R-Value Calculator (Insulation & U-Value)
Build a wall, roof or floor from its layers to get the total R-value and U-value, in US and SI units. Advanced adds λ or your own R per layer, heat loss, yearly heating cost and the thickness you need for a target.
Pick a typical build-up, then change the layers to match yours, from the outside in or the inside out. The order doesn't change the total.
Typical values — use the product's rated R or λ from its data sheet. Every value can be changed. Small thickness changes matter: 3.5 in is 89 mm.
Use the wall, roof or floor area without windows and doors. In summer, put the hotter outside temperature in to see heat coming in.
Yearly heating (optional)Degree-days for your town come from your weather service or a degree-day website, for the base temperature you heat to. 100% is an electric heater; a heat pump with a COP of 3 is 300%; a gas boiler is often 85–95%.
Use the target your designer, energy rating or local rules give you. This page doesn't hold any code minimums because they differ by country, climate and building part.
How your R-value is worked out
Each layer's R = thickness × R per inch (or thickness ÷ λ). Layers add up. U = 1 ÷ R.
- 1
Each layer's R
3.5 in × R-3.2= R-11.2 fibreglass+ 4 more layers
- 2
Add them up
5 layers in a row= R-13.24 (US)
- 3
US to SI
13.24 ÷ 5.678= 2.33 m²·K/W
- 4
U-value
U = 1 ÷ 2.33= 0.429 W/m²·K(0.0755 US)
Read more: the formula, a worked example, the material table, heat loss and degree-days
Heat moves through a wall, roof or floor one layer after another, so each layer's resistance adds to the next, like water passing through a row of filters. This is the simple "layers in series" method. It does not count studs, joists, wall ties or gaps in the insulation, which let more heat through, so the real figure is usually a little lower than the one here.
The formula
R of a layer (US) = thickness in inches × R per inch
R of a layer (SI) = thickness in metres ÷ λ (W/m·K)
air films and air gaps = a fixed R
total R = R of layer 1 + R of layer 2 + … + R of the last layer
RSI (m²·K/W) = US R ÷ 5.678263
U-value = 1 ÷ total R
heat loss (W) = U (W/m²·K) × area (m²) × (inside − outside temperature in °C)
BTU/h = W × 3.412
heat a year (kWh) = U × area × heating degree-days (°C·days) × 24 ÷ 1000
energy bought = heat a year ÷ heating efficiency; cost = energy bought × price per kWh
thickness for a target = (target R − R of the other layers) ÷ R per inch (or × λ)
Worked example
A timber-frame wall, from the outside in: outside air film RSI 0.04, ½ in plywood at R-1.25 per inch, 3.5 in fibreglass batt at R-3.2 per inch, ½ in drywall at R-0.9 per inch and the inside air film RSI 0.13. In US units the films are 0.04 × 5.678263 = R-0.23 and 0.13 × 5.678263 = R-0.74. The plywood is 0.5 × 1.25 = R-0.625, the batt 3.5 × 3.2 = R-11.2 and the drywall 0.5 × 0.9 = R-0.45. Added up: 0.227 + 0.625 + 11.2 + 0.45 + 0.738 = R-13.24 (US). In SI that is 13.24 ÷ 5.678263 = 2.33 m²·K/W, so the U-value is 1 ÷ 2.332 = 0.429 W/m²·K (0.0755 in US units). The wall is 4.5 in (114.3 mm) thick.
With the Advanced defaults, 10 m² of this wall with 20 °C inside and 0 °C outside loses 0.429 × 10 × 20 = 85.8 W, or 85.8 × 3.412 = 293 BTU/h. With 2,500 °C·days a year it lets out 0.42886 × 10 × 2,500 × 24 ÷ 1000 = 257.3 kWh of heat. With an electric heater (100%) at $0.15 per kWh, that is about $38.60 a year. To reach R-20 (US), the other layers give 13.24 − 11.2 = R-2.04, so the batt needs (20 − 2.04) ÷ 3.2 = 5.61 in, which is 2.11 in more than now.
Typical values for common materials (check your product)
| Material | R per inch (US) | λ (W/m·K) | RSI per 100 mm |
|---|---|---|---|
| Fibreglass batt | 3.2 | 0.0451 | 2.22 |
| Mineral wool (rock wool) | 3.8 | 0.038 | 2.63 |
| Cellulose (blown) | 3.5 | 0.0412 | 2.43 |
| EPS foam board | 4 | 0.0361 | 2.77 |
| XPS foam board | 5 | 0.0288 | 3.47 |
| Polyiso (PIR) board | 6 | 0.024 | 4.16 |
| Spray foam, open cell | 3.7 | 0.039 | 2.57 |
| Spray foam, closed cell | 6.5 | 0.0222 | 4.51 |
| Plywood | 1.25 | 0.1154 | 0.87 |
| OSB | 1.4 | 0.103 | 0.97 |
| Drywall (plasterboard) | 0.9 | 0.1603 | 0.62 |
| Softwood timber | 1.25 | 0.1154 | 0.87 |
| Brick | 0.2 | 0.7211 | 0.14 |
| Concrete block (hollow) | 0.14 | 1.0302 | 0.1 |
| Concrete | 0.08 | 1.8028 | 0.06 |
| Cement plaster / render | 0.2 | 0.7211 | 0.14 |
| Air film or gap | RSI (m²·K/W) | R (US) |
|---|---|---|
| Air film inside (wall) | 0.13 | 0.74 |
| Air film inside (ceiling, heat going up) | 0.1 | 0.57 |
| Air film outside | 0.04 | 0.23 |
| Sealed air gap (25 mm / 1 in or more) | 0.18 | 1.02 |
These are typical round figures, not ratings. Real products vary with density, brand, age and moisture: foam boards made with blowing agents lose some R over the years, and wet or squashed insulation does much worse. Use the R-value or λ (lambda) printed on the product or its data sheet. The air film figures are the usual ISO 6946 surface resistances for walls (heat flow sideways) and ceilings (heat flow up); US ASHRAE tables use about R-0.68 inside and R-0.17 outside.
US and SI units
- US R-value is in ft²·°F·h/BTU. It is the number printed on US and Canadian batts, like R-13 or R-19.
- SI R-value (RSI) is in m²·K/W. 1 RSI = 5.678263 US R. Australia, New Zealand, the UK and Europe use it.
- λ (lambda) or k is the thermal conductivity of the material, in W/m·K. Lower is better. R per inch = 0.14423 ÷ λ.
- U-value is 1 ÷ R: the heat that gets through each m² (or ft²) for each degree of difference. Lower is better.
Degree-days
Heating degree-days add up, day by day, how far the outside temperature was below a base temperature (often 15.5 °C, 18 °C or 65 °F) over a year. Your national weather service or a degree-day website lists them for your town. °F·days are 1.8 times °C·days for the same weather. The yearly figure here is only the heat through this one area; windows, the roof, the floor and draughts add more.
Tips
- Seal air leaks first: gaps and draughts can lose more heat than a thin wall.
- Insulation only works when it fills the space: gaps, folds and squashed batts cut the R-value.
- Insulation outside the studs or block (a continuous layer) stops the heat that leaks through the frame.
- For a new build or a grant, ask your designer for the U-value method your local rules accept.
Common questions
How do you work out the R-value of a wall?
Work out the R-value of each layer and add them up. For a material, R = thickness in inches × its R per inch (US), or thickness in metres ÷ its thermal conductivity λ (SI). Air films and air gaps have a fixed R. Layers that sit one behind the other add straight up, so a wall of R-0.23, R-0.63, R-11.2, R-0.45 and R-0.74 is R-13.24 in total.
What is the difference between R-value and U-value?
R-value is how well a layer or a wall resists heat flow: higher is better. U-value is how much heat gets through: lower is better. U = 1 ÷ R. So a wall of R 2.33 m²·K/W has a U-value of 1 ÷ 2.33 = 0.429 W/m²·K.
How do I convert US R-value to SI (RSI)?
Divide the US R-value by 5.678 to get RSI in m²·K/W, or multiply RSI by 5.678 to get US R. So R-13 (US) is about RSI 2.29, and RSI 3 is about R-17 (US).
How do I get R-value from thermal conductivity (λ or k)?
R = thickness in metres ÷ λ in W/m·K. 100 mm of insulation with λ 0.035 has R = 0.1 ÷ 0.035 = 2.86 m²·K/W, which is R-16.2 in US units. Data sheets in Europe and the UK usually give λ; in the US and Canada they give R per inch or the R of the product.
How much heat is lost through a wall?
Heat loss in watts = U-value × area in m² × the temperature difference in °C (or K). A 10 m² wall with U 0.429 W/m²·K, 20 °C inside and 0 °C outside, loses 0.429 × 10 × 20 = 85.8 W. Multiply watts by 3.412 for BTU per hour.
Do wooden studs lower the R-value?
Yes. This calculator adds the layers as if each one runs across the whole wall. Studs, joists, steel ties and mortar joints are better at carrying heat than insulation, so the real wall is a bit worse. Many people knock 10–25% off a timber-frame wall's figure for the studs, or use a full method such as ISO 6946 or ASHRAE's parallel path method.
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