Insulation Calculator

What this calculator does

Attic insulation is specified in R-value, not inches, and the quantity that matters is the gap between what you already have and what you want. This calculator works out that gap, converts it to depth, and — the part most calculators skip — adjusts bag coverage for the R-value you are actually installing.

Why coverage is not a fixed number

A bag holds a fixed volume of material. Spread it thin across a large area and you get a low R-value; pile it deep across a small one and you get a high R-value. So coverage and R-value trade off directly:

coverage at your R = bag coverage at R-1 ÷ R gap

This is why every bag label quotes coverage at a stated R-value, and why reading “covers 40 sq ft” without checking which R-value that refers to leads to ordering roughly half what you need.

A practical example: a 1,200 square foot attic going from R-19 to R-49 needs an R-30 top-up. If a bag covers 1,000 square feet at R-1, it covers 33 square feet at R-30 — so 36 bags.

R-value per inch, by material

Material	R per inch	Notes
Blown fiberglass	~2.5	Light, does not settle much, resists moisture
Blown cellulose	~3.5	More R per inch, recycled, settles somewhat
Fiberglass batts	~3.2	Cheap, easy to fit badly around obstructions

Cellulose reaches a target in less depth, which matters in a shallow attic. Blown fiberglass handles occasional moisture better. Batts are the worst choice for a top-up over existing material, because they only perform when in full contact with no gaps — and an attic floor full of joists, wiring and boxes guarantees gaps.

Air sealing comes first

This is the most useful thing on this page and it is not a quantity.

Insulation slows conduction — heat moving through material. It does almost nothing about convection, which is air moving through gaps. A typical US attic leaks air around recessed lights, plumbing stacks, wiring penetrations, the top plates of interior walls and, worst of all, a bare attic hatch.

Adding insulation over unsealed penetrations buries the problem without fixing it, and the installed R-value will underperform its rating. Seal first with foam and caulk, weatherstrip the hatch, then insulate. The sealing usually costs a fraction of the insulation and often delivers more.

Diminishing returns are real

Heat loss is proportional to 1/R, so each added increment buys less than the last:

• R-0 to R-13 removes roughly 92% of conductive loss.
• R-13 to R-30 removes about 6% more.
• R-30 to R-49 removes roughly 1.5% more.
• R-49 to R-60 is a fraction of a percent.

That is not an argument against reaching code — code levels are set where they are for good reason, and the marginal cost of blown insulation is low. It is an argument against paying a premium to go far beyond it while air leaks and uninsulated ducts remain untouched.

What this leaves out

• Baffles at the eaves, which keep insulation out of the soffit vents. Block those and you trade an energy problem for a moisture problem.
• Ventilation. Attic ventilation must be maintained; insulation is not supposed to reach the roof deck.
• Thermal bridging. Joists conduct heat around the insulation between them, so a real assembly performs somewhat below its nominal R.
• Knee walls and sloped ceilings, which are different assemblies entirely.
• Duct and pipe insulation, frequently a larger loss than the attic floor in houses with ductwork above the ceiling.
• Existing problems. Wet or mould-affected insulation must be removed, not covered.

Doing it alongside other work

If the roof is being replaced, that is the cheapest moment to deal with the attic — the roofing calculator sizes that job, and the access is already paid for. Any ceiling opened up for the work is covered by the drywall calculator.

For US recommendations by climate zone, the Department of Energy’s insulation guidance is the primary source, and many utilities offer rebates that materially change the payback — worth checking before buying, since some require pre-approval.