Thermal resistance is thickness divided by conductivity. Conductivity, written as lambda and measured in watts per metre per kelvin, is a property of the material: mineral wool sits around 0.038 to 0.044, expanded polystyrene near 0.038, polyisocyanurate board around 0.022, and a vacuum insulated panel below 0.008. Divide the thickness in metres by that figure and you have R-value in m²K/W — so 150 mm of mineral wool at 0.038 W/mK gives R = 3.95.
U-value is the reciprocal of total resistance and is the number building regulations actually specify. Total resistance means the insulation plus the surface air films and any other layers, which is why a wall never quite achieves the U-value the insulation alone would suggest. Adding roughly 0.18 m²K/W for internal and external surface resistances to the example above gives a U-value of 0.242 W/m²K.
The practical consequence is diminishing returns. Going from no insulation to 100 millimetres removes most of the heat loss; going from 200 to 300 millimetres removes very little more, because you are adding resistance to something that is already resistive. That is why loft insulation recommendations plateau around 270 to 300 millimetres rather than continuing upward.
The formula
thickness- Insulation depth in millimetres
λ- Thermal conductivity in W/mK, from the product datasheet
R- Thermal resistance in m²K/W — higher is better
U- Heat transfer per m² per kelvin — lower is better
ΔT- Temperature difference across the element
How it works, step by step
- Enter the insulation thickness in millimetres.
- Pick the material, or enter your own conductivity from the datasheet.
- Set the area and the design temperature difference.
- The gauge shows R-value; U-value and heat loss appear below.
Worked examples
150 mm of mineral wool
R is 0.150 ÷ 0.038 = 3.95 m²K/W. Adding 0.18 for surface resistances gives a total of 4.13 and a U-value of 0.242 W/m²K. Across 40 m² at a 24 K difference that is 233 W of heat loss.
PIR board does the same job thinner
To match R = 3.95 with PIR at λ 0.022 you need only 87 mm — about 42 per cent less depth. That is why board is used where floor build-up or reveal depth is constrained, despite costing more per square metre.
How to read your score
Frequently asked questions
Is a higher R-value better, or a lower one?
Higher R is better because it measures resistance to heat flow. U-value is the inverse and lower is better. Building regulations are written in U-values, product marketing tends to quote R-values, and confusing the two leads people to buy the wrong thing.
Why does the calculated U-value differ from the manufacturer figure?
Because a real element has more layers than the insulation: plasterboard, blockwork, render, air gaps and the surface films all add resistance, while timber studs or joists bridge it and reduce it. A full calculation weights the bridged and unbridged areas, which usually lands somewhat worse than the insulation alone.
How much loft insulation is enough?
UK guidance settles around 270 to 300 millimetres of mineral wool. Beyond that the extra resistance saves so little heat that it will not repay the material cost, and you risk compressing the layer or blocking eaves ventilation.
Do air gaps help or hurt?
A sealed, unventilated cavity adds a useful amount of resistance, roughly 0.18 m²K/W for a 25 millimetre gap. A ventilated or draughty gap adds almost nothing because air moves through it and carries heat with it, which is why sealing matters as much as thickness.
Insulation R-value reference
| Thickness | Mineral wool λ0.044 | Batt/EPS λ0.038 | Phenolic λ0.032 | PIR λ0.022 |
|---|---|---|---|---|
| 25 mm | 0.57 | 0.66 | 0.78 | 1.14 |
| 50 mm | 1.14 | 1.32 | 1.56 | 2.27 |
| 75 mm | 1.70 | 1.97 | 2.34 | 3.41 |
| 100 mm | 2.27 | 2.63 | 3.12 | 4.55 |
| 150 mm | 3.41 | 3.95 | 4.69 | 6.82 |
| 200 mm | 4.55 | 5.26 | 6.25 | 9.09 |
| 270 mm | 6.14 | 7.11 | 8.44 | 12.27 |
| 300 mm | 6.82 | 7.89 | 9.38 | 13.64 |
Add surface and other layer resistances before taking the reciprocal for a U-value.