What Is Cavity Wall Insulation and Which Properties Have Cavities?
A cavity wall consists of two parallel leaves of masonry (typically brick) separated by a gap (the cavity) of 50–100mm. The cavity was originally left empty to prevent moisture from the outer leaf reaching the inner leaf — a significant improvement over the solid walls of Victorian properties. However, an empty cavity also provides no thermal insulation — the air in the cavity convects, transferring heat from the warm inner leaf to the cold outer leaf.
Cavity wall insulation (CWI) fills the cavity with an insulating material — mineral wool fibres, EPS beads, or polyurethane foam — to reduce this convective heat transfer. The result is a wall with a U-value of approximately 0.35–0.55 W/m²K, compared to 1.0–1.5 W/m²K for an empty cavity and 2.0 W/m²K for a solid brick wall.
Most Hertfordshire properties built between 1920 and 1990 have cavity walls. Properties built before 1920 (Victorian and Edwardian) are almost always solid wall — these require a different approach, as described in our Retrofit Insulation Guide. Properties built after 1990 usually have cavity walls that were already filled at construction — but the fill quality may have degraded and is worth checking.
The Pre-Installation Survey: What a Good Surveyor Should Check
A pre-installation survey is the most important step in a CWI project. A thorough survey should check the following:
Cavity width and condition
The cavity must be at least 50mm wide and free of debris. A borescope inspection confirms the cavity is clear and identifies any existing partial fill.
Wall tie condition
Iron wall ties in pre-1970s properties may be corroding. Corroded ties can expand and crack the outer leaf. Insulation can accelerate corrosion by trapping moisture. If ties are corroded, they must be replaced before CWI is installed.
Existing damp
Any existing damp in the wall must be identified and treated before CWI is installed. CWI over an active damp problem will make the problem worse. See our guide on how to tell if you have rising damp.
Exposure rating
Properties in exposed locations (east-facing walls, hillside sites, areas with high driving rain index) are at higher risk of moisture bridging. The BBA (British Board of Agrément) exposure zone maps should be consulted. Some locations in Hertfordshire are classified as "severe" exposure — CWI may not be appropriate for these properties.
Existing fill
Some properties have partial fills or degraded fills from previous CWI installations. A borescope inspection will identify these. Overfilling a partially-filled cavity can cause problems.
Construction details
Window reveals, door frames, and other construction details must be checked to ensure the insulation can be installed without creating cold bridges or moisture pathways.
Comparing CWI Materials: Mineral Wool, EPS Beads, and Polyurethane Foam
| Material | Thermal Performance | Cost (3-bed semi) | Best For | Risks |
|---|---|---|---|---|
| Mineral wool (blown) | 0.040 W/mK | £400–£700 | Standard cavities, sheltered locations | Can absorb moisture; requires dry cavity |
| EPS beads | 0.035 W/mK | £500–£800 | Exposed locations, narrower cavities | Beads can settle slightly over time |
| Polyurethane foam | 0.025 W/mK | £700–£1,200 | Narrow cavities, highest performance | Difficult to remove; irreversible |
What If Cavity Wall Insulation Is Not Suitable for Your Property?
If the pre-installation survey identifies that CWI is not appropriate — due to exposure, existing damp, or wall tie condition — the alternatives are internal wall insulation (IWI) or external wall insulation (EWI). These are more expensive and disruptive, but they are the only options for solid-wall properties and for cavity-wall properties where CWI is not suitable.
IWI involves fixing insulated plasterboard to the inside face of the external wall. It reduces the internal floor area by 80–100mm per wall and requires all skirting boards, architraves, and electrical sockets to be repositioned. It is typically done as part of a broader refurbishment — see our Property Refurbishment service page for details.
EWI involves fixing insulation boards to the outside face of the wall and finishing with a render or cladding system. It is more expensive than IWI but maintains the internal floor area and eliminates more thermal bridges. It may require planning permission in conservation areas. For a full comparison of IWI and EWI, see our Retrofit Insulation Guide.
TCM's Approach to Cavity Wall Surveys
TCM Building & Maintenance carries out pre-CWI surveys as part of our broader property maintenance and refurbishment service. We check cavity width and condition with a borescope, assess wall tie condition, identify existing damp, and review the exposure rating for the property's location. We will not recommend CWI for a property where the risks outweigh the benefits.
For a client in Borehamwood with a 1950s semi, our survey identified corroded wall ties in the east-facing wall (the most exposed elevation). We replaced the wall ties before installing EPS bead insulation in the protected west and south elevations, and recommended IWI for the east elevation. The client's heating bills reduced by approximately £280 per year — and the wall tie replacement prevented what could have become a serious structural problem.
If you are considering cavity wall insulation and want an independent survey before committing to an installation, contact TCM for a free assessment.
Frequently Asked Questions
How do I know if my Hertfordshire home has an unfilled cavity wall?+
The most reliable method is a borescope inspection — a small camera is inserted through a drilled hole in the mortar joint to view the cavity. A qualified surveyor can also check the wall thickness (cavity walls are typically 270–300mm thick, solid walls 225mm or less) and the construction date (pre-1920 properties are almost always solid wall; post-1920 properties are usually cavity). Some properties have partial fills or degraded fills that need to be identified before new insulation is installed.
Can cavity wall insulation cause damp problems?+
Yes, if installed incorrectly or in unsuitable conditions. The main risks are: (1) moisture bridging — if the cavity is already damp or the property is in an exposed location, insulation can bridge moisture from the outer to the inner leaf; (2) degraded wall ties — old iron wall ties may be corroding, and insulation can accelerate this; (3) blocked cavities — debris in the cavity can prevent even fill distribution. A pre-installation survey by a qualified surveyor is essential to identify these risks.
What is the difference between mineral wool, EPS beads, and polyurethane foam for cavity wall insulation?+
Mineral wool (glass wool or rock wool) is the most common and cheapest material — it is blown into the cavity as loose fibres. EPS (expanded polystyrene) beads are slightly more expensive but have a higher thermal resistance and are better in wet conditions. Polyurethane foam provides the highest thermal performance but is more expensive and difficult to remove if problems arise. The choice depends on the cavity width, exposure rating, and budget. Your installer will recommend the most appropriate material for your property.
Is there a grant available for cavity wall insulation in Hertfordshire?+
Yes. The Great British Insulation Scheme (GBIS) provides free or subsidised cavity wall insulation for properties with an EPC rating of D or below, or for households receiving certain benefits. The Energy Company Obligation (ECO4) provides similar support for lower-income households. Hertfordshire County Council's Warm Homes programme also provides grants for eligible households. Contact your energy supplier or visit the Simple Energy Advice service to check eligibility.
How long does cavity wall insulation last?+
Cavity wall insulation installed correctly with appropriate materials should last 25+ years. Mineral wool and EPS bead insulation are inert materials that do not degrade over time, provided they remain dry. Polyurethane foam can shrink slightly over time, potentially leaving gaps. If you are concerned about the performance of existing cavity wall insulation, a borescope inspection can check the fill quality and identify any voids or degradation.
What is the typical payback period for cavity wall insulation in Hertfordshire?+
For a three-bedroom semi-detached property in Hertfordshire with an unfilled cavity, the typical payback period for cavity wall insulation is 3–5 years at 2026 energy prices. Annual savings are typically £200–£400 depending on the property size, heating system, and current energy tariff. With grant funding, the payback period can be under 2 years.
Related Guides
Retrofit Insulation in Hertfordshire
The complete guide to all insulation options for existing homes
How to Tell If You Have Rising Damp
Identify damp before it undermines your insulation
EPC Rating Guide Hertfordshire
How CWI improves your EPC score and property value
Loft Insulation Guide Hertfordshire
Combine CWI with loft insulation for maximum savings
Want an Independent Survey Before Proceeding?
TCM Building & Maintenance provides independent pre-CWI surveys for Hertfordshire homeowners — checking cavity condition, wall ties, damp, and exposure before you commit to an installation.