Before starting the installation, make sure you’re prepared and have everything you need.

Safety Precautions

Working with fiberglass insulation requires some safety measures:

• Check for proper ventilation in the work area.
• Use gloves and safety glasses.
• Wear a dust mask or respirator.
• Wear long sleeves and pants to protect your skin.

Tools and Materials

Gather the following tools and materials:

• 3-mil polyethylene sheeting (for vapor barrier)
• Fiberglass insulation batts
• Ladder for reaching high places, if needed
• Low-expansion foam insulation
• Staple gun and staples
• Straight edge or 2-by-4 board
• Tape measure
• Utility knife

Installing Fiberglass Insulation

Once you have your materials and safety gear ready, follow these steps.

Installing in Open Stud Bays

Here’s how to install the insulation in open stud bays: 

1. Measure the height of the stud bay and cut the insulation about an inch longer than needed.
2. Gently push the insulation into the stud bay, starting at the top.
3. Make sure the insulation fits snugly against the top and bottom plates and side studs.

Avoid compressing the insulation—this reduces its effectiveness.

Working Around Obstacles

When you’re installing insulation around electrical boxes, pipes, or other obstacles:

1. Split the batt in half lengthwise.
2. Tuck one half behind the obstacle.
3. Place the other half in front. Cut around the obstacle as needed.
4. Make sure there are no gaps or compressed areas around the obstacle.

If you have vertical obstructions, split the batt vertically and install it on either side of the obstacle, creating a snug fit.

Insulating Around Windows and Doors

Proper insulation around windows and doors prevents drafts. Here are our recommendations:

1. Cut small pieces of insulation to fit snugly in the spaces around window and door frames.
2. Use low-expansion foam insulation to seal any gaps.
3. Apply the foam carefully, starting at the top and working your way down.
4. Allow the foam to expand and cure before trimming any excess.

Installing Blockers

Blockers help prevent air movement between the living space and the attic. To install them:

1. Cut pieces of faced insulation to fit between the floor joists above the top plate of exterior walls.
2. Install the blockers with the vapor barrier facing the living space.
3. Make a tight fit to prevent air leakage.

Adding a Vapor Barrier to Your Insulation

Many climates require a vapor barrier to protect walls and installation from moisture issues within the walls.

When To Use a Vapor Barrier

Vapor barriers are typically required in colder climates where the temperature difference between indoor and outdoor air can cause condensation within walls. Check your local building codes to see if a vapor barrier is necessary in your area.

Installing the Vapor Barrier

If a vapor barrier is needed:

1. Use 3-mil polyethylene sheeting.
2. Cut the sheeting to fit from floor to ceiling, with extra length for overlap.
3. Staple the sheeting to the face of the wall studs, overlapping seams by at least 6 inches.
4. Seal any penetrations or seams with tape designed for vapor barriers.

Common Insulation Mistakes To Avoid

When you’re installing fiberglass insulation, avoid making these common mistakes:

• Pressing the insulation down too much—this reduces its effectiveness.
• Skipping the vapor barrier if required in your area.
• Not measuring and cutting accurately to avoid wasting materials.
• Failing to check for gaps or spaces, which can reduce the insulation’s performance.
• Not wearing protective gear to stay safe.
• Neglecting to seal around electrical boxes and other openings to keep the insulation snug.
• Forgetting to check local building codes before you begin to stay compliant.