Insulation R-Value Calculator
R-value calculator — enter the thickness and thermal conductivity of each material layer to get the total thermal resistance (R-value) and the U-value of a wall, roof or floor build-up. Add up to four layers. Runs in your browser.
Insulation R-Value Calculator
How to Use the R-Value Calculator
Enter layer 1
Thickness in mm and the material’s k-value.
Add more layers
Up to four — insulation, board, blockwork.
Read total R
The layer resistances are summed automatically.
Check U-value
Compare against the regs’ maximum U-value.
How Insulation Adds Up
Whether a wall, roof or floor keeps heat in comes down to two numbers that are simply reciprocals of each other: the R-value and the U-value. R-value measures thermal resistance — how strongly the build-up opposes heat flow — and higher is better. U-value measures the rate at which heat passes through, and lower is better. Because they are inverses, U equals one divided by the total R. This calculator lets you build up an assembly layer by layer and reports both, so you can see how a design performs and how it compares against the maximum U-values that building regulations set.
The underlying physics is reassuringly simple. The R-value of a single layer is its thickness in metres divided by its thermal conductivity, the k-value, in watts per metre-kelvin. A lower k means a better insulator: mineral wool sits around 0.038, rigid PIR boards around 0.022, timber around 0.13, and dense concrete well above 1. Crucially, thermal resistances in series simply add, so the total R-value of a wall made of plasterboard, insulation and blockwork is just the three layer R-values summed. That additive behaviour is why this tool can take up to four layers and give an immediate total, and why R-value scales directly with thickness — doubling an insulation layer doubles its contribution, which is exactly why topping up loft insulation is so effective.
A couple of honest limits keep the result in perspective. This calculator sums the solid material layers you enter; a full building-physics calculation also adds the resistance of the thin air films on each surface and of any air cavities, all of which raise the total R-value, and it accounts for thermal bridging through studs and joists, which lowers real-world performance. It also offers both the SI R-value (m²·K/W) that the rest of the world uses and the US R-value (multiply SI by about 5.678), so you can work to either convention. Use it to understand the principle, to compare build-ups, and to see the dramatic effect of a better insulant or more thickness — then rely on a specialist’s full U-value calculation for regulatory compliance. As always, the whole calculation runs in your browser, so nothing you enter leaves your device.
Thermal resistances add in series — which is why every extra layer, and every extra millimetre of insulation, counts directly toward the total.
10 Facts About R-Value
R-value measures thermal resistance — higher is better.
U-value is the inverse of R-value — lower is better.
R of a layer = thickness ÷ conductivity (k).
Layered R-values simply add together.
Mineral wool has a k around 0.038 W/m·K.
PIR/PUR boards insulate better, k ≈ 0.022.
SI R-value × 5.678 ≈ the US R-value.
Building regs set maximum U-values, not R.
Air gaps and surfaces add extra resistance.
This calculator runs in your browser — nothing is uploaded.
Frequently Asked Questions
- R-value is a measure of thermal resistance — how well a material or build-up resists heat flow. The higher the R-value, the better the insulation. For a single layer it is calculated as the thickness (in metres) divided by the material’s thermal conductivity, known as its k-value. This calculator computes it for each layer and sums them.
- They are inverses. R-value measures resistance to heat flow (higher is better); U-value measures the rate of heat transfer (lower is better). U-value equals one divided by the total R-value. Building regulations are usually written as maximum U-values for walls, roofs and floors, so the calculator gives both.
- Thermal resistances add in series, so you simply sum the R-value of each layer to get the total for the build-up. A wall with plasterboard, insulation and blockwork has a total R-value equal to the three individual R-values added together. This tool lets you enter up to four layers and adds them automatically.
- The k-value, in watts per metre-kelvin, describes how readily a material conducts heat — lower means a better insulator. Mineral wool is around 0.038, rigid PIR/PUR boards around 0.022, timber around 0.13, and dense concrete well over 1. You can usually find a product’s k-value on its datasheet.
- As rough guides in W/m·K: mineral/glass wool 0.035–0.040, EPS 0.036, XPS 0.034, PIR/PUR 0.022–0.026, timber 0.13, plasterboard 0.16, dense concrete block 1.1–1.3, brick 0.77. Always prefer the figure on the actual product datasheet, as formulations vary between manufacturers.
- They use different units. The SI R-value (m²·K/W) is what this calculator computes by default; the US R-value uses imperial units (ft²·°F·h/BTU) and is numerically larger. To convert, multiply the SI R-value by about 5.678. The tool shows both so you can compare against either standard.
- No — it sums the solid material layers you enter. A complete building-physics calculation also adds the thermal resistance of internal and external air films and any ventilated or unventilated air cavities, which increase the total R-value. For regulatory compliance, those surface and cavity resistances should be included by a specialist.
- Because R-value scales directly with thickness for a given material: doubling the thickness of an insulation layer doubles its R-value. That is why adding depth to loft insulation is so effective, and why thin high-performance boards like PIR achieve the same R-value as much thicker mineral wool — their lower k-value compensates for less thickness.
- The layer arithmetic is exact for the values you enter, but a real assembly’s performance also depends on air films, cavities, thermal bridging through studs or joists, and workmanship, none of which this simple sum captures. Use it to compare build-ups and understand the principle; rely on a full U-value calculation for compliance.
- Completely free, with no account or limit. It works offline once the page has loaded and collects no data.
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