# Insulation Calculator

URL: https://speedycalculator.com/calculators/construction/insulation/
Last updated: 2026-10-25

> The insulation calculator adds up the R-value of up to two layers of insulation from the material and thickness, converts to RSI and U-value, and estimates the heat flow through the area for a temperature difference.

## Inputs

- Area (ft², m², yd², acre)
- Layer 1 material
- Layer 1 thickness (in, cm, mm, ft, m)
- Layer 2 material
- Layer 2 thickness (in, cm, mm, ft, m)
- Temperature difference (°F, °C / K)

## Results

- Total R-value
- U-value
- Heat flow through area

## Formula

```
R = Σ R per inch × thickness (in)
RSI = R ÷ 5.678; U = 1 ÷ R
Heat flow (BTU/h) = area (ft²) × ΔT (°F) ÷ R
```

## Explanation

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

MaterialR per inchRSI per 25 mmFiberglass batt3.20.56Blown fiberglass (loose fill)2.50.44Blown cellulose3.50.62Mineral wool batt3.30.58Expanded polystyrene (EPS)40.70Extruded polystyrene (XPS)50.88Polyisocyanurate board61.06Open-cell spray foam3.60.63Closed-cell spray foam6.51.14Cotton (denim) batt3.50.62Straw bale1.50.26Softwood lumber1.250.22Concrete0.080.01

### 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

FieldWhat to enterAreaUnits: ft², m², yd², acreLayer 1 materialChoose one: Fiberglass batt, Blown fiberglass (loose fill), Blown cellulose, Mineral wool batt …Layer 1 thicknessUnits: in, cm, mm, ft, mLayer 2 materialChoose one: Fiberglass batt, Blown fiberglass (loose fill), Blown cellulose, Mineral wool batt …Layer 2 thicknessUnits: in, cm, mm, ft, mTemperature differenceUnits: °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]

## Worked 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).

## FAQ

**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).


## Sources

- [U.S. Department of Energy – Insulation](https://www.energy.gov/energysaver/insulation)