Insulation Calculator

How Insulation R-Value Works

R-value measures thermal resistance, meaning how well insulation resists the flow of heat. A higher R-value means better insulation. R-values are additive, so layering insulation adds their R-values together.

Heat loss formula:

Heat loss in BTU (British Thermal Units) per hour = Area (sq ft) × Temperature difference (°F) ÷ Total R-value

Worked example:

• Attic area: 1,200 sq ft
• R-value of existing insulation: R-19
• Indoor temperature: 68°F, Outdoor: 10°F → ΔT = 58°F

Heat loss = 1,200 × 58 ÷ 19 = 3,663 BTU/hr

Adding R-19 more (total R-38):

Heat loss = 1,200 × 58 ÷ 38 = 1,832 BTU/hr, a 50% reduction

Notice how the second R-19 buys you far less than the first. Going from nothing to R-19 cuts the loss enormously; doubling R-19 to R-38 only halves what is left. That diminishing return is why attics get deep insulation and walls do not, and why chasing R-60 in a mild climate rarely pays back.

Batts are sized to fit standard framing, so buy for your cavity depth rather than for a target R-value alone. An R-30 batt crushed into a 2x6 wall loses much of the rating you paid for. Air sealing first, insulation second: a small gap around a recessed light or a top plate will undo more performance than a couple of R-points ever gain back.

DOE (Department of Energy) recommended R-values by climate zone:

R-value per inch by insulation type:

Thickness needed:

Inches needed = Target R-value ÷ R per inch

To achieve R-49 with blown cellulose (R-3.5/inch):

Thickness = 49 ÷ 3.5 = 14 inches

Bag coverage falls as the target R-value rises, and it falls in proportion

This is the single most expensive mistake on an insulation order, so it is worth being blunt about. A bag of blown insulation holds a fixed amount of material. Spread it twice as deep and it covers half the floor. So coverage is inversely proportional to the R-value you are aiming for:

Coverage per bag = (coverage at R-30) × 30 ÷ (your target R-value)

Blown cellulose covers roughly 32 sq ft per bag at R-30. That same bag covers about 50 sq ft at R-19, and only about 25 at R-38. Blown fiberglass is lighter per bag and anchors near 26 sq ft at R-30.

The number printed in big type on the front of the bag is quoted at a shallow R-value, and people read it, divide their attic area by it, and order roughly half of what the job needs. Turn the bag over. The coverage table on the back gives a separate square-footage figure for every R-value, and that is the row you want. If your brand’s table disagrees with this calculator, follow your brand.

The R-13 row is the one place the formula stops applying. Straight arithmetic would give 74 sq ft for cellulose, but a bag spread that thin no longer settles to the density the R-value assumes, so the calculator holds it at 70. Nobody blows an attic to R-13 anyway; that row is there for a floor or a knee wall.

Batts do not work this way, because a batt is a manufactured slab at a fixed thickness rather than loose material you spread. A batt package covers whatever its label says and nothing else, which is why the two insulation types are counted separately below.