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CalcTheBuild

Insulation R-Value & Savings Calculator

Find out whether an insulation upgrade is worth it. Enter the area, the R-value you have now and the one you want, choose a material, and see the thickness needed, your yearly energy savings and the payback period.

sq ft
R

Include the existing assembly. Uninsulated ≈ R-2 (RSI 0.35).

R
°F·days

Annual HDD for your location (e.g. Chicago ≈ 6,300 °F·d; London ≈ 2,800 °C·d).

$
%

Use 250–350% for heat pumps.

$/sq ft

Your estimate

Annual savings
$363.65
Simple payback
4.8 years
Insulation thickness to add
11.9 in
R-value to add
R-38
Heat loss reduction
78%
Energy saved per year
3,030 kWh
Installed cost
$1,750.00
10-year savings
$3,636

Savings cover conductive heat loss through this area only, before any rebates or tax credits.

How to use this calculator

  1. Enter the area of the attic, loft, wall or floor you plan to insulate.
  2. Enter the current R-value (or RSI in metric). An uninsulated wall is about R-4; an attic with 3–4″ of old batts is about R-11.
  3. Enter your target R-value. US building codes call for R-38 to R-60 in attics depending on climate zone.
  4. Pick the insulation material, your heating degree days and the price you pay for heating energy.
  5. Add the installed cost per square foot or square metre to see the payback period.

How the savings are calculated

Heat flows through an assembly at a rate of 1 ÷ R (the U-value). The annual heat loss is area × U-value × heating degree days × 24 hours. We work it out for your current and target R-values; the difference, divided by the efficiency of your heating system, is the energy you save.

Required thickness is the R-value you need to add, divided by the R-value per inch of the material you choose.

Pro tips

  • Seal air leaks before you insulate. Air sealing often saves as much energy as the insulation itself.
  • Check for federal, state or utility rebates. In the US, the 25C tax credit has covered part of insulation costs.
  • Compressing batts reduces their R-value. Don't squeeze R-19 batts into a 2×4 wall cavity.

Frequently asked questions

What is the difference between R-value and RSI?

R-value is the imperial measure of thermal resistance (ft²·°F·h/BTU). RSI is the metric equivalent (m²·K/W). RSI = R × 0.176, so R-30 is about RSI 5.3.

How do I convert a U-value to an R-value?

In metric, RSI = 1 ÷ U-value. A UK loft U-value of 0.16 W/m²K equals RSI 6.25, or about R-35.5 in imperial units.

How much attic insulation do I need?

The US Department of Energy recommends R-30 to R-49 in warm climates and R-49 to R-60 in cold climates. In the UK, 270 mm of mineral wool is the usual recommendation for lofts.

Where can I find heating degree days for my area?

Free tools such as degreedays.net and national weather services publish annual heating degree day totals for most towns. Use a base temperature of 65 °F (18 °C).

These results are estimates for planning and budgeting. Actual quantities depend on site conditions, products and workmanship. Check structural and code requirements with a licensed professional before you build.