Internal vs External Wall Insulation: Complete Comparison 2024
Choosing between internal (IWI) and external wall insulation (EWI) is a critical decision that affects costs, performance, and your home's longevity. Here's everything you need to know to make the right choice.
At a Glance: IWI vs EWI
| Factor | Internal (IWI) | External (EWI) |
|---|---|---|
| Typical Cost | £40-£70/m² (Lower) | £80-£150/m² (Higher) |
| Floor Space Lost | 5-15cm per wall (Significant) | None (Adds to exterior only) |
| Living Disruption | High (Room-by-room work, dust) | Low (Work outside, stay inside) |
| Thermal Performance | Good (U-value ~0.25-0.30) | Excellent (U-value ~0.15-0.20) |
| Condensation Risk | Higher (Interstitial condensation) | Lower (Walls stay warmer) |
| Planning Permission | Not usually required (Internal work) | Often required (Appearance change) |
| Heritage Buildings | High risk (Moisture trapping) | Preferred (if allowed) (Breathable systems) |
| Property Value Impact | Neutral to negative (Space loss concern) | Positive (Weatherproofing + EPC) |
| Lifespan | 25-40 years (Interior environment) | 30-60 years (Depends on render) |
| Reversibility | Easier (Can remove boards) | Difficult (Major work to revert) |
When to Choose Internal Wall Insulation (IWI)
IWI makes most sense when external insulation is impossible or impractical. It's the "compromise" solution that delivers energy savings without changing your home's exterior.
Ideal Scenarios for IWI:
- ✓
Listed Buildings / Conservation Areas
EWI would alter protected facades. IWI preserves external appearance while improving energy performance.
- ✓
Properties with Attractive Brickwork
Victorian, Edwardian features worth preserving. EWI would cover ornamental brickwork, stone lintels, decorative details.
- ✓
Semi-Detached or Terraced Homes (Neighbors Won't Cooperate)
EWI on one property but not adjoining creates aesthetic mismatch and potential party wall issues. IWI is independent.
- ✓
Properties Close to Boundary Lines
EWI thickness (150-200mm+) may breach planning setbacks. IWI doesn't affect property boundaries.
- ✓
Single Room or Partial Insulation
Want to insulate one or two rooms only (e.g., cold bedroom). IWI allows selective application. EWI typically whole-house.
- ✓
Budget Constraints (But See True Cost Analysis Below)
Lower upfront cost. However, factor in space loss value and higher condensation risk—total cost of ownership may favor EWI long-term.
Key Advantages of IWI:
Practical Benefits:
- • No scaffolding required (lower cost)
- • Can be done room by room (spread cost over time)
- • Doesn't affect gutters, windows, or external features
- • Easier to reverse if problems occur
Planning & Aesthetic:
- • No planning permission (usually)
- • Preserves original facade
- • No neighbor buy-in needed (semi/terraced)
- • Acceptable in conservation areas
IWI is only suitable if ALL these conditions are met:
- Walls are completely dry with no existing damp issues
- Adequate floor space to lose 100-150mm per external wall
- Vapor control layer will be installed (essential for condensation prevention)
- Ventilation will be upgraded simultaneously (MVHR or enhanced extract fans)
- NOT a heritage building with lime plaster/mortar (unless expert-approved system)
- Family prepared for disruption and temporary room unavailability
When to Choose External Wall Insulation (EWI)
EWI is the gold standard for solid wall insulation—superior thermal performance, lower condensation risk, and adds weatherproofing. Choose EWI whenever planning and budget permit.
Ideal Scenarios for EWI:
- ✓
Detached Properties (Full Exterior Access)
Can insulate entire thermal envelope. No party wall complications. Maximizes energy savings.
- ✓
Homes with Render/Pebble Dash (No Special Brickwork)
Nothing visually special to preserve. EWI improves appearance while insulating. Can choose attractive new finish.
- ✓
Properties Needing External Repairs Anyway
Render failing, walls need repointing, cracks. EWI + repairs done together—cost synergy. Scaffolding needed anyway.
- ✓
Whole Terrace Coordinating EWI Together
Neighbors agree to insulate simultaneously. Shared scaffolding reduces individual cost. Uniform appearance maintained.
- ✓
Heritage Buildings (Where Permitted with Breathable Systems)
Wood fiber boards + lime render allow historic walls to breathe. Preferred over IWI moisture-trapping. Requires conservation officer approval.
- ✓
Maximizing Property Value
EWI adds £10K-£25K to property value (studies show 3-7% increase). Plus weatherproofing extends building life 20-30 years.
- ✓
Long-Term Home (10+ Years)
Higher upfront cost justified by decades of benefits. Payback period 8-15 years—excellent ROI if staying put.
Key Advantages of EWI:
Thermal & Technical:
- • Superior U-values (0.15-0.20 vs 0.25-0.30 for IWI)
- • Eliminates thermal bridging at joist ends
- • Wall thermal mass retained (better temperature stability)
- • Dramatically lower condensation risk
- • Can achieve Passivhaus standards
Practical & Financial:
- • No interior space loss
- • Family can live normally during work
- • Weatherproofs walls (extends building life)
- • Increases property value significantly
- • Often eligible for better grant funding
Before committing to EWI, verify:
- Planning permission status (check with local authority—usually permitted development but not always)
- Party wall agreements (if semi-detached or terraced—neighbors' written consent may be needed)
- Property boundary setbacks (EWI can't encroach on public footpaths or neighboring land)
- Access for scaffolding (narrow side access can significantly increase costs)
- Window/door reveals (need extending—add £200-£400 per opening)
- Coordination with roof work (if roof needs replacing soon, do together for cost savings)
True Cost Comparison: Beyond Upfront Price
While IWI appears cheaper initially, full lifecycle analysis often shows EWI provides better value—especially when factoring in space loss, property value impact, and longevity.
| Property Type | IWI Cost | EWI Cost | Difference |
|---|---|---|---|
| 2-Bed Mid-Terrace (80m² external wall) | £3,200 - £5,600 | £6,400 - £12,000 | +£3,200 - £6,400 |
| 3-Bed Semi (110m²) | £4,400 - £7,700 | £8,800 - £16,500 | +£4,400 - £8,800 |
| 4-Bed Detached (160m²) | £6,400 - £11,200 | £12,800 - £24,000 | +£6,400 - £12,800 |
IWI Hidden Costs & Value Loss:
- −
Floor Space Loss Value
100mm off each wall in 4m x 4m room = 0.8m² lost. At £4,000/m² property value, that's £3,200 value reduction per room.
- −
Radiator Relocation
Extending pipework for each radiator: £150-£300 each (6-10 radiators in typical house)
- −
Electrical Sockets/Switches
Moving out to new wall surface: £30-£60 per outlet × 20-30 = £600-£1,800
- −
Condensation Remediation (If Occurs)
30% of poorly installed IWI develops condensation: £5,000-£15,000 to remove and fix
EWI Added Value Benefits:
- +
Property Value Increase
Studies show 3-7% increase (£9K-£21K on £300K property). Buyers pay premium for well-insulated, low-running-cost homes.
- +
Weatherproofing Walls
Protects brickwork from water ingress, extends wall life by 20-30 years. Saves future repointing/render costs (£3K-£8K).
- +
Higher Energy Savings
Better U-values = 10-15% more savings than IWI. Over 25 years, this adds up to £2,500-£5,000 extra savings.
- +
Longer Lifespan
60 years vs 40 for IWI. Cost spread over longer period = better annual value.
Return on Investment (25-Year Analysis):
IWI (3-Bed Semi Example):
• Installation cost: £6,000
• Hidden costs (radiators, sockets, space loss): +£4,000
• Energy savings (£420/year × 25): +£10,500
• Property value impact: Neutral to −£5,000
Net Position (25 years): £500 gain to −£4,500 loss
EWI (3-Bed Semi Example):
• Installation cost: £12,000
• Hidden costs: Minimal (£500)
• Energy savings (£485/year × 25): +£12,125
• Property value impact: +£15,000
Net Position (25 years): +£14,625 gain
*Figures are indicative. Actual results vary by property, energy prices, installation quality, and maintenance. Energy savings based on average solid wall home (EPC E to C).
Recommendations by Property Type
Pre-1919 Solid Wall Properties (Victorian, Edwardian)
✓ First Choice: External Insulation (Breathable System)
- • Wood fiber boards (140-200mm) + lime or silicate render
- • Allows walls to breathe—critical for lime mortar/plaster
- • May require listed building consent / conservation area approval
⚠ Second Choice (If EWI Impossible): No Insulation or Minimal IWI
- • Standard IWI very high risk of moisture trapping in historic buildings
- • If must use IWI: sheep's wool or wood fiber (breathable) + vapor-open systems ONLY
- • Expert specification essential (heritage building specialist)
- • Consider accepting modest thermal performance gains from loft/draft-proofing alone
1930s-1960s Cavity Wall Properties
If Cavity is Genuinely Unsuitable:
EWI preferred (detached properties). IWI acceptable if semi/terraced and neighbors won't cooperate on EWI.
Note: Many ECO scheme installers wrongly claim cavities "too narrow"—get independent survey. Standard cavity (50-75mm) is perfectly adequate for cavity bead insulation.
Post-2000s Properties
Solid wall insulation rarely cost-effective:
Modern properties already meet reasonable insulation standards (U-value 0.30-0.45). Focus on:
- • Loft insulation upgrade (cheapest, highest ROI)
- • Draft-proofing and air-tightness
- • Solar panels / heat pumps (if considering major investment)
Solid wall insulation payback period >30 years for already-moderate U-values. Not financially prudent.
Decision Helper: Which is Right for You?
Is your property listed or in a conservation area?
NO → Continue ⬇
Does your property have attractive period features (ornamental brickwork, stone details)?
NO → Continue ⬇
Is your property detached OR can neighbors coordinate EWI together?
NO (semi/terraced, neighbors won't cooperate) → Continue ⬇
Can you afford £8,000-£24,000 for EWI?
NO → Continue ⬇
Can you afford to lose 100-150mm of floor space per external wall?
NO, rooms too small → Save for EWI or reconsider if insulation is truly necessary
Still Unsure Which to Choose?
Get expert, unbiased advice from a RICS surveyor who specializes in solid wall insulation
Independent advice • Property-specific recommendations • No obligation