Cavity wall insulation is one of the most cost-effective measures to improve the energy efficiency of a home. Uninsulated cavity walls typically account for around 30–35% of all heat lost from a house.
What Does Cavity Wall Insulation Cost?
Cost per m² of Wall Area
| Material | DIY | Specialist (including installation) |
|---|---|---|
| EPS beads (polystyrene) | Not possible (specialist required) | €15 – €30 per m² |
| Mineral wool (blown) | Not possible (specialist required) | €20 – €40 per m² |
| PUR foam (injected) | Not possible | €25 – €45 per m² |
Note: Cavity wall insulation is a specialist trade. Blowing or injecting insulation into a cavity wall requires specialist equipment and expertise. It is not a DIY task.
Total Costs for a Detached House (120 m² External Wall)
| Material | Cost (specialist, all-in) | Payback Period |
|---|---|---|
| EPS beads | €1,800 – €3,600 | 5–8 years |
| Mineral wool | €2,400 – €4,800 | 7–10 years |
| PUR foam | €3,000 – €5,400 | 8–12 years |
Material Comparison: Which Insulation for Cavity Walls?
| Material | R-value (80 mm cavity) | Breathable | Dimensionally stable | Water uptake |
|---|---|---|---|---|
| EPS beads (polystyrene) | 2.0–2.5 m²K/W | No | Yes | Low |
| Mineral wool (blown) | 2.0–2.5 m²K/W | Yes | Yes | Low |
| PUR foam | 3.0–3.5 m²K/W | No | Very good | Very low |
| Cellulose (blown) | 2.0–2.5 m²K/W | Yes | Yes (if kept dry) | Medium |
Recommendation: EPS beads are the most cost-effective and most commonly used solution. Mineral wool has better fire resistance and is breathable (vapour open).
Is Your Home Suitable for Cavity Wall Insulation?
Cavity wall insulation is only possible in homes with a cavity (two-skin) wall construction. These are typically houses built between the 1920s and 1990s.
Checklist:
– [ ] House built between approx. 1920–1990
– [ ] External wall at least 250 mm thick (indicates cavity construction)
– [ ] Cavity is clear (no previous insulation, no moisture damage)
– [ ] External render and masonry in good condition
Pre-survey: A specialist will drill a small endoscope hole (20 mm) to check the cavity width and whether it is clear.
Grants for Cavity Wall Insulation
UK: ECO4 Scheme and Great British Insulation Scheme
| Grant Scheme | Available Support |
|---|---|
| ECO4 (low income households) | Up to 100% funded for eligible households |
| Great British Insulation Scheme | Partial funding for most households |
| Local Authority Flex (LADS) | Additional funding via local councils |
Check the government’s Simple Energy Advice website (gov.uk) for current eligibility and application details.
How Cavity Wall Insulation Works
Step 1: Survey and Quotes
- Commission an independent surveyor to assess condition (cost €100–€300)
- Get at least 3 quotes from registered installers
- Check references – poorly installed cavity wall insulation can cause moisture problems
Step 2: Pre-Installation Survey
The specialist:
1. Checks the condition of the external wall
2. Endoscopes the cavity
3. Checks for gaps, mortar bridges and moisture
Step 3: Blowing / Injecting the Insulation
- Grid of drill holes from outside (30–40 mm) through the outer leaf
- Insulation blown or injected until the cavity is completely filled
- Holes plugged with stoppers or mortar
- Facade cleaned
Step 4: Post-Installation Check
A good installer carries out thermographic imaging after installation to verify all cavities are filled.
Common Mistakes
| Mistake | Consequence | Solution |
|---|---|---|
| No condition survey beforehand | Insulates a damp wall – mould problems | Check condition before insulating |
| Inadequate injection pressure | Cavity not completely filled | Demand thermographic check |
| Unsuitable masonry | Insulation falls through joints | Specialist assesses first |
FAQs
Can I do cavity wall insulation myself?
No. Blowing or injecting insulation requires specialist equipment (blowing machine) and expertise. Incorrectly installed cavity wall insulation can cause serious moisture damage.
Which homes are suitable?
Homes with cavity (two-skin) wall construction, typically built between the 1920s and 1990s. Solid wall homes (single brick without a cavity) need external or internal wall insulation instead.
How long does cavity wall insulation last?
EPS beads and mineral wool last 50+ years when correctly installed. They do not slump and barely lose insulating performance.
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