how thick should loft insulation be
Loft insulation thickness is key to slashing heat loss—up to 25% of your home's warmth escapes through the roof without it. UK building regs now push for 270mm as the sweet spot for most homes, balancing energy savings with practicality.
Current UK Guidelines
The latest standards (as of 2026) recommend 270mm for mineral wool or fiberglass rolls in cold lofts, hitting a U-value of 0.16 W/m²K to meet regs and cut bills. New builds might go thicker—350-400mm —for stricter 0.11 W/m²K targets. Always layer it: 100mm between joists, then 170mm on top for best coverage without compressing.
By Insulation Type
Different materials pack varying thermal punch, so thickness adjusts:
Material| Recommended Thickness| Notes
---|---|---
Mineral/Glass Wool| 270mm| Most common; affordable but bulky.139
Rigid Boards (PIR)| 150mm| Slimmer, higher R-value per inch; great for
tight spaces.37
Cellulose| 220mm| Eco-friendly blown-in option.9
Slabs| 250mm| Firmer, less sag over time.9
Rigid boards shine where headroom's tight, like warm lofts under the roof pitch.
Real-World Factors
- Existing Insulation : Top up to total 270mm—don't double-layer same type if compressed.
- Loft Type : Cold lofts (unheated space) get full 270mm at joist level; warm lofts need roof-hugging combos.
- Climate/Location : Harsher winters? Go thicker. Check local EPC for tailored advice.
- Overdoing It? 400mm+ rarely adds value and risks moisture if ventilation's poor—270mm usually maxes ROI.
Quick Install Tips
- Clear debris and check for wiring/pipes.
- Lay first layer between joists (uncompressed!).
- Add second layer across joists.
- Ventilate eaves to dodge condensation.
- Pro install? Grants like ECO4 might cover it in 2026.
"270mm will usually suffice to meet minimum requirements." – Expert retrofit advice
TL;DR : Aim for 270mm mineral wool as standard—saves £200-400/year on bills. Measure your loft, pick material wisely, and consult regs for peace of mind.
Info from public web sources like regs guides and forums.