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
How do you choose insulation for a metal-sheet roof?
In short
Start by checking whether the roof is single skin or double skin. A single-skin roof usually takes roll insulation laid on wire mesh over the purlins as the roofing goes on; a double-skin roof takes slabs placed between the upper and lower steel sheets. Then compare the R-value at the thickness you can actually install, the type of foil facing, how moisture is handled and the building's fire requirements. No single insulation suits every roof.
Why a metal roof gets so hot
Metal sheet is thin and conducts heat well. In the sun it heats up quickly and passes that heat down, both by conduction through the sheet and by radiation from its underside into the space below. With nothing in the way, that heat reaches people and machinery directly. For the overall picture see how heat enters buildings.
Insulation under the roof slows this heat down. ROCKWOOL's documentation explains that in buildings without air-conditioning roof insulation reduces solar heat coming through the roof, and in air-conditioned buildings it reduces cooling energy.
Look at the roof first: single or double skin
| Roof build-up | What it is | Insulation usually used |
|---|---|---|
| --- | --- | --- |
| Single skin | One layer of metal sheet fixed to the purlins; common in factories, warehouses, carports and houses | Roll insulation under the roof sheet, supported by wire mesh and foil over the purlins |
| Double skin | Upper and lower steel sheets joined by spacers, leaving a gap for insulation and a vapour-control layer | Semi-rigid slabs fitted to the gap |
| Existing roof with a ceiling | The roof is not being opened | Insulation laid on the ceiling — see ceiling insulation |
Single-skin roofing is the economical system used on most industrial buildings. Double-skin roofing is a higher-performance system for heat, sound and fire; the underside is a steel sheet, so it looks finished and the insulation is hidden.
Roll or slab: what is the difference?
- Roll / blanket — unrolls in long continuous lengths with few joints and goes down fast over large areas; suited to single-skin roofs where it is laid across the purlins.
- Slab / board — holds its shape and can be cut to fit; suited to the gap between the sheets of a double-skin roof or to the spaces between purlins.
Higher density makes insulation stiffer and better at keeping its shape, but it does not always mean better thermal performance. See insulation density.
Common installation methods on single-skin roofs
Manufacturers' guides describe these methods:
- Wire mesh over the purlins, insulation laid across them — done together with the roofing, so it suits new buildings or re-roofing.
- Wire mesh under the purlins, insulation laid between them — possible in new buildings and in existing buildings that are already roofed.
- Tie wires supporting the insulation — usable on steel or timber structures; follow the manufacturer's guidance on wire and purlin spacing.
When insulation is laid across the purlins and the roof sheet is fixed on top, the insulation is squeezed thin along each purlin, and heat passes there more easily than elsewhere. Some manufacturers recommend roof fixings that do not press on the insulation at the purlins. Read more in thermal bridges and gaps and compression.
Where does the foil go, and what does it do?
The aluminium-foil facing on roof insulation does several jobs at once:
- Reduces radiant heat — this works when the foil faces an air space. Foil pressed tightly against the roof sheet or another material conducts heat instead. See reflective foil.
- Slows water vapour — it helps keep humid air out of fibrous insulation. See water dripping under a metal roof.
- Acts as the finished surface — it encloses the insulation and is what you see from below.
On a single-skin roof without a ceiling, the foil therefore usually sits on the underside of the insulation, facing the space below. Insulation comes with foil on one side, on both sides or fully encapsulated. Follow the guide for the specific product for the correct orientation, and seal the joints continuously with foil tape. See sealing insulation joints.
What to compare
- R-value at the real thickness — R-value = thickness ÷ λ. An example from SCG SUPER COOL-G documentation: 25 mm (16 kg/m³) gives R = 0.658, while 75 mm (12 kg/m³) gives R = 1.786 m²·K/W. Thickness matters more than density.
- The thickness that will actually fit — it depends on purlin spacing, the depth of the gap and the fixing method. See insulation thickness.
- Facing and moisture control — in humid or steamy spaces the vapour-control layer must be continuous.
- Behaviour in fire — check the material's test results in the documents and the building's requirements. A material classification is not the fire-resistance rating of the roof system. See fire performance terms.
- Sound — fibrous insulation under the roof absorbs sound and reduces rain noise to some degree. See rain noise on a metal roof.
- Weight and structure — have the designer confirm that the roof structure can carry the insulation and mesh.
Our insulation for this job
- Single skin, stone wool roll — ROCKWOOL Cool 'n' Comfort RL: RL920 (40 kg/m³) for open, well-ventilated areas and RL940 (60 kg/m³) for residential roofs.
- Single skin, foil-encapsulated glass wool — SCG SUPER COOL-G, for the roofs of large buildings, laid over or between purlins on wire mesh.
- Single skin, glass wool with foil on one side — Microfiber MicroRoof FLS, available in many densities and thicknesses.
- Double skin, stone wool slab — ROCKWOOL Cool 'n' Comfort SL, fitted between the upper and lower roof sheets.
ROCKWOOL Rock Air RL (30 kg/m³) is another lightweight stone wool roll; please ask about the items and sizes currently available before ordering.
Roof work needs experienced installers
Installing roof insulation is work at height, and some roof sheets and translucent sheets cannot carry a person's weight. Use a contractor with fall-protection equipment and experience, keep the insulation dry throughout the work, and do not roof over wet insulation.
Common mistakes
- Choosing by density alone without checking R at the thickness used.
- Leaving foil joints open, or not repairing torn foil.
- Not covering the whole area, leaving gaps along the ridge and eaves.
- Expecting foil pressed against the roof sheet to reflect heat like foil facing an air space.
- Comparing advertised "% heat reduction" figures that were measured under different conditions. See how insulation saves air-conditioning energy.
Sources
- ROCKWOOL — Cool 'n' Comfort Blanket and Slab Data Sheet (single- and double-skin roof systems; applications of the RL and SL series)
- ROCKWOOL Thailand — Cool 'n' Comfort product page
- SCG — SCG Thermal Insulation for Roof brochure (installation methods, foil facings, R-value tables)
- Florida Solar Energy Center (FSEC) — Radiant Barriers: A Question and Answer Primer (foil must face an air space)
- Green Building Advisor — Insulating a Metal Building (insulation compressed at purlins and girts)
- Journal of Industrial Technology, Suan Sunandha Rajabhat University, Vol. 11 No. 1 (2023) — Thermal performance of thermal insulation installed under metal sheet roofs
Related products
Categories and example products for this topic
- ROCKWOOL Cool 'n' Comfort RL — Stone wool roll for single-skin metal roofs, in 40 and 60 kg/m³ grades
- ROCKWOOL Cool 'n' Comfort SL — Stone wool slab fitted between the two sheets of a double-skin metal roof
- SCG SUPER COOL-G — Foil-encapsulated glass wool laid over or between purlins on wire mesh
- Microfiber MicroRoof FLS — Glass wool roll, foil one side; densities 12–48 kg/m³, 25–75 mm thick
Building thermal insulation · Stone wool
ROCKWOOL Cool 'n' Comfort SL
- Thermal conductivity (λ)
- 0.034–0.035 W/m·K
- Thermal resistance (R)
- 1.43–1.47 m²·K/W
- Sound absorption (NRC)
- 1.00–1.05
Building thermal insulation · Glass wool
SCG SUPER COOL-G
- Thickness
- 25–75 mm
- Density
- 12–24 kg/m³
- Thermal conductivity (λ)
- 0.035–0.042 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