What is the best cavity wall insulation material?

EPS bead with adhesive is the most popular UK cavity fill — it's moisture-resistant, won't slump, and has a good lambda value (0.033). Mineral wool is cheaper but can absorb moisture in exposed locations. Cellulose fibre is the greenest option (made from recycled paper). UF foam fills completely but has largely fallen out of favour due to formaldehyde concerns and shrinkage issues.

How many injection holes are needed?

Injection holes are typically drilled on a 1m grid pattern in the external wall mortar joints. A typical semi-detached house with 60 m² of wall area needs around 60-80 holes. The holes are 22-25mm diameter, drilled in mortar joints (not bricks), and filled with mortar-coloured plugs after injection. The process takes 2-4 hours for a typical house.

What cavity width is needed for blown insulation?

A minimum cavity width of 50mm is needed for most blown insulation materials. Standard UK cavities are 50-100mm. Wider cavities give better thermal performance — a 100mm cavity filled with EPS bead achieves roughly U = 0.35 W/m²K, compared to about 0.55 for 50mm. Some older properties have irregular cavities — a borescope inspection is done before work starts.

Can cavity wall insulation cause damp?

In most cases, properly installed CWI does not cause damp. However, in exposed locations (driving rain zones 3-4 in the UK), mineral wool CWI can bridge the cavity and transfer moisture to the inner leaf. EPS bead is less prone to this problem. Properties with defective external walls, missing wall ties, or in very exposed coastal locations should be assessed carefully before CWI installation.

Is cavity wall insulation guaranteed?

CIGA (Cavity Insulation Guarantee Agency) provides 25-year guarantees for CWI installed by registered contractors. This is independent of the installer, so even if the company ceases trading, the guarantee remains valid. Always use a CIGA-registered installer to ensure the guarantee applies. If problems arise, CIGA will arrange remedial work at no cost.

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Blown-In Insulation Calculator — Cavity Fill Volume & Bags

Calculate blown-in cavity wall insulation quantities from wall area and cavity width. Covers EPS bead, mineral wool, cellulose, and UF foam.

Wall Area

External wall area minus openings

Cavity Width

50-100mm typical UK cavity

Insulation Material

Wall Height

For injection hole calculation

Price Per m² Installed (optional)

How We Calculate This

This calculator works out the volume of cavity to fill, the weight of insulation material needed, and the number of injection holes required for blown-in cavity wall insulation.

The formula

Cavity volume = Wall area × Cavity width (m) × Wastage

Material weight = Volume × Density (kg/m³)

Injection holes = (Wall length ÷ Spacing) × (Wall height ÷ Spacing)

For example, 50 m² wall with 75mm cavity, EPS bead (density 15 kg/m³): Volume = 50 × 0.075 = 3.75 m³ + 5% = 3.94 m³. Weight = 3.94 × 15 = 59 kg. 8 bags of 0.5 m³. Holes on 1m grid: ~50 holes.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.