Loft insulation provides best ROI. 25-35% of heat escapes through roof - easiest and cheapest to fix. Target R-38 to R-60 for lofts. Cost: £1.50-£3.50 per m² installed. Typical savings: 15-25% on heating bills. Payback: 2-7 years. Second priority: basement/crawl space, then walls.

Depends on climate zone. Cold climates: Loft R-49 to R-60, walls R-18 to R-21. Moderate climates: Loft R-38 to R-49, walls R-13 to R-15. Warm climates: Loft R-30 to R-38, walls R-13. Check current insulation first - adding to R-19 loft is much more valuable than R-30 to R-40.

Spray foam is superior (R-6.5 per inch, air sealing) but 3-4x cost of fibreglass (R-3.2 per inch). Best strategy: spray foam for rim joists, knee walls, and problem areas; fibreglass for large areas like lofts. Cellulose is middle ground - good R-value, eco-friendly, moderate cost.

DIY loft insulation is feasible with fibreglass batts or blown cellulose rental (£50/day). Safety: wear mask, goggles, gloves, long sleeves - insulation irritates skin/lungs. Avoid covering recessed lights or blocking ventilation. Wall and spray foam insulation requires professional installation. DIY saves 30-50% on labour (£500-1,500) but takes 1-3 days.

Quality varies by type. Fibreglass/cellulose: lasts 80-100 years if dry and undisturbed, but settles 10-20% over decades (reducing R-value). Spray foam: 80+ years with no settling. Moisture is enemy - wet insulation loses 50%+ R-value and grows mould. Check loft insulation every 5-10 years for settling, moisture damage, or pest infestation. Add more if compressed or degraded.