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.

Do not fill the gap between the jamb and framing with too much foam too quickly. Otherwise, the jamb could bow and bind the sash. Instead, apply a single thin bead of minimally expanding polyurethane foam to the gap between the window and the framing. Allow the bead to expand and cure for one hour before adding more, and repeat until the cavity is completely filled. If you’re using fiberglass insulation instead of foam, cut unfaced batting slightly larger than the gap and push it in with a putty knife, then cover the seam with aluminum tape to prevent air leaks. If the gap is too narrow for either foam or fiberglass, seal it with a bead of caulk.

Pro Tip: When using expanding foam around windows and doors, make sure you’re choosing the right product. As the This Old House team advises: “If you’re doing a window or a door, you actually look for the can that says window or door on it.” Standard expanding foam can exert so much pressure that it will push the jamb inward, locking the window or door in place. Low-expansion formulas designed specifically for windows and doors prevent this costly mistake.

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.

Pro Tip: Tom Silva, This Old House general contractor, prefers to use 6-mil polyethylene sheeting over paper-faced batts for vapor barrier duty. Cut the sheeting to fit between the joists and seal all seams with foil tape to prevent moisture from seeping into the insulation. Whether using loose fill or batts, put the vapor barrier closest to the warm side of your installation — facing the house’s interior in cold climates and the attic’s interior in hot climates.

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.

Pro Tip: Tom Silva warns that sloppy framing is a common culprit behind poorly performing batts: “If the studs are 16.5 inches on center and you’re using batts made for 16 inches, you’ll have a crack on each side that air can move through.” In cases like this, he says it’s best to use cavity-filling foam or blown-in insulation instead of forcing batts to fit.

Don’t block soffit vents: Stuffing insulation along the eaves can cover soffit vents, cutting off the airflow from soffits to the ridge vent that keeps the roof cool and prevents ice dams. Before adding attic insulation, staple plastic or foam baffles to the roof sheathing near the eaves to maintain at least a half-inch to two-inch space between the insulation and the underside of the roof deck.