Room in Roof Insulation Calculator
Calculate U-values for room-in-roof insulation in loft conversions. When converting a loft to habitable space, insulation must go between and over the rafters to meet Part L requirements of 0.16 W/m²K for roof extensions.
U-Value Calculator
Check Part L 2021 compliance for your insulation project
Tips for Room-in-Roof Insulation
- Between-rafter + over-rafter insulation achieves best U-values
- Use breathable membrane over insulation before tiles
- Allow 50mm ventilation gap above insulation in cold roofs
- PIR boards maximise headroom with thinner profiles
Room-in-Roof Insulation Cost Estimates
| Roof Area | Between Rafters | + Over Rafters | Project Type |
|---|---|---|---|
| 20m² | £800-1200 | £1500-2200 | Small dormer |
| 35m² | £1400-2100 | £2600-3800 | Single room conversion |
| 50m² | £2000-3000 | £3800-5500 | Double bedroom |
| 70m² | £2800-4200 | £5300-7700 | Full floor conversion |
Prices include materials and labour. VAT may apply for new builds.
Common Mistakes to Avoid
Insufficient thickness between rafters
Problem: Cannot achieve target U-value
Solution: Add over-rafter or under-rafter insulation layer
No ventilation gap
Problem: Condensation, timber rot, mould
Solution: Maintain 50mm gap above insulation in cold roofs
Gaps at rafter edges
Problem: Thermal bridging, cold spots, draughts
Solution: Cut boards tightly, seal with low-expansion foam
No vapour control layer
Problem: Moisture migration into insulation
Solution: Install VCL on warm side of insulation
Forgetting dormer cheeks
Problem: Cold walls in dormers, condensation
Solution: Insulate dormer walls to wall U-value standard
Understanding Room-in-Roof Insulation
Room-in-roof insulation is required when converting a loft into habitable living space. Unlike simple loft insulation laid on the ceiling, room-in-roof insulation follows the roof slope between and over the rafters, creating a warm, usable space.
For standard loft insulation (uninhabited lofts), insulation goes on the ceiling. For conversions, consider combining with internal wall insulation for dormers.
Example: 35m² Loft Conversion
Rafter depth: 100mm
Solution: 100mm PIR between rafters + 25mm PIR under rafters
U-value achieved: ~0.14 W/m²K (exceeds 0.16 target)
Cost: £2500-4000 installed (insulation only)
Frequently Asked Questions
Related Calculators
Building Regulations
Loft conversions require full Building Regulations approval including:
- • Structural calculations
- • Fire safety (30-min doors, escape)
- • Thermal performance
- • Staircase regulations
Other Calculators
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