The job
Modelled, not recorded. Every figure here is the answer the Cavity Wall & Stud Bay Insulation Fill Calculator gives for these inputs, written down in both unit systems and re-run whenever the site is built. No job was carried out, so there is no measured overrun on this page.
The house is a two-storey semi with a masonry cavity wall, a construction common in British houses of the inter-war and post-war years. The job is the two elevations an installer fills from outside in one visit: the gable end and the rear wall, 14.5 m (47.57 ft) of wall together, 5.1 m (16.73 ft) from the damp-proof course to the top of the cavity under the eaves. The front wall is a separate visit and a separate run.
The two walls hold 5 windows and doors between them, 7.2 m² in all, which the calculator takes off before it works out a volume. Each opening also adds holes to the drilling grid, because the fill has to be put in under a cill and beside a frame where the regular grid does not reach.
Everything else is left at the calculator's own figures, which are the figures in its first source, a BBA Agrément certificate for one blown mineral wool product: the cavity width the installer typically finds, 65 mm; the installed density, 25 kg/m³; the bale, 15.5 kg; and the hole grid, 1.35 m along a row with rows the same distance apart. A different product means a different certificate and different figures in all four.
The inputs, exactly as entered
| Input | Value | Why |
|---|---|---|
| Wall Length | 14.5 m (47.57 ft) | The gable end and the rear wall added together, measured along the face of the brickwork. |
| Wall Height to Fill | 5.1 m (16.73 ft) | From the damp-proof course to the top of the cavity, where the fill stops below the eaves; on a two-storey house that is most of the elevation. |
| Openings Area (windows and doors) | 7.2 m² (77.5 sq ft) | The windows and the back door added together, each measured across its frame. |
| Number of Openings | 5 | Every window and door in the two walls; each one adds holes around it. |
| Cavity Width or Stud-Bay Depth | 65 mm (2.56 in) | Left at the calculator's own default. |
| Stud Thickness (0 for a masonry cavity) | 0 mm (0 in) | Left at the calculator's own default. |
| Hole Centres Along a Row (stud spacing if framed) | 1.35 m (53.15 in) | Left at the calculator's own default. |
| Row Spacing Up the Wall (height between blocking if framed) | 1.35 m (4.43 ft) | Left at the calculator's own default. |
| Extra Holes per Opening | 2 | Left at the calculator's own default. |
| Installed Density (from the certificate or chart) | 25 kg/m³ (1.56 pcf) | Left at the calculator's own default. |
| Bag Mass | 15.5 kg (34.17 lb) | Left at the calculator's own default. |
| Waste Allowance (%) | 0 | Left at the calculator's own default. |
The working, step by step
1. Wall area to fill, openings deducted
66.75 m² (718.5 sq ft)
Length times height, less the openings, leaves 66.75 m² of wall with a cavity behind it to fill. This is the area the certificate's density and the installer's grid are both applied to.
2. Fill area, stud faces deducted
66.75 m² (718.5 sq ft)
In a framed wall the faces of the studs would come off here, because the fill goes between them; a masonry cavity has no studs, so the stud thickness is left at zero and the area is unchanged, 66.75 m². The row is kept so the same calculator serves both kinds of wall.
3. Fill volume
4.34 m³ (153.2 ft³)
The area times the cavity width is the space the fill must occupy: 4.34 m³. It is the step most sensitive to the survey, because the width is read from the trial holes and a cavity built wider than the drawing is common in houses of this age.
4. Fill mass at the installed density, allowance included
108.5 kg (239.1 lb)
That volume at the certificate's installed density gives the mass of fibre to blow in, 108.5 kg, with any allowance added; this job has none. Dividing by the bale and rounding up gives the bale count in the headline.
5. Holes along each row
11 holes
The drilling grid starts along the wall: at the certificate's spacing a row needs 11 holes across the two elevations. The holes go through the outer leaf only and are made good after the work.
6. Rows of holes up the wall
4 rows
Rows are stacked up the height at the same spacing, which takes 4 rows on walls this tall. A single-storey wall would need fewer, and a gable that rises into a point loses holes near the apex, which this rectangle does not model.
7. Extra holes at openings
10 holes
Each window and door adds the extra holes the calculator's default allows, 10 holes here, for the short bays under a cill and beside a frame that the regular grid would leave unfilled.
Bags of fill at the installed density
7 bags
The material is 7 bags of the certified product, and the work is 54 holes in the outer leaf. Both are planning figures for a job an approved installer carries out after the site assessment the certificate requires: the assessment decides whether the wall is suitable at all, which depends on the wall's exposure, the state of the cavity and the outer leaf, none of which the calculator can see. Read the bale count as what a correct installation uses, so a quotation can be compared against it.
What to buy
| Quantity | Figure | What it is for |
|---|---|---|
| Bags of fill at the installed density | 7 bags | Bales of the certified fill at the certificate's installed density; supplied by the installer, not bought by the householder. |
The allowance, and what it does here
The calculator's allowance is zero by default, because the installed density is a target for the fibre in the wall and the bale count already rounds up. What is really lost on site is fill blown into the loft where the cavity is open at the top, fibre left in the hose, and a part-used bale; the variant below adds a margin for that. It does not cover a cavity full of rubble or mortar droppings, which is a reason found at the survey not to fill the wall, not a percentage to add.
The common mistake
Using the cavity width from the drawing instead of the holes
The fill volume is the area times the cavity width, so the width is the input that decides the order, and it is also the one most often assumed. Cavities in older houses vary, and the width found through the first survey holes can differ from a drawing or from the house next door. A wider cavity than expected means more bales, and the installation stops short when they run out. Take the width from the trial holes on each elevation, and enter the widest one if they differ.
Change the job, and see what moves
The same walls, with a wider cavity found at the survey
Cavity Width or Stud-Bay Depth: 75 mm (2.95 in) → 9 bags
If the survey holes show a cavity of 75 mm instead, the same walls take 9 bags. The drilling grid does not change at all; only the material does, which is why the width is the figure to confirm before the bales are loaded onto the van.
The same walls, allowing for fill lost in the hose and the loft
Waste Allowance (%): 10 → 8 bags
Allow 10 per cent for fill lost into the loft and left in the hose and the count becomes 8 bags. The mass rises by that share and the bales round up once more, so on a job this size a modest allowance is worth one bale.
What to run next
- Cavity Barrier Run and Crossing Count Calculator
Where the cavity meets a party wall or an opening, cavity barriers are part of the same detail, counted by where the cavity is crossed.
- R-Value Calculator
The filled wall's thermal resistance, added up layer by layer, is what the insulation is for and the figure a building control notice asks about.
- Insulation R-Value by Climate Zone Calculator (IECC 2021 and NBC 2020)
Sets the filled wall against the minimum the reader's code tabulates for a wall of this kind.
Sources
The method and every constant behind these figures belong to the Cavity Wall & Stud Bay Insulation Fill Calculator, version 1.0.0, last verified 2026-09-28. It rests on:
- BBA Agrément Certificate 11/4857, Product Sheet 2 Issue 4 (12 December 2023), Knauf Insulation Supafil 34 (Retrofit): a site assessment by a trained assessor before installation (A.1); target mean installed density 25 kg/m³ (±5) over the whole wall (9.1.4); bales of about 15.5 kg (11.1); nominal cavity not less than 50 mm, and over 200 mm referred to the certificate holder (9.1.2, 9.1.3); 22 mm holes in a diamond pattern at about 1.35 m centres, bottom row about 800 mm above the DPC, top holes within 350 mm of the top and not more than 1 m apart, extra holes under cills and around features (A.5); ventilation openings and flues sleeved before and checked after (9.1.5, A.7); up to 12 m without a waiver (9.1.14)
- BBA Agrément Certificate 88/2033, Product Sheet 1, fourth issue (26 July 2021), Knauf Insulation Supafil 40: target mean installed density 18 kg/m³ (section 1.2) and bales of about 17.6 kg (section 3.1) — a second product from the same maker, which is why the density and the bag mass are the certificate's, not the page's
- BBA Agrément Certificate 04/4167, Product Sheet 1, third issue (4 June 2020), Springvale Ecobead Platinum bonded EPS bead: target mean installed density 12 kg/m³ (±2) (section 1.2); 22 mm holes at about 600 mm intervals, holes about 250 mm below openings and between them, rows not more than 2.5 m apart (18.1–18.3); bead delivered in sacks or bulk containers (3.1)
- GreenFiber INS515LD application coverage chart (manufacturer's data sheet): 30 lb nominal bag, minimum net weight 28.5 lb; dry dense-pack sidewall at 3.5 pcf minimum installed density; a 2x4 wall at 3.5 in takes 1.02 lb/sq ft, 29.39 sq ft a bag before framing and 32.4 sq ft at 16 in centres; framing factor 9.375% at 16 in centres and 6.25% at 24 in
- US DOE Building America Solution Center (PNNL): 'Dense-Pack Insulation' — cellulose dense-packed at 3.5 to 4.5 lb/ft³ (56 to 72 kg/m³); 'Blown Insulation for Cavities of Existing Exterior Walls' — a 1 in or 2 in hole in the centre of each stud cavity about 6 in below the ceiling, an additional hole below any blocking, and knob-and-tube wiring not to be surrounded by insulation
- Ofgem / ECO Reporting Working Group, Guidance on suitable photographic evidence of retrofitted CWI, Version 1.0 (October 2020): standard drill holes of approximately 18–25 mm in the mortar joints, a pattern determined by the product manufacturer or certification body, concentrated round windows and doors and under waste pipes and vents; lance installations drilled at the corners of elevations, typically 27–40 mm
- BS 8208-1:1985, Guide to assessment of suitability of external cavity walls for filling with thermal insulants — Part 1: Existing traditional cavity construction — withdrawn by BSI on 1 July 2009 and still cited in Approved Document D (1992 edition incorporating the 2002 and 2010 amendments) for foam fill; the withdrawn guide behind the survey this page leaves to the installer
- NFPA 70, National Electrical Code, Article 394.12 — concealed knob-and-tube wiring is not permitted in hollow wall spaces insulated by loose, rolled or foamed-in-place insulation that envelops the conductors
- US EPA Renovation, Repair and Painting Rule, 40 CFR Part 745 Subpart E: the rule applies to renovations performed for compensation in target housing (§745.82), and work in housing built before 1978 that disturbs more than 20 sq ft of painted exterior surface is not minor repair (§745.83)
- The Building Regulations 2010 (England), regulation 3(1)(e): the insertion of insulating material into the cavity wall of a building is building work; the Cavity Insulation Guarantee Agency (CIGA) registers the guarantee a UK installer issues
Frequently asked questions
- How many bags of cavity wall insulation does a semi-detached house need?
- For the gable and rear walls here, 14.5 m of wall 5.1 m high with the openings taken off, the calculator gives 7 bags at the certificate's density and a cavity of 65 mm. The front wall is counted separately.
- How many holes are drilled for cavity wall insulation?
- On these two walls the calculator's grid gives 54 holes: 4 rows of 11 holes, plus 10 holes around the openings, at the spacing the certificate sets.
- Does a wider cavity need more insulation?
- Yes, in proportion. At 75 mm the same walls take 9 bags instead of 7 bags, with the same number of holes.