Building thermal insulation · Glass wool
SCG STAY COOL
- Thickness
- 75–150 mm
- Thermal resistance (R)
- 1.79 m²·K/W (75 mm)
What is thermal resistance (R-value)?
In short
Thermal resistance (R-value) shows how strongly a layer of material at a given thickness resists heat flow. The higher it is, the more heat it resists. For a uniform layer, R = thickness (m) ÷ λ. The SI unit is m²·K/W; US documents use ft²·°F·h/Btu, which gives very different numbers, so never compare the two directly.
- 1λ (thermal conductivity, W/m·K): a property of the material; lower means less heat conducted — independent of thickness
- 2R (thermal resistance, m²·K/W): for a layer of given thickness, R = thickness ÷ λ; the same material in a thicker layer gives a higher R
- 3U (W/m²·K): the heat transfer rate through a whole wall or roof, all layers and surface air films included; lower means less heat passes
How is R-value different from λ?
- λ (thermal conductivity) is a property of the material, independent of thickness. Lower is better.
- R belongs to a layer of a set thickness. Higher is better.
So if the same material is made twice as thick, its R-value doubles.
How is R calculated?
For a uniform layer: R = d ÷ λ, where d is the thickness in metres.
Hypothetical example (not data for any product): insulation with λ = 0.040 W/m·K, 75 mm thick, gives R = 0.075 ÷ 0.040 ≈ 1.88 m²·K/W.
When layers are stacked, their R-values add up. For example, one layer of R 1.88 plus another of R 0.50 (hypothetical) gives R 2.38 m²·K/W. This is the basis for working out the U-value of a whole wall or roof.
SI and imperial units — do not mix them
| System | Unit | Where you see it |
|---|---|---|
| --- | --- | --- |
| SI (sometimes written RSI) | m²·K/W | European and Asian documents and most manufacturers in Thailand |
| Imperial (US) | ft²·°F·h/Btu | US documents and websites, e.g. "R-13", "R-30" |
1 m²·K/W is about 5.678 ft²·°F·h/Btu. So insulation rated R 1.88 in SI units would be written as roughly R-10.7 in imperial units, although it is the same product. If an R figure looks unusually high, check the unit first.
An R figure without a unit cannot be compared
If a document only says "R-10" without a unit, ask the seller or manufacturer which unit it uses and at what thickness it was measured.
How to compare insulation by R-value
- Compare at the thickness you will actually use, not by λ alone.
- Use the same unit, and make sure the λ used in the calculation was measured at a similar mean temperature.
- Think about installation — insulation that is compressed will not give its full rated R-value, and gaps between pieces reduce the overall result.
- A material's R is not the result for the whole roof or wall — the whole building element is described by the U-value, which includes every layer and the air surfaces.
A thin foil sheet has almost no R-value from its own material. Its effect depends on the air space in front of the foil surface and on the direction of heat flow, so do not compare it with bulk insulation by an R figure for the sheet alone. Read how reflective foil works.
For the R-value of a specific product, see the manufacturer's datasheet at that thickness, or ask us. To choose a thickness, read how to choose insulation thickness.
Sources
- ISO 7345:2018 — Thermal performance of buildings and building components — Physical quantities and definitions
- ISO 6946:2017 — Building components and building elements — Thermal resistance and thermal transmittance — Calculation methods
- ASTM C168 — Standard Terminology Relating to Thermal Insulation
- U.S. Department of Energy / ENERGY STAR — Insulation Fact Sheet (R-values add up; effect of compression)
- Virginia Cooperative Extension — ENERGY SERIES: What about Radiant Barriers? (radiant barriers and R-value)
- NIST Special Publication 811 — Guide for the Use of the International System of Units (unit conversion)
Related products
Categories and example products for this topic
- SCG STAY COOL — R 1.79 m²·K/W at 75 mm (SI units, insulation only)
- ROCKWOOL Rock Chill — R 12.17 hr·ft²·°F/Btu at 75 mm - imperial units, do not compare directly with SI
- ROCKWOOL Cool 'n' Comfort RL — R at 50 mm thickness: 1.43 (RL920) and 1.47 (RL940) m²·K/W
- Microfiber MicroRoof FLS — Has a table of R-values by thickness and density, based on λ at 24 °C
Building thermal insulation · Stone wool
ROCKWOOL Rock Chill
- Thickness
- 75 mm
- Width
- 600 mm
- Length
- 4,000 mm
Building thermal insulation · Stone wool
ROCKWOOL Cool 'n' Comfort RL
- Thickness
- 25–50 mm
- Density
- 40–60 kg/m³
- Thermal conductivity (λ)
- 0.034–0.035 W/m·K
Building thermal insulation · Glass wool
Microfiber MicroRoof FLS
- Thickness
- 25–75 mm
- Density
- 12–48 kg/m³
- Thermal conductivity (λ)
- 0.032–0.042 W/m·K