Insulation is rated by its R-value: the higher the number, the slower heat passes through. R-values of layers in series simply add up. Energy codes specify minimum R-values by climate zone – for example R-38 to R-60 in attics and R-13 to R-21 in walls in much of the U.S.
How to use the Insulation Calculator
- Area
- Layer 1 material
- Layer 1 thickness
- Layer 2 material
- Layer 2 thickness
- Temperature difference
- Every result field is editable: type into Total R-value and Layer 1 thickness plus every other field update to match.
Results update as you type, and every field accepts the units shown in its menu.
How it works
Choose the material and thickness for each layer (set layer 2 to zero thickness if there is none), enter the area and temperature difference. Type a target R-value into the result to find the thickness of layer 1 you need.
R-value per inch
| Material | R per inch | RSI per 25 mm |
|---|---|---|
| Fiberglass batt | 3.2 | 0.56 |
| Blown fiberglass (loose fill) | 2.5 | 0.44 |
| Blown cellulose | 3.5 | 0.62 |
| Mineral wool batt | 3.3 | 0.58 |
| Expanded polystyrene (EPS) | 4 | 0.70 |
| Extruded polystyrene (XPS) | 5 | 0.88 |
| Polyisocyanurate board | 6 | 1.06 |
| Open-cell spray foam | 3.6 | 0.63 |
| Closed-cell spray foam | 6.5 | 1.14 |
| Cotton (denim) batt | 3.5 | 0.62 |
| Straw bale | 1.5 | 0.26 |
| Softwood lumber | 1.25 | 0.22 |
| Concrete | 0.08 | 0.01 |
Tips for an accurate estimate
- Keep units consistent – the calculator handles conversions, but check every field’s unit before reading a result.
- Use values from standards, data sheets or test pieces whenever you have them; typical values are only a starting point.
- Apply appropriate safety factors and have structural, electrical and pressure work checked by a qualified professional.
- Record the inputs you used so the calculation can be reviewed later.
Common mistakes to avoid
- Confusing diameter and radius, or nominal and actual sizes.
- Using allowable stresses and ultimate strengths interchangeably.
- Treating rule-of-thumb results as a substitute for code tables or an engineered design.
Inputs explained
| Field | What to enter |
|---|---|
| Area | Units: ft², m², yd², acre |
| Layer 1 material | Choose one: Fiberglass batt, Blown fiberglass (loose fill), Blown cellulose, Mineral wool batt … |
| Layer 1 thickness | Units: in, cm, mm, ft, m |
| Layer 2 material | Choose one: Fiberglass batt, Blown fiberglass (loose fill), Blown cellulose, Mineral wool batt … |
| Layer 2 thickness | Units: in, cm, mm, ft, m |
| Temperature difference | Units: °F, °C / K |
What each result means
- Total R-value = [Layer 1 material]×[Layer 1 thickness]÷0.0254 + [Layer 2 material]×[Layer 2 thickness]÷0.0254
- U-value = 1÷[Total R-value]
- Heat flow through area = [Area]÷0.09290304×[Temperature difference]÷[Total R-value]
Formula
R = Σ R per inch × thickness (in)RSI = R ÷ 5.678; U = 1 ÷ RHeat flow (BTU/h) = area (ft²) × ΔT (°F) ÷ RWorked example
Inputs — Area: 1,000 ft²; Layer 1 material: Fiberglass batt; Layer 1 thickness: 3.5 in; Layer 2 material: Extruded polystyrene (XPS); Layer 2 thickness: 1 in; Temperature difference: 40 °F.
Results — Total R-value = 16.2 R (ft²·°F·h/BTU); U-value = 0.0617 BTU/h·ft²·°F; Heat flow through area = 2,469 BTU/h.
Key terms
- R-value
- Thermal resistance, ft²·°F·h/BTU in the U.S.
- RSI
- Thermal resistance in m²·K/W (R ÷ 5.678).
Frequently asked questions
How do I add R-values?
Layers in series add: R-13 batts plus R-5 foam board make R-18 (ignoring framing).
What R-value do I need?
It depends on climate zone and assembly; attics typically need R-38 to R-60, walls R-13 to R-21, floors R-19 to R-30 (U.S. IECC).
Does compressing insulation reduce its R-value?
Yes – a batt squeezed into a thinner cavity loses R-value, though its R per inch rises slightly.
What formula does the Insulation Calculator use?
R = Σ R per inch × thickness (in).