Building thermal insulation · Glass wool
SCG STAY COOL
- Thickness
- 75–150 mm
- Thermal resistance (R)
- 1.79 m²·K/W (75 mm)
How to choose insulation thickness
In short
Thickness directly sets the R-value: with the same material, twice the thickness gives twice the R-value, but the heat saved grows less and less as you keep adding thickness. The right thickness depends on your goal, the space available, the weight the structure can carry, your budget and, for cold pipes and ducts, the design conditions for preventing condensation. There is no single correct thickness for every job.
- 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 much less heat does extra thickness give?
Heat through an insulation layer is inversely proportional to its R-value. Doubling R halves the heat through that layer, but the extra saving at each step gets smaller.
| Hypothetical example (insulation layer only) | Relative R | Heat through (vs first step) | Extra reduction vs previous step |
|---|---|---|---|
| --- | --- | --- | --- |
| 1× thickness | 1 | 100% | — |
| 2× thickness | 2 | 50% | 50 points |
| 3× thickness | 3 | about 33% | about 17 points |
| 4× thickness | 4 | 25% | about 8 points |
This is a mathematical principle, not the result for any product or building. A real building also has other layers, air surfaces, thermal bridges, windows and air leaks. The first layers of thickness usually pay off most; after that, weigh whether more thickness is worth the cost and the space.
What to check before choosing a thickness
1. The goal of the job
Comfort, lower air-conditioning bills, meeting a figure set by the designer (such as the roof U-value), or preventing condensation on a pipe surface — each goal is worked out differently.
2. Space available
The height of the roof space, the gap between purlins and roof sheet, or the depth of a wall cavity limits the thickness you can fit. Do not squeeze thick insulation into a narrower space: compressed insulation does not give its full rated R-value.
3. Weight the structure can carry
Thicker or denser insulation weighs more, especially when laid on a ceiling or on T-bar ceiling tiles. Check with the installer or the ceiling manufacturer that the frame can take the extra load. See how to insulate a ceiling properly.
4. The material's λ
A material with a lower λ needs less thickness for the same R-value, which helps where space is tight. Always compare by R at the real thickness.
5. Budget and the thicknesses on sale
Each product comes in a set of standard thicknesses, e.g. SCG STAY COOL ceiling insulation or ROCKWOOL Cool 'n' Comfort RL stone-wool roof insulation. See the manufacturer's documents for the thicknesses available and the R-value of each.
Cold pipes and AC ducts: thickness depends on design conditions
For surfaces colder than the surrounding air — refrigerant pipes, chilled-water pipes and cold air ducts — the insulation must be thick enough to keep its outer surface warmer than the dew point of the surrounding air, or condensation will form. The thickness needed therefore depends on the temperature inside the pipe, the temperature and humidity of the surrounding air, the pipe size and the insulation's λ.
No ready-made thickness for every pipe
A thickness commonly used in one place may not be enough in a more humid room or on a colder pipe. Have the designer or manufacturer calculate it for the real conditions. Read dew point and condensation and how to choose pipe insulation.
Common mistakes
- Choosing by thickness alone, without checking that product's λ or R-value
- Buying thick insulation but not covering the whole area, leaving gaps at edges and joints
- Squeezing thick insulation into a narrow space until it is flat
- Copying the thickness from one job to another with different conditions
If you would like help choosing a thickness, tell us where it will go, the space available and the goal. Browse thermal insulation.
Sources
- ISO 6946:2017 — Building components and building elements — Thermal resistance and thermal transmittance — Calculation methods
- U.S. Department of Energy / ENERGY STAR — Insulation Fact Sheet (R depends on type, thickness and density; effect of compression)
- ASTM C680 — Standard Practice for Estimate of the Heat Gain or Loss and the Surface Temperatures of Insulated Flat, Cylindrical, and Spherical Systems (surface temperature of insulated systems)
- Green Building Advisor / Energy Vanguard — articles on the diminishing returns of adding insulation (supporting the principle)
Related products
Categories and example products for this topic
- SCG STAY COOL — Two thicknesses, 75 and 150 mm; R 1.79 m²·K/W at 75 mm (insulation only)
- ROCKWOOL Cool 'n' Comfort RL — Maker's R-value at 50 mm: 1.43 (RL920) and 1.47 (RL940) m²·K/W
- Microfiber MicroRoof FLS — 25, 50 and 75 mm in several densities - compare R-values in the size table
- AEROFLEX Tube Insulation — Cold pipes: elastomeric tube with 6–50 mm wall thicknesses to match design conditions
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
Pipe insulation · Elastomeric foam
AEROFLEX Tube Insulation
- Tube bore
- 6–165 mm
- Wall thickness
- 6–50 mm
- Service temp.
- -57 – 125 °C