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
What is a thermal bridge?
In short
A thermal bridge is a part of a roof, wall or ceiling where heat passes far more easily than through the insulated area around it — because a heat-conducting material crosses the insulation layer, or because the insulation is interrupted or thinner there. Examples are steel purlins, metal framing, screws and gaps between insulation slabs. The result is that the real assembly insulates less than the R-value printed on the insulation. The remedy is to make the insulation layer as continuous as possible.
A picture to keep in mind
Heat always takes the easiest path. Think of insulation as a dam: if metal pipes pass through it at many points, water flows through the pipes however thick the rest of the dam is. Steel conducts heat several hundred times better than insulation, or more, so a small piece of steel crossing the insulation carries far more heat than its share of the area.
Where they occur
| Location | How it happens |
|---|---|
| --- | --- |
| Steel purlins of a metal-sheet roof | Roll insulation laid across the purlins is squeezed thin by the roof sheet along each purlin, and the purlin touches the hot sheet directly |
| Metal framing of lightweight walls and ceilings | Insulation sits only between the frame members; heat travels through the frame itself |
| Screws, fixing pins, hanger wires | Many small pieces of metal pierce the insulation layer at points |
| Concrete columns, beams, slab edges, balconies | Structure runs from outside to inside with no insulation in between |
| Joints and gaps between insulation slabs | The insulation is interrupted and hot air can slip through |
| Around recessed lights, access hatches, ceiling edges | Insulation has to be held back or cut out |
Designers sort thermal bridges into repeating ones such as framing and purlins, linear ones along structural junctions such as slab edges and wall corners, and point ones such as fixings.
Why it matters
- The assembly's R-value is lower than the insulation's — the R-value on the label belongs to the insulation alone. The heat that actually enters the building depends on the whole assembly, expressed by a U-value that includes the framing.
- The effect is especially large with metal framing — Building Science Corporation, summarising North American walls, reports that steel studs take away roughly half or more of the effectiveness of cavity insulation, while timber framing loses much less. Actual figures depend on stud spacing, steel thickness and system details, so treat this as a trend, not a value for every wall.
- Thicker cavity insulation gives diminishing returns — because the heat that bypasses through the framing does not fall with it.
- Local hot or cold spots appear — in cold rooms or air-conditioned rooms, a spot where metal carries the cold through to the surface can be below the dew point of the surrounding air, so drips form in lines along framing or at screw heads.
Why continuous insulation matters
Continuous insulation is an insulation layer that runs across the face of the structure without being cut by the framing, so every heat path has to cross at least one layer of insulation. Principles that apply to everyday work:
- Have at least one layer the structure does not interrupt — for example a roll laid continuously across the purlins, or an extra layer over the face of the framing.
- Do not let insulation be squeezed thin at supports — use spacers or fixings as the roofing and insulation manufacturers recommend. Some insulation manufacturers' documents call for roof-sheet fixings that do not press on the insulation at the purlins.
- Butt joints tightly, and with two layers stagger the joints. See gaps and compression.
- Use thermal breaks — a low-conductivity material between two metal parts, where the designer specifies.
- Keep the foil facing continuous so humid air cannot reach cold metal. See sealing insulation joints.
Real buildings always need structure and fixings. The aim is to cut the number and size of shortcuts, not to reach zero. Where a required U-value must be met, have the designer calculate it with the structure included.
Examples with commonly used products
- Single-skin metal roof — roll insulation laid continuously across the purlins, such as stone wool ROCKWOOL Cool 'n' Comfort RL or foil-encapsulated glass wool SCG SUPER COOL-G. Details in metal roof insulation.
- Framed lightweight wall — slabs filling the frame cavity, such as ROCKWOOL Thermalrock S, with slab edges butted tightly to the frame. See hot west-facing walls.
Sources
- Building Science Corporation — BSD-011: Thermal Control in Buildings
- Building Science Corporation — BSI-005: A Bridge Too Far
- Green Building Advisor — Insulating a Metal Building
- SCG — SCG Thermal Insulation for Roof brochure (installation over purlins)
- ROCKWOOL — Cool 'n' Comfort Blanket and Slab Data Sheet
Related products
Categories and example products for this topic
- ROCKWOOL Cool 'n' Comfort RL — Stone wool roll for single-skin roofs, laid over foil and mesh on the purlins
- SCG SUPER COOL-G — Foil-encapsulated glass wool roll laid over or between purlins on wire mesh
- ROCKWOOL Thermalrock S — Stone wool slab 1,200 × 600 mm in several densities, for framed cavities
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 Thermalrock S
- Thermal conductivity (λ)
- 0.034–0.036 W/m·K
- Sound absorption (NRC)
- 0.90–1.05