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Insulation Calculator

An insulation calculator works out how much insulation to buy — square feet of batts or rolls, or bags of blown-in for an attic — from the area and, for blown-in, your target R-value. It handles the part people miss: blown-in coverage shrinks as the R-value climbs, because you're laying a deeper blanket.

Your measurements

Result

Whether you're filling stud bays with batts or topping up an attic with blown-in, this turns your area and R-target into a clear shopping quantity.

How it is calculated

Batts/rolls: area × waste multiplier ÷ coverage per roll. Blown-in: area × allowance ÷ the per-bag coverage at the chosen R-value (deeper layers cover less — ≈60 sq ft/bag at R-30 down to ≈30 at R-60 for blown fibreglass). Coverage varies by product and machine; confirm on the bag.

Steps

Choose batts/rolls or blown-in.
Enter the area to insulate.
For blown-in, pick the target R-value (R-38, R-49…).
For batts, enter the coverage per roll or bundle.
Read the square feet or bag count, with waste added.

Worked example

A 500 sq ft attic to R-38 needs about 11 bags of blown-in fibreglass (≈47 sq ft per bag at R-38, +5%).

Common questions

How much insulation do I need?

Batts: your framed area plus a little waste. Blown-in: area divided by the per-bag coverage at your target R-value — the calculator does both.

Why does R-value change the number of bags?

A higher R means a deeper, denser layer, so each bag covers fewer square feet. R-49 needs far more bags than R-30 for the same attic.

What R-value do I need?

It depends on climate and location — attics often target R-38 to R-60. Check your local energy code or the US DOE zone map.

Batts or blown-in?

Batts suit open stud/joist bays and walls; blown-in is fast and seamless for attics and irregular spaces. Many attics use blown-in over existing batts.