How to Insulate an Attic

What you’ll need

Materials:

• Blown-in insulation bags (fiberglass or cellulose)
• Expanding foam sealant (Great Stuff or similar)
• Rafter baffles / eave baffles
• Depth markers (cardboard rulers or paint sticks)

Tools:

• Insulation blowing machine (rent free at most home centers with bag purchase)
• N95 or P100 respirator
• Safety goggles
• Work gloves and long sleeves
• Headlamp and work lights
• Tape measure

Time: 4–8 hours for a 1,000–1,500 sq ft attic with two people
Cost: $400–$900 in materials (DIY)

How much insulation to buy

Use the Insulation Calculator with your attic sq ft and target R-value. As a rule of thumb:

• R-38: ~25–28 bags (25 lb bags) per 1,000 sq ft
• R-49: ~32–36 bags per 1,000 sq ft
• R-60: ~40–45 bags per 1,000 sq ft

Add 10% for waste. The store can usually return unopened bags.

Step-by-step instructions

Step 1 — Measure and calculate

Measure your attic floor area. Check existing insulation depth. Calculate how many bags you need for your target R-value zone (see R-Value Guide if unsure).

Step 2 — Gather materials and equipment

Most big box stores offer free blower rental when you buy a minimum number of bags (typically 10+). Buy safety gear before the day: N95 respirator, goggles, disposable coveralls or old clothes, and knee pads.

Step 3 — Seal air leaks first (critical step)

This is the step most DIYers skip — and it makes the biggest difference. Before adding any insulation:

• Seal all recessed light cans with fire-rated covers or foam
• Seal around plumbing and electrical penetrations through the top plate
• Seal the attic pull-down stair frame with weatherstripping and a rigid foam cover
• Foam-seal the gap between interior partition wall top plates and the attic floor sheathing

Air sealing before insulating improves results by 30–40% compared to insulation alone.

Step 4 — Install rafter baffles at eaves

Staple foam or cardboard baffles at each rafter bay where the roof meets the exterior wall. Baffles maintain a clear airway from soffit vents to the ridge vent. Without them, blown-in insulation will block soffit vents and cause condensation and rot.

Step 5 — Mark your target depth

Cut 12-inch strips of cardboard and label them with your target depth (e.g., “15 inches for R-38”). Staple one to a joist or lean one against a rafter every 6–8 feet across the attic. These depth gauges help you avoid under-insulating spots.

Step 6 — Blow in the insulation

• Start at the far end of the attic and work backward toward the hatch
• Hold the hose about 12–18 inches above the joists
• Sweep in slow, overlapping rows
• Check depth markers as you go
• Aim slightly over target — insulation can settle 5–10%

Step 7 — Verify depth and R-value

After finishing, measure depth in at least 8–10 spots. Calculate your actual R-value: inches × R-per-inch for your product. Standard blown-in fiberglass is R-2.2 to R-2.5 per inch at settled depth.

Common mistakes to avoid

• Skipping air sealing — biggest mistake; insulation alone doesn’t stop air infiltration
• Blocking soffit vents — always install baffles before blowing
• Under-estimating bags — buy 10% extra and return unopened ones
• Working without a respirator — fiberglass particles are irritating and potentially harmful
• Covering junction boxes — keep electrical junction boxes accessible, not buried under insulation