Loft insulation is the highest-return single upgrade available in most existing UK dwellings — a well-executed installation can reduce heat loss through the roof by 25–30% of whole-house energy demand. But “loft insulation” covers several distinct strategies, and choosing the wrong one for your roof type creates problems ranging from condensation to planning headaches.
Mineral Wool Rolls: Types and Grades
Glass wool and rock wool rolls are the standard product for cold loft floors. They are sold in rolls that typically cover 5–10 m² per pack, in widths of 370 mm (for rafters/joists at 400 mm centres) and 570 mm (for 600 mm centres), with lengths varying by pack size and thickness.
| Product | λ (W/mK) | Density (kg/m³) | Common thicknesses (mm) | Supply cost (£/m²) |
|---|---|---|---|---|
| Glass wool standard roll | 0.044 | 11–14 | 100, 150, 200 | £2–£4 |
| Glass wool enhanced roll | 0.032–0.036 | 16–22 | 100, 150, 200 | £4–£7 |
| Rock wool loft roll | 0.040 | 23–30 | 100, 150, 200 | £3–£6 |
| Rock wool acoustic/rigid batt | 0.036 | 45–60 | 50, 75, 100 | £5–£9 |
Glass wool rolls are significantly lighter and cheaper to handle; rock wool batts are stiffer and often preferred where some foot traffic across the loft floor is anticipated between joists (they don’t compress as easily). For pure thermal performance, either glass wool or rock wool at the right thickness achieves the same U-value.
The 270 mm Depth Explained
The commonly cited “270 mm” target for cold loft insulation comes from government and EST guidance that maps to approximately 0.16 W/m²K using standard glass wool (λ 0.044). The installation method is a two-layer approach:
- First layer (100 mm): laid between the ceiling joists, with the insulation sitting in the joist bays. This layer fills the depth of the structural timbers.
- Second layer (170 mm): laid across the joists at 90°, covering both the first layer and the tops of the joists (which are thermal bridges). This cross-laid approach eliminates cold bridging through the timber.
The resulting 270 mm total achieves approximately 0.15–0.16 W/m²K for a typical UK suspended ceiling construction. For new-build compliance with Part L 2021’s 0.11 W/m²K target, additional depth or a higher-performance product is needed:
| Target U-value | Glass wool (λ 0.044) depth | Enhanced glass wool (λ 0.032) depth |
|---|---|---|
| 0.20 W/m²K | ~220 mm | ~160 mm |
| 0.16 W/m²K | ~270 mm | ~200 mm |
| 0.13 W/m²K | ~335 mm | ~245 mm |
| 0.11 W/m²K (Part L new-build) | ~395 mm | ~290 mm |
For retrofit (existing buildings), Approved Document L for work to existing dwellings specifies a target of 0.16 W/m²K where practically achievable — 270 mm of standard glass wool sits comfortably within this. Insulation to 0.11 W/m²K is not typically required in retrofit unless Part O or SAP calculations make it necessary for compliance.
Cold Loft vs Warm Loft
The choice of strategy affects everything that follows — ventilation, vapour control, usable space and cost.
Cold loft (insulate at ceiling level): insulation sits on the ceiling/floor of the loft void. The loft space itself is cold (unheated, ventilated to the outside). This is the standard and cheapest approach for unused loft spaces.
- Ventilation requirement: 25 mm equivalent continuous eaves ventilation per BS 5250:2021
- No VCL (vapour control layer) needed at ceiling level in most cases; the permeable loft space allows moisture to escape
- Keep insulation clear of eaves — do not block ventilation path
Warm loft (insulate at rafter level): insulation is installed between and/or above rafters, bringing the loft space inside the thermal envelope. Used when the loft is to be used as habitable space, or when rafter-level insulation is more practical (e.g. where the ceiling follows the pitch).
- Requires a vapour control layer (VCL) on the warm side of insulation
- Thermal bridge through rafters must be addressed (insulate between and below rafters, or use rigid boards above deck)
- No requirement for cross-loft ventilation (the rafter void is sealed)
- Significantly more expensive than cold loft approach
Room-in-Roof: PIR Between and Below Rafters
When converting a roof space to habitable use, mineral wool rolls in the rafter bays alone are usually insufficient to meet Part L targets without unacceptable depth. A hybrid approach is standard:
- Between rafters: 100 mm rock wool or glass wool batt (or PIR board cut to fit)
- Below rafters: 37.5 mm or 52.5 mm PIR-backed plasterboard (e.g. 12.5 mm plasterboard + 25 mm or 40 mm PIR)
This combination typically achieves 0.15–0.18 W/m²K depending on rafter depth and spacing. For the full 0.11 W/m²K target in a rafter-depth-constrained space, PIR board above the deck plus insulation between rafters is the reliable route.
PIR between rafters (cut-to-fit boards): 60–100 mm PIR at £8–£14/m² supply; must be cut accurately and edges taped to form continuous thermal layer.
PIR-backed plasterboard (warm side): 52.5 mm (12.5 + 40 mm) at £12–£18/m²; provides both insulation and a level surface ready for decoration; VCL is incorporated by taping joints.
Vapour Control
For cold loft construction, a VCL is not normally required at ceiling level provided the loft space is adequately cross-ventilated. The permeable nature of plasterboard and mineral wool allows incidental moisture to escape upward into the ventilated void.
For warm loft and room-in-roof construction, a VCL (typically 500-gauge polythene or foil-backed insulated plasterboard with taped joints) is essential on the warm side (below) of the insulation. Without it, warm moist air from the dwelling diffuses into the insulation layer and condenses at the cold boundary, risking interstitial condensation, timber decay and reduced insulation performance.
Condensation risk assessments (Glaser method, or more accurately using WUFI hygrothermal modelling) should be carried out for any warm roof design where a conventional VCL placement is not straightforward.
Part L U-value Targets (England, 2021)
| Building type | Element | Target (W/m²K) |
|---|---|---|
| New dwelling | Roof | 0.11 |
| New dwelling | Flat roof | 0.11 |
| Existing dwelling — major renovation | Roof | 0.16 |
| Existing dwelling — retained roof | Threshold (if disturbed) | 0.16 |
Wales, Scotland and Northern Ireland each have separate Building Regulations with broadly similar targets; check the relevant national document before specifying.
Cost Summary
| Scenario | Material cost (£/m²) | Coverage |
|---|---|---|
| 270 mm cold loft (100 mm + 170 mm glass wool) | £5–£9 | Retrofit to 0.16 W/m²K |
| 400 mm cold loft (enhanced glass wool) | £8–£14 | New-build to 0.11 W/m²K |
| Room-in-roof hybrid (rock wool + PIR plasterboard) | £22–£35 | Habitable rafter-level to 0.15–0.18 W/m²K |
| Warm flat roof PIR (150 mm) | £20–£28 | New-build to 0.11 W/m²K |
Installation costs for cold loft roll: £3–£7/m² (accessible lofts); £5–£10/m² (awkward access, deep or double-layer). Room-in-roof installation: £20–£40/m² additional above material cost.
Relevant Standards
- BS 5250:2021 — management of moisture in buildings (ventilation requirements)
- BS EN 13162 — factory-made mineral wool products
- Approved Document L (2021) — conservation of fuel and power
- Approved Document C — resistance to weather (relevant to roof vapour control)
- PAS 2035:2023 — retrofitting dwellings for energy efficiency (retrofit assessor guidance)
- CIBSE Guide A — environmental design (condensation risk methodology)
