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
Why is rain so loud on a metal roof, and how do I reduce it?
In short
Raindrops hitting a metal sheet make the thin, light steel vibrate across its whole area, so the roof radiates sound downwards like a drum skin. There are two places to deal with it: at the roof, with an insulation blanket held tight against the underside of the sheet to damp the vibration, or an insulated sandwich roof panel; and at the ceiling, with an airtight ceiling without openings and sound insulation above it — plus a second ceiling layer if it is still too loud. The result depends on the whole roof and ceiling system, so no product can promise a specific dB reduction.
Why is rain louder on a metal roof than on tiles?
Rain noise is impact sound. Each drop strikes the roof surface; the sheet that takes the hit vibrates and radiates sound both upwards and downwards. Metal roofing is thin steel with little mass per square metre and high stiffness, so it vibrates easily and radiates sound well. Heavier roofs such as tiles or a concrete slab vibrate less.
The noise you hear in the room therefore comes in two steps:
- The roof sheet vibrates and radiates sound into the roof space
- The sound passes through the ceiling (or through openings in it) into the room
Rooms without a ceiling, open to the roof — such as loft rooms or carports — get the full noise because there is no second layer.
Rain noise can be measured in the laboratory with artificial rain under ISO 10140 (an annex of ISO 10140-1 covers rainfall sound on roofs and roof/ceiling systems, and ISO 10140-5 specifies the artificial rain source; this replaced the former ISO 140-18). The result belongs to the roof system tested as a whole, not to one insulation product.
Fix it at the roof: reduce vibration at the source
Steel roofing manufacturer Lysaght recommends laying an insulation blanket over the foil laminate or support mesh before the roofing goes on, with the laminate pulled tight enough to hold the blanket hard against the underside of the sheet. The blanket damps the vibration where the rain strikes. If the blanket is not in contact with the sheet, the noise reduction is smaller.
- Suits new houses or re-roofing, because it must be installed before the sheets are fixed.
- These blankets also reduce heat from the roof, for example SCG SUPER COOL (foil-encapsulated glass wool for metal roofs) or ROCKWOOL Cool 'n' Comfort RL (stone wool roll for single-skin metal roofs).
- Sandwich (insulated) roof panels, with insulation between two steel skins, are another option for new work. Acoustic performance depends on the panel model — ask the roofing manufacturer for data.
- Keep the insulation dry, especially at roof edges and gutters. Insulation that stays damp loses performance and can damage the coating on the underside of the roofing.
Fix it at the ceiling: stop the sound reaching the room
In a finished house the roof usually cannot come off, so the practical route is to turn the ceiling into a sound-insulating layer. The logic is the same as a sound-insulating wall: mass, airtightness and insulation in the cavity (why cavity insulation improves sound insulation).
- The ceiling must be airtight. A plastered, jointed flat ceiling insulates sound better than a T-bar ceiling whose tiles rest on a grid with gaps around them. Ceiling vents, access hatches and downlight holes all let sound through (gaps and compression).
- Lay sound insulation over the whole ceiling, for example glass or stone wool slabs such as ROCKWOOL Safe 'n' Silent Pro, butted together with no gaps.
- If it is still loud, add a second ceiling. Hang a second ceiling frame below the existing ceiling, fill the gap between the two with insulation, then board and joint it. Two separated ceiling layers work like a double wall. Room height drops by the depth of the system.
- Use surface-mounted lights instead of cutting holes for recessed fittings, so the sound-insulating layer stays intact.
- Sound can still travel by other paths, such as wall sections above the ceiling or vents connected to the roof space (flanking sound transmission).
New house or existing house — what can be done?
| Situation | Options to consider |
|---|---|
| --- | --- |
| New house / re-roofing | Insulation tight under the roof sheet, or sandwich panels, and plan the sound-insulating ceiling at the ceiling stage — usually cheaper than fixing it later |
| Existing house, roof stays | Seal leaks in the ceiling, add sound insulation above it, and for more effect add a second ceiling layer |
| Bedrooms or rooms that need to be especially quiet | Focus on an airtight double ceiling, and also check windows and walls (outside noise in the bedroom) |
Check loads and safety first
Insulation and a second ceiling add weight to the roof framing and ceiling frame. Have an engineer or knowledgeable contractor confirm the structure can take the extra load. Roof work is work at height, and working in the roof space carries the risk of falling through the ceiling — use experienced installers. See the precautions in how to insulate a ceiling properly.
Common misconceptions
- "Any thermal insulation stops rain noise." Not necessarily. Thin foil or thin foam helps with heat, but acoustic effect varies a lot. Rain-noise reduction depends on where it is installed and on the system, not only on the material. See the two functions in soundproofing vs sound absorption.
- Laying insulation on a T-bar ceiling with gaps around the tiles and expecting silence — sound leaks through the gaps; insulation helps only so much.
- Cutting in downlights or vents after building a sound-insulating ceiling — this undoes much of the investment.
- Trusting dB figures without test conditions — sound insulation values belong to the tested roof or ceiling system. See what STC and Rw mean.
A bonus: insulation under the roof and above the ceiling also reduces heat. Read how to choose roof insulation and how heat gets into a building, or browse sound insulation · thermal insulation.
Sources
- ISO 10140-1 (annex on rainfall sound for roofs and roof/ceiling systems) and ISO 10140-5 (artificial rain source) — laboratory measurement of rain noise
- Fraunhofer IBP — Rain noise of roof constructions (testing to ISO 140-18 / ISO 10140; influence of sheet material and insulation)
- Lysaght (BlueScope) — Rain Noise Considerations for Steel Roofs (blanket must be held hard against the underside of the roofing to damp vibration)
- Lysaght (BlueScope) — Using Blankets and Batts as Insulation (protecting blankets from moisture at roof edges)
- Fielders — Roofing Acoustics (insulation against the underside of a metal deck damps rain and hail impact noise)
Related products
Categories and example products for this topic
- ROCKWOOL Cool 'n' Comfort RL — Stone wool roll under single-skin metal roofs, made for heat and sound absorption
- SCG SUPER COOL-G — Foil-wrapped glass wool for roofs; maker lists it for heat and rain noise on metal roofs
- ROCKWOOL Cool 'n' Comfort SL — Stone wool slab fitted between the two sheets of a double-skin roof, for heat and noise
- ROCKWOOL Safe 'n' Silent Pro — At the ceiling: stone wool in the void above an airtight ceiling, as part of the system
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 · 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
Sound insulation · Stone wool
ROCKWOOL Safe 'n' Silent Pro
- Thickness
- 25–100 mm
- Density
- 40–100 kg/m³
- Sound absorption (NRC)
- 0.90–1.00