Question 1

How much insulation do I need for my attic?

Accepted Answer

Most attics need R-38 to R-60 insulation, depending on your climate zone. For fiberglass batts, that means 12 to 19 inches of thickness. For blown-in cellulose, you need about 16 to 25 inches. Calculate your attic area in square feet, then divide by the coverage per bag (typically 40-50 sq ft at the desired R-value) to get the number of bags needed. Add 10% for waste and gaps around joists and obstacles.

Question 2

What R-value do I need for my climate zone?

Accepted Answer

The DOE recommends R-38 to R-49 for attics in warm climates (zones 1-3), R-49 for moderate climates (zones 4-5), and R-49 to R-60 for cold climates (zones 6-8). For walls, R-13 to R-21 is typical depending on cavity depth. For floors over unconditioned spaces, R-25 to R-30. Basement walls need R-10 to R-15 continuous insulation. Always check local building codes, which may require higher values.

Question 3

How many bags of blown-in insulation do I need?

Accepted Answer

Blown-in insulation bags typically cover 40-50 square feet at R-30, 25-35 sq ft at R-38, and 18-25 sq ft at R-49. To calculate: divide your total area by the coverage per bag at your target R-value, then add 10% for waste. For example, 1,000 sq ft at R-38 with 30 sq ft per bag = 1,000 / 30 = 34 bags, plus 10% = 37 bags. Coverage varies by brand, so always check the manufacturer's chart on the bag label.

Question 4

How much does insulation cost per square foot?

Accepted Answer

Fiberglass batts cost $0.25-$1.50 per sq ft installed. Blown-in cellulose costs $1.00-$2.50 per sq ft. Spray foam (closed-cell) costs $1.50-$3.50 per sq ft or $3-$7 per board foot. Rigid foam board costs $0.50-$2.00 per sq ft. A typical 1,000 sq ft attic costs $1,000-$2,500 for fiberglass, $1,500-$3,500 for cellulose, or $3,000-$7,000 for spray foam. DIY installation can save 40-60% on labor costs.

Question 5

What is the difference between fiberglass, cellulose, and spray foam insulation?

Accepted Answer

Fiberglass batts are the most affordable ($0.25-$1.50/sq ft), easy to install DIY, and fire-resistant, but can leave air gaps. Blown-in cellulose is made from recycled paper ($1.00-$2.50/sq ft), fills irregular spaces better, and has good soundproofing, but can settle 20% over time. Spray foam (closed-cell, $1.50-$3.50/sq ft) creates an air seal, has the highest R-value per inch (R-6 to R-7), and acts as a vapor barrier, but requires professional installation. For most attics, blown-in cellulose or fiberglass offers the best cost-to-performance ratio.

Question 6

How do I calculate insulation for walls vs attics?

Accepted Answer

For walls with 2x4 studs, the cavity depth is 3.5 inches, allowing R-13 to R-15 fiberglass batts. For 2x6 studs, the cavity is 5.5 inches, allowing R-19 to R-21. Calculate wall area as height x width minus 15% for windows and doors. For attics, measure the full ceiling footprint (length x width) since the insulation sits on the attic floor. Use continuous insulation (rigid board on exterior) to reduce thermal bridging through studs, which can reduce effective R-value by 20-25%.

Question 7

Should I remove old insulation before adding new?

Accepted Answer

In most cases, you can add new insulation on top of existing insulation if it is dry, clean, and evenly distributed. Do NOT cover recessed light fixtures, knob-and-tube wiring, or exhaust fan vents without proper clearance. If old insulation is wet, moldy, compressed, or vermiculite (which may contain asbestos), remove it professionally first. Vermiculite insulation requires a lab test before disturbing &mdash; do not handle it yourself if it tests positive for asbestos.

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