Internal Wall Insulation Calculator (IWI)
Calculate U-values for internal wall insulation on solid walls. Our calculator helps you determine the right thickness of insulated plasterboard or stud frame systems to meet Part L 2021 targets for solid wall properties.
U-Value Calculator
Check Part L 2021 compliance for your insulation project
Tips for Internal Wall Insulation
- IWI reduces room size - allow 50-100mm per insulated wall
- Address any damp issues before installing IWI
- Consider thermal bridging at window reveals, floors, and ceilings
- Leave a small gap behind radiators or move them to maintain heat output
IWI Cost Estimates
Professional installation costs by wall area (2025 prices):
| Wall Area | Direct Bond | Stud Frame | Scope |
|---|---|---|---|
| 20m2 | GBP 600-900 | GBP 1000-1500 | Single room |
| 40m2 | GBP 1200-1800 | GBP 2000-3000 | Two rooms |
| 60m2 | GBP 1800-2700 | GBP 3000-4500 | Three rooms |
| 100m2 | GBP 3000-4500 | GBP 5000-7500 | Whole house |
Excludes decoration, electrics relocation, and reveal insulation.
Common Mistakes to Avoid
Installing over damp walls
Problem: Trapped moisture causes mould and rot
Solution: Diagnose and fix damp source before any insulation
Ignoring thermal bridging
Problem: Cold spots cause condensation and mould
Solution: Insulate reveals, returns, and floor/ceiling junctions
Blocking ventilation
Problem: Moisture build-up in room and walls
Solution: Maintain trickle vents and extract fans
DIY without vapour control
Problem: Interstitial condensation damages insulation
Solution: Install VCL on warm side or use foil-faced boards
Forgetting services
Problem: Difficult access for future electrical/plumbing work
Solution: Plan service routes before fixing insulation
Understanding Internal Wall Insulation
Internal wall insulation is the primary option for solid wall properties (pre-1920s construction) where external wall insulation is not suitable due to planning restrictions, cost, or building constraints.
Solid walls have U-values of 1.5-2.0 W/m2K compared to the Part L target of 0.28 W/m2K. IWI typically reduces this to 0.30-0.40 W/m2K - a major improvement even if not achieving full compliance.
IWI vs EWI Comparison
IWI costs less upfront but reduces room size and requires careful moisture management. EWI preserves internal space and addresses thermal bridges better but costs more and changes external appearance. For listed buildings, IWI is often the only option.
Example: Victorian Terrace Living Room (20m2 wall)
Before: 225mm solid brick (U-value 2.1)
After: 50mm PIR + 12.5mm plasterboard (U-value 0.35)
Space lost: 62.5mm per wall = 0.25m2 floor area
Cost: GBP 800-1200 professional, GBP 300-500 DIY
Annual saving: GBP 100-180 per room
For complete home energy improvements, combine IWI with loft insulation and floor insulation for maximum savings.
Frequently Asked Questions
Related Calculators
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Need a Survey?
Professional damp and insulation surveys recommended before IWI installation.
Need Professional Advice?
Get in touch for custom calculations or project guidance.