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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.
External wall area minus openings
50-100mm typical UK cavity
For injection hole calculation
≈1m across the wall (staggered diamond pattern)
≈1.5m up the wall (Ofgem ECO drill pattern)
5% standard for blown-in
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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 × Installed density (kg/m³)
Injection holes = (Wall length ÷ Horizontal spacing) × (Wall height ÷ Vertical spacing)
For example, a 50 m² wall with a 75mm cavity in EPS bonded bead (installed density 12 kg/m³ per BBA 04/4167): Volume = 50 × 0.075 = 3.75 m³, + 5% wastage = 3.94 m³. Weight = 3.94 × 12 ≈ 47 kg. Bonded bead is supplied loose and priced per installed m³, so order about 4 m³. On the staggered 1m × 1.5m drill pattern, a 2.4m-high wall gives 21 holes per row × 2 rows ≈ 42 holes.
Thermal conductivity (λ) used: EPS bonded bead 0.033 W/m·K (Springvale Ecobead, BBA 04/4167), blown glass mineral wool 0.034 (Knauf Supafil 34), cellulose 0.038 (Warmcel), UF foam 0.040. Achieved U-value is indicative and assumes a typical dense masonry wall — confirm with a full BS EN ISO 6946 calculation.
Frequently Asked Questions
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.
Injection holes are drilled in a staggered (diamond) pattern in the external wall mortar joints — roughly 1m apart horizontally and 1.5m apart vertically per the Ofgem ECO drill-pattern guidance, so the fill zones from adjacent holes overlap and leave no voids. A typical semi-detached house with 60 m² of wall needs around 40-55 holes on this pattern (extra holes are added around windows, doors and where extensions meet). The holes are about 22mm diameter, drilled into mortar joints (not bricks), and filled with mortar-coloured plugs after injection. The process takes 2-4 hours for a typical house.
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 fully filled with EPS bonded bead (λ 0.033) in a typical brick/block wall achieves roughly U = 0.27-0.30 W/m²K, compared with about 0.45-0.50 W/m²K for a filled 50mm cavity (the practical retrofit limit). These figures are calculated to BS EN ISO 6946 and bracket both the calculator's indicative output and manufacturer system data; the calculator above shows the U-value for your exact cavity width. Some older properties have irregular cavities — a borescope inspection is done before work starts.
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.
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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Last updated: March 2026
Verified against UK standards · estimates only, confirm with your supplier.