Insulation Calculator & R-Value Guide

How to Use This Insulation Calculator

Choose batt/roll or blown-in insulation. Enter your area in square feet. For batts, select the R-value for your application, R-13 for 2×4 walls, R-19 for 2×6 walls, R-30 for attics, R-38 for cold-climate attics. For blown-in, pick your target R-value and material type. The calculator shows units needed and recommended depth.

Insulation Guide: Choosing the Right R-Value and Type

Proper insulation is the single best investment you can make in home energy efficiency. The Department of Energy estimates that 90% of homes in the US are under-insulated. Here's what you need to know to get it right.

R-Value by Location

• Attic: R-30 to R-60. The attic is where most heat escapes, it's the most important place to insulate well.
• 2×4 Walls: R-13 to R-15 (the cavity only fits 3.5 inches). Consider rigid foam sheathing on the exterior for additional R-value.
• 2×6 Walls: R-19 to R-21.
• Floors over unheated space: R-19 to R-30.
• Basement/crawlspace walls: R-10 to R-19 (use rigid foam board, fiberglass batts can trap moisture here).

Batts vs Blown-In

Batts are ideal for open walls during new construction and accessible attics with standard joist spacing. They're DIY-friendly and cost $0.30-0.50 per sq ft. Blown-in is better for existing attics with irregular joist spacing, around obstructions, and for topping up existing insulation. It fills gaps more completely than batts but requires a blowing machine (free rental with purchase at most home centers). Cost is similar: $0.40-0.70 per sq ft.

Insulation Materials: Fiberglass, Cellulose, Spray Foam & More

• Fiberglass (batts or blown): Most common. R-2.2 to R-3.8 per inch depending on density. Non-combustible, doesn't absorb water. The tiny glass fibers are skin and lung irritants, wear PPE during installation. Standard kraft-faced batts include a vapor barrier.
• Cellulose (blown-in): Made from recycled newspaper treated with borate for fire and pest resistance. R-3.2 to R-3.8 per inch, slightly higher than fiberglass. Settles 10-20% over the first year, so initial depth must account for settlement. Better at blocking air infiltration than fiberglass.
• Spray Foam (closed-cell): R-6.0 to R-7.0 per inch, the highest R-value per inch. Acts as its own air barrier and vapor barrier. Expensive ($1.50-3.00/board foot installed) and requires professional installation. Ideal for rim joists, crawl spaces, and hard-to-air-seal areas.
• Spray Foam (open-cell): R-3.5 to R-3.8 per inch. Less expensive than closed-cell ($0.75-1.50/board foot). Not a vapor barrier, not recommended for below-grade applications. Excellent for soundproofing interior walls.
• Mineral Wool (Rockwool): R-3.0 to R-3.3 per inch. Fire-resistant to 2,150°F, water-repellent, and excellent soundproofing. More expensive than fiberglass but easier to cut precisely and doesn't irritate skin.

Air Sealing: The Critical Step Most DIYers Skip

Insulation slows heat transfer; air sealing stops air movement. They're different things. Fiberglass batts lose up to 50% of their R-value when wind can blow through them. Before insulating, seal all penetrations between the living space and attic or exterior: electrical boxes, plumbing vents, recessed lights, chimney chases, and top plates. Use expanding foam (Great Stuff) for gaps under 1", rigid foam board + canned foam for larger openings, and aluminum flashing + high-temp caulk around chimneys and flues. The EPA estimates that air sealing alone can reduce heating and cooling costs by 15%, often more than adding insulation alone.

Vapor Barriers: When You Need One (and When You Don't)

A vapor barrier prevents moisture from migrating through walls and condensing inside the insulation cavity. In cold climates (Zones 5-7), the vapor barrier goes on the WARM side of the wall (interior) to prevent indoor humidity from condensing in the cold wall cavity. In hot, humid climates (Zones 1-2), the vapor barrier goes on the EXTERIOR side. In mixed climates (Zones 3-4), vapor barriers can trap moisture and do more harm than good, use vapor-retardant paint instead. Kraft-faced batts have a built-in asphalt-impregnated paper vapor barrier; unfaced batts need a separate 6-mil polyethylene sheet.