Internal Wall Insulation Problems: Damp, Mould & Condensation Issues

What is Internal Wall Insulation (IWI)?

Internal Wall Insulation involves fitting insulation boards to the inside of external walls, then covering them with plasterboard. It was promoted under ECO4 and GBIS schemes as a less disruptive alternative to external wall insulation for solid-walled properties.

Why Does Internal Wall Insulation Cause Problems?

Government-funded IWI installations have created thousands of damp and mould cases due to:

1. Interstitial Condensation

The most serious issue with IWI is condensation forming between the insulation and the original wall:

• Warm indoor air reaches the dew point within the wall structure
• Moisture condenses on the cold side of the insulation
• Water becomes trapped with no escape route
• Mould grows hidden behind the insulation boards
• Timber battens and structural elements rot

2. Missing or Inadequate Vapour Barriers

Proper IWI requires vapour control layers to prevent moisture reaching the cold wall:

• Many ECO4/GBIS installations skipped vapour barriers to cut costs
• Incorrect barrier placement (wrong side of insulation)
• Poor sealing around edges, sockets, and penetrations
• Use of wrong membrane type for the application

3. Inadequate Ventilation

IWI dramatically reduces a room's ability to "breathe":

• Reduced wall surface area for moisture evaporation
• Installers rarely upgrade ventilation when required
• Existing trickle vents become insufficient
• Condensation accumulates with nowhere to go

4. Thermal Bridging

Cold spots where insulation is interrupted create condensation hotspots:

• Around window and door reveals (often left uninsulated)
• At ceiling and floor junctions
• Where internal partition walls meet external walls
• Behind radiators and fitted furniture

5. Loss of Living Space

While not a moisture issue, IWI reduces room sizes:

• Typically 100-150mm lost per external wall
• Affects furniture placement and room proportions
• Can reduce property value if poorly executed

Signs Your IWI Has Failed

Look out for these warning signs of internal wall insulation problems:

Immediate Red Flags (0-12 months after installation)

• Black mould in corners - especially external wall corners and ceiling junctions
• Condensation on walls - walls feel damp or show water droplets
• Musty smell - persistent odour even with ventilation
• Cold walls despite heating - insulation not performing as expected

Medium-Term Problems (1-3 years)

• Wallpaper peeling - moisture behind plasterboard causing adhesion failure
• Paint blistering or flaking - especially on external walls
• Plasterboard damage - discolouration, softening, or sagging
• Increased energy bills - moisture reducing insulation effectiveness
• Worsening mould - spreading to furniture, clothes, and belongings

Severe Long-Term Damage (3+ years)

• Structural timber decay - joists and battens rotting behind boards
• Wall tie corrosion - if moisture reaches cavity (in cavity walls with IWI)
• Health problems - respiratory issues from prolonged mould exposure
• Electrical issues - moisture affecting sockets and wiring

IWI Problems by Property Type

Victorian/Edwardian Solid Walls

These properties are particularly vulnerable:

• Original lime mortar and breathable materials now sealed
• Often have existing damp issues that IWI makes worse
• High ceilings and room proportions significantly reduced
• Period features (cornicing, picture rails) disrupted

1930s-1950s Properties

• May have cavity walls where IWI creates moisture trap
• Existing cavity wall insulation can compound problems
• Often have asbestos behind walls (removal costs)

Flats and Apartments

• Limited external wall area makes ventilation critical
• Often have restrictive lease terms for modifications
• Removal complicated by shared walls and access issues

ECO4 and GBIS Internal Wall Insulation Failures

The National Audit Office's damning 2024 report found that government-funded IWI installations routinely failed to meet basic building standards:

• 98% failure rate across all wall insulation measures
• No pre-installation surveys - unsuitable properties approved automatically
• Inadequate installer training - many had never fitted IWI before
• Zero quality checks - Trustmark oversight "not fit for purpose"
• Rushed installations - entire houses insulated in a single day
• Missing critical components - vapour barriers, ventilation upgrades skipped

Your Options: Removal, Remediation, or Live With It?

Option 1: Complete Removal (£3,000-£12,000)

When to consider:

• Severe condensation and mould throughout property
• Structural damage to timber or plasterboard
• Health issues from mould exposure
• IWI installed without vapour barriers

What's involved:

• Strip all insulation boards and plasterboard
• Remove contaminated timber battens
• Treat or replace affected wall areas
• Reinstate walls with proper plastering
• Redecorate affected rooms

Option 2: Targeted Remediation (£1,500-£5,000)

When to consider:

• Problems limited to specific areas (corners, reveals)
• Recent installation with minor defects
• Willing to accept some reduction in performance

What's involved:

• Add mechanical ventilation (MVHR or MEV system)
• Install dehumidifiers in affected rooms
• Repair thermal bridges at junctions
• Add vapour barriers where missing (if accessible)
• Improve sealing around penetrations

Option 3: Manage the Problem (£200-£1,000/year ongoing)

When to consider:

• Cannot afford removal costs
• Problems are mild and manageable
• Temporary measure while seeking compensation

What's involved:

• Run dehumidifiers continuously (adds £200-£400/year to bills)
• Regular mould treatment and cleaning
• Increase heating to maintain higher temperatures
• Maximum ventilation (windows open, extractor fans on)
• Regular inspections for worsening damage

Getting Help: Next Steps

Frequently Asked Questions

Can internal wall insulation be fixed without complete removal?

In some cases, yes. If the problem is condensation due to poor ventilation, adding mechanical ventilation (MVHR system) can help. However, if vapour barriers are missing or insulation is already saturated with moisture, removal is usually the only effective solution.

Will my home insurance cover IWI removal?

Standard home insurance typically excludes damage from faulty installations or maintenance issues. However, if the installer has liability insurance and you can prove negligence, you may have a claim. Document everything and seek legal advice.

How long does IWI removal take?

For an average 3-bedroom semi-detached house:

• Single room: 2-3 days
• Whole ground floor: 5-7 days
• Entire property: 2-3 weeks

Add extra time for structural repairs, replastering, and redecoration.

Can I claim compensation from the installer?

Yes, if:

• Installation was within the last 6 years (legal guarantee period)
• Installer is still trading and insured
• You can prove defective workmanship caused the damage
• Work was done under Trustmark or similar accreditation

Many ECO4/GBIS installers have ceased trading, making claims difficult. Government compensation schemes are currently being discussed but not yet available.

Is internal wall insulation always bad?

No. When properly designed and installed with:

• Correct vapour control layers
• Adequate ventilation upgrades
• Proper thermal bridging details
• Suitable property assessment
• Quality materials and workmanship

IWI can be effective. The problem is that government scheme installations routinely skip these critical requirements to maximize installer profits.

Should I buy a house with internal wall insulation?

Be cautious, especially if installed post-2020 under ECO4/GBIS:

• Insist on a Level 3 building survey (not just homebuyer's)
• Request specialist damp and timber survey
• Check for Trustmark or installer guarantees
• Look for signs of condensation and mould
• Factor removal costs into offer if problems found

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