Insulation Calculator
How much insulation your attic or walls need to hit target R-value, what it costs, what it saves every year — and how fast it pays for itself.
Estimates for planning. Savings depend on your climate, air sealing, and how your house actually performs — insulation only works if air leaks are sealed first. Attics also need ventilation and safe clearances around recessed lights, chimneys and flues. A blower-door energy audit beats any calculator.
Usually close to the footprint of the floor below.
Perimeter × wall height, minus windows and doors.
Prices are DIY material only; installed prices run higher.
Used to estimate savings — the attic is typically 25–35% of a home's heat loss.
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| Detail | Amount |
|---|---|
| R-value added | — |
| Material cost | — |
| After tax credit | — |
| Estimated annual saving | — |
| Simple payback | — |
How Insulation Savings Are Calculated
Heat loss through a surface is proportional to 1 ÷ R-value. Doubling R doesn't double your savings — it halves the remaining loss, which is why the first inches matter enormously and the last inches barely register.
- Depth needed = (target R − current R) ÷ R-per-inch of your material.
- Loss reduction = 1 − (Rold ÷ Rnew), counting the assembly itself. Even a bare wall has about R-4 from drywall, sheathing, siding and air films, and a bare ceiling about R-3 — so an empty cavity is never truly R-0. Going R-19 → R-49 cuts attic loss about 58%.
- Dollar saving = that reduction × the share of your bill lost through that surface (attics ≈ 30%, walls ≈ 25%).
- Payback = net cost after the tax credit ÷ annual saving.
Worked example — 1,200 sq ft attic, R-19 → R-49, blown fiberglass, $1,500/yr bill: 12 inches added, about $1,080 in material, ~$260/yr saved → roughly 2.9 years payback after the credit.
Why the First Inches Pay and the Last Ones Don't
| Upgrade | Heat loss removed | Verdict |
|---|---|---|
| R-0 → R-19 | ~86% of what was escaping | Enormous — do this immediately |
| R-19 → R-38 | 46% of remaining | Excellent return |
| R-38 → R-49 | 21% of remaining | Still worth it in cold climates |
| R-49 → R-60 | 17% of remaining | Marginal outside cold zones |
R-Value Per Inch by Material
| Material | R per inch | Inches for R-49 | Notes |
|---|---|---|---|
| Blown fiberglass | 2.5 | ~20" | Cheapest per R; settles slightly |
| Fiberglass batts | 3.2 | ~15" | Easy DIY between joists |
| Blown cellulose | 3.5 | ~14" | Better air blocking, recycled content |
| Closed-cell spray foam | 6.5 | ~8" | Air seals too; several times the price |
Before You Buy
- Air seal first. Insulation slows conduction; it does nothing about air leaking around wires, pipes and top plates. Sealing before insulating is what separates a good result from a disappointing one — and it counts toward the same tax credit.
- Don't block soffit vents. Use baffles at the eaves; a smothered attic traps moisture and rots the roof deck.
- Keep clearance around heat sources — recessed lights that aren't IC-rated, chimneys, and flues need dedicated spacing.
- Never lay faced batts over existing insulation. The vapor barrier ends up mid-assembly and traps moisture. Unfaced or blown only.
- Insulate before sizing a heat pump. A tighter house needs a smaller unit — sometimes a full ton less — which cuts the equipment bill and improves cold-weather performance.
Frequently Asked Questions
What R-value should my attic be?
R-49 to R-60 across most of the US; R-30 to R-49 in the warmest zones. Many older homes sit at R-19 or below.
How much does attic insulation save?
R-19 → R-49 cuts attic loss ~60%. On a $1,500/yr bill that's often $150–$300 a year, paying back in 3–6 years.
Is there a tax credit?
30% of insulation and air-sealing cost, capped at $1,200 per year, through at least 2032. The cap resets annually.
How many inches for R-49?
~20" blown fiberglass, ~14" cellulose, ~8" closed-cell foam.
Insulate before a heat pump?
Usually yes — a tighter house needs a smaller unit, and both carry their own credits.
Can I add over old insulation?
In an attic, yes, if it's dry and uncompacted; R-values add. Never faced batts on top.
References
- US Department of Energy — insulation R-value recommendations by climate zone: energy.gov
- ENERGY STAR — sealing and insulating, and federal tax credits: energystar.gov
- Oak Ridge National Laboratory — insulation performance research: ornl.gov