Quantities
Cavity wall insulation: volume and bags
- where
- cavity volume to fill (m³)
- gross external wall area (m²)
- area of windows and doors (m²)
- cavity width (mm)
- mass of fill material (kg)
- installed density (kg/m³)
- waste and blow-back allowance (%)
- bags required
- mass per bag (kg)
Explain this equation
Works out the volume of a cavity to be filled and the bags of blown fibre or beads needed.
The net wall area times the cavity width (converted to metres) is the volume to fill. Each system is certified to be installed at a set density, so volume times density gives the mass, increased for waste and blow-back. Dividing by the bag weight and rounding up gives bags.
Volume equals area times width; mass equals volume times density.
Sense check: 65 m² of net wall with a 75 mm cavity is about 4.9 m³; at 25 kg/m³ plus 5% that is about 128 kg, 8 bags of 16.6 kg.
Watch out: cavity widths vary, so measure at several drill holes. Check the wall is suitable for the exposure zone before filling.
Estimates the cavity volume and the number of bags of blown fibre or bead needed to fill a cavity wall.
Enter your values to see the result.
Good to know
Measure the cavity width at several drill holes or with a borescope; 50–75 mm is typical for 1930s–1980s houses. Installed density and bag mass must come from the system's BBA or other third-party certificate and the installer's data: the density and bag mass shown are example values only. Cavity wall insulation must be installed by a registered installer to the system certificate and PAS 2030, after a pre-installation survey of exposure, cavity condition and wall ties. The Part L target for cavity fill in an existing wall is 0.55 W/m²K.
Common mistake
Using the nominal cavity width from drawings. Old cavities vary along the wall and are often partly blocked with mortar snots, which a survey must find before filling.
Limits
Volume estimate only. Excludes debris, varying widths and fill above the top of the wall. Does not decide whether the wall is suitable for filling (exposure zone, rain penetration, render condition).
Sources
- BRE BR 443 (3rd edition, 2019) Conventions for U-value calculations (Rsi 0.13 / 0.10 / 0.17 and Rse 0.04 m²K/W; air-gap levels 0 / 0.01 / 0.04; timber fractions; ground λ clay 1.5, sand/gravel 2.0, rock 3.5 W/mK; ignore corrections under 3%)
- Approved Document L Volume 1: Dwellings, 2026 edition (England): Table 3.2 new or replacement elements in existing dwellings (roof 0.15, wall 0.18, floor 0.18 W/m²K); Table 3.3 existing elements: threshold / improved U-values roof 0.35/0.16, cavity fill 0.70/0.55, internal or external wall insulation 0.70/0.30, floor 0.70/0.25; para 3.14 15-year payback and 0.7 backstop. In force 24 March 2027; the same values appear in Tables 4.2 and 4.3 of the 2021 edition, which applies until then
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow BS EN ISO 6946, BS 5250, PAS 2035, Building Regulations Part L and manufacturers’ data. Whole-house retrofit needs a qualified retrofit assessor and coordinator.