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Waste is set to 0% by hand. Pick a tier above to replace it, or keep your own figure.
The length of the elevation being filled, measured along its face.
Run one elevation at a time, corner to corner or party line to corner, and add the results: a gable, a front and a back are three runs, because each is drilled and filled as its own panel. A semi-detached or terraced house stops at the party line, where the installer fits a cavity brush or barrier so the fill does not run into the neighbour's wall.
Tools needed: Tape measure
Masonry: the damp-proof course to the top of the cavity. Framed: one storey's bay, plate to plate.
In a masonry wall the fill runs from the damp-proof course to where the cavity closes under the roof, or up into the gable. A gable apex is a triangle: run it on its own, with half the apex height as its average height. In a platform-framed wall each storey's bays are closed by plates and the floor between, so run each storey on its own with that storey's height. The 12 m (39.4 ft) limit in the masonry certificates cited here is on the height of the building, not of this run: the page flags a run taller than that, and a building over 12 m (39.4 ft) with shorter runs is one the certificates cover only with the certificate holder's height restriction waiver.
The combined area of the windows and doors in this run, taken off the wall before the volume is worked out.
Width times height of each frame opening, added together. A cavity closes at every reveal, so the space the fill cannot reach is the opening itself; the reveals and lintel areas are left in, which is one reason the volume errs a little high.
How many windows and doors there are in this run; each one adds the extra holes set below.
Count every frame in the elevation, and the air bricks, vents and waste pipes that cross the cavity if the installer drills under them too. A door has no wall under it, so it takes fewer extra holes than a window; count two doors as one opening if you want the figure closer.
The clear gap to fill: the width measured at the survey holes, 65 mm (2.56 in) in the masonry example, or a framed wall's stud depth, 3.5 in (89 mm) for a 2x4.
In masonry, use the clear width the borescope survey measured between the leaves, not the width the age of the house suggests; the masonry certificates cited here cover a nominal cavity from 50 mm (1.97 in). In a framed wall it is the actual depth of the stud: 3.5 in (89 mm) for a 2x4 and 5.5 in (140 mm) for a 2x6, less any foam board already inside the bay.
Tools needed: Borescope or cavity gauge, Tape measure
Leave at 0 mm (0 in) for a masonry cavity; in a framed wall, the stud's face width, 1.5 in (38 mm) for dimensional lumber.
The studs take a slice of every framed wall that the fill cannot occupy, and the share is the stud's thickness over the spacing between stud centres: 1.5 in (38 mm) at 16 in (406 mm) centres is 9.375%, the figure a dense-pack coverage chart adjusts for. Plates, headers and blocking are not taken off.
Masonry: the certificate's spacing along a row, 1.35 m (4.43 ft) for Supafil 34. Framed: the stud centres, 16 in (406 mm) or 24 in (610 mm).
Read it off the drilling pattern in the certificate for the system being installed. Supafil 34 staggers its holes in a diamond at about 1.35 m (4.43 ft) centres; Ecobead Platinum drills at about 600 mm (23.6 in) along a row. In a framed wall there is one hole in every bay, so the spacing is the stud spacing, and the page also uses it to work out the share of the wall the studs take.
Masonry: the vertical centres of the certificate's rows. Framed: the height of each bay section between blocking, or the storey height if there is none.
The page counts one row of holes for every row spacing of wall height, and at least one. Supafil 34 spaces its rows at about 1.35 m (4.43 ft); Ecobead Platinum allows up to 2.5 m (8.2 ft) between rows. In a framed wall each run of blocking closes the bay and needs its own hole below it, so a storey with a row of mid-height blocking is two sections: enter half the storey height.
Holes added for each window or door on top of the regular grid, under a cill, beside a frame, or in a short bay above or below it.
Masonry certificates ask for holes under cills and in the piers between openings so the fill closes round them; a framed wall's bays above a header and below a sill are separate spaces, each with its own hole. Two a window is a starting figure, not a published one: count them off the certificate's drilling drawing, or the installer's marked-up elevation, and change it.
The density the fill is blown to in the wall: 25 kg/m³ (1.56 lb/ft³) for Supafil 34, 3.5 lb/ft³ (56 kg/m³) for dense-pack cellulose.
Use the installed density the product's certificate or dense-pack chart states, never the loose density printed for a loft. The masonry certificates cited here set a target mean over the whole wall: 25 kg/m³ (1.56 lb/ft³) for Supafil 34, 18 kg/m³ (1.12 lb/ft³) for Supafil 40 and 12 kg/m³ (0.75 lb/ft³) for Ecobead Platinum bonded bead. Dense-pack cellulose in a stud wall goes in at 3.5 to 4.5 lb/ft³ (56 to 72 kg/m³). For an injected foam, use its certificate's installed density.
The mass of one bag or bale as sold: 15.5 kg (34.2 lb) for a Supafil 34 bale, 30 lb (13.6 kg) nominal for a GreenFiber INS515LD cellulose bag.
Take it from the bag or the certificate. Supafil 34 comes in bales of about 15.5 kg (34.2 lb) and Supafil 40 bales weigh about 17.6 kg (38.8 lb); GreenFiber INS515LD cellulose, sold through home centres, comes in bags of a nominal 30 lb (13.6 kg), with a minimum net weight of 28.5 lb (12.9 kg) on its chart. Other brands pack differently, so enter the mass printed on the bag being blown. Bonded bead often arrives in bulk containers rather than bags; read the mass line then, and ignore the bag count.
A share added to the fill mass for material lost in the hose, blown into the loft and cleared out, or left in a part-used bag.
Unsourced, which is why it opens at zero: none of the certificates or charts cited here gives a figure, and the installed density already describes a full wall. Set it from your own delivery tickets against the walls they filled.
Bags of fill at the installed density
7 bags
The mass is the fill volume at the installed density entered, and the bags are that mass over the bag mass entered, rounded up. The holes are a grid estimate: what gets drilled is the pattern the system's certificate or the manufacturer's instructions draw for this elevation, which the installer marks up after the survey.
- Wall area to fill, openings deducted
- 221 ft²
- Fill area, stud faces deducted
- 200.28 ft²
- Fill volume
- 58.42 ft³
- Fill mass at the installed density, allowance included
- 204.45 lb
- Holes along each row
- 25 holes
- Rows of holes up the wall
- 1 row
- Extra holes at openings
- 6 holes
- Injection holes in all
- 31 holes
They open the calculator with your figures already in it
Cavity Wall & Stud Bay Insulation Fill Calculator: 7 bags — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- 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/ft², 29.39 ft² a bag before framing and 32.4 ft² 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
Inputs used
- Wall Length
- 33 ft
- Wall Height to Fill
- 8 ft
- Openings Area (windows and doors)
- 43 sq ft
- Number of Openings
- 3
- Cavity Width or Stud-Bay Depth
- 3.5 in
- Stud Thickness (0 for a masonry cavity)
- 1.5 in
- Hole Centres Along a Row (stud spacing if framed)
- 16 in
- Row Spacing Up the Wall (height between blocking if framed)
- 8 ft
- Extra Holes per Opening
- 2
- Installed Density (from the certificate or chart)
- 3.5 pcf
- Bag Mass
- 30 lb
- Waste Allowance (%)
- 0
Intermediate steps
- Wall area to fill, openings deducted
- 221 ft²
- Fill area, stud faces deducted
- 200.28 ft²
- Fill volume
- 58.42 ft³
- Fill mass at the installed density, allowance included
- 204.45 lb
- Holes along each row
- 25 holes
- Rows of holes up the wall
- 1 row
- Extra holes at openings
- 6 holes
- Injection holes in all
- 31 holes
Confidence note: The mass is the fill volume at the installed density entered, and the bags are that mass over the bag mass entered, rounded up. The holes are a grid estimate: what gets drilled is the pattern the system's certificate or the manufacturer's instructions draw for this elevation, which the installer marks up after the survey.
What this calculation does not cover
- Whether this wall should be filled at all is not in the arithmetic. Driving-rain exposure, the clear width hole by hole, mortar snots and rubble bridging the leaves, a partial-fill board already inside, the state of the ties, missing cavity trays and damp already on the inner leaf are the installer's survey to establish before anything is drilled: the site assessment by a trained assessor that the system's certificate calls for (BBA 11/4857 for Supafil 34, section A.1), a practice set out in BS 8208-1:1985, a guide BSI withdrew in 2009. The page takes a wall that survey has cleared and counts what goes into it.
- Some wall voids must never be filled, and nothing here tells them apart: the air space behind a brick veneer on a timber or steel frame, and the gap between a masonry outer leaf and a timber-frame inner wall, are drainage and ventilation spaces. The page counts material for a two-leaf masonry cavity or for closed stud bays; which of those a wall is belongs to the surveyor.
- Air bricks and sleeves feeding an underfloor void or a combustion appliance, flue terminals and extract ducts that cross the void have to be sleeved or closed off before filling and checked clear afterwards, as the certificates cited require. The volume does not deduct them and the hole grid does not route round them.
- In a framed wall, knob-and-tube wiring must not be buried: the National Electrical Code does not permit it in a hollow wall insulated with loose or foamed-in-place fill that surrounds the conductors, so an electrician deals with it before any fill goes in. When the work is done for pay on a home built before 1978, opening painted siding in a way that disturbs more than 20 sq ft (1.86 m²) of it falls under the EPA's Renovation, Repair and Painting rule, which calls for a certified firm working lead-safe.
- The holes are a rectangular grid, holes along each row times rows up the wall, plus a fixed number an opening. A certificate draws its own pattern: Supafil 34 staggers 22 mm (0.87 in) holes in a diamond at about 1.35 m (4.43 ft), starts about 800 mm (31.5 in) above the damp-proof course and keeps the top holes within 350 mm (13.8 in) of the top of the cavity and no more than 1 m (3.28 ft) apart along it, or 1.35 m (4.43 ft) under a gable slope, so its top row usually takes more holes than the grid gives it; Ecobead Platinum drills at about 600 mm (23.6 in) with rows up to 2.5 m (8.2 ft) apart. The grid can land a row short or a row over; the marked-up elevation governs.
- Volume is the gap times the net area, with the studs' share taken off a framed wall. Plates, headers, blocking, window reveals where the leaves close together, and a cavity that narrows or widens along its length are left in, so a real wall usually takes a little less than this; a wall that takes far less is one whose cavity was already partly blocked.
- The mass follows the installed density typed in, and nothing is added for settling: the masonry certificates set a target mean over the whole wall, 25 kg/m³ (1.56 lb/ft³) for Supafil 34 and 12 kg/m³ (0.75 lb/ft³) for Ecobead Platinum, and dense-pack is blown to a minimum such as 3.5 lb/ft³ (56 kg/m³) so that it holds its place. A loft chart's lower density counts too few bags for the same wall.
- Nothing here stands for a guarantee or for building-control sign-off. In England and Wales, putting insulation into a cavity wall is building work notified to building control, directly or through an installer registered with a competent person scheme, and a UK installer registers the guarantee, commonly through the Cavity Insulation Guarantee Agency (CIGA). Whether either is in place is the installer's paperwork, not this estimate.
Estimated cost — your price
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Part of a bigger job
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also measuresMasking and Protection Materials
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-28 · v1.0.0
Regulatory standards & verification citations10
- 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/ft², 29.39 ft² a bag before framing and 32.4 ft² 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
Which documents these citations point at
- Approved Documents to the Building Regulations (England) — D (United Kingdom)Practical guidance on meeting the Building Regulations, one lettered part per subject — Part A structure, Part B fire, Part K protection from falling, Part L conservation of fuel and power, and the rest.
- National Electrical Code (NFPA 70) (United States)Electrical installations — conductor sizing and protection, load calculation, wiring methods, grounding and working clearances.
A code or standard has force only where a jurisdiction has adopted it, usually with local amendments. This site holds no adoption data for any authority, so check what is in force with the authority where you build. Any section cited above without an edition should be checked against the edition in force where you build. What it would take to know.
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Protection this work requires
- Stop and check first. Textured ceiling coatings, board behind panels, floor tiles and bitumen adhesive in older buildings — built before asbestos was banned or phased out where you are — can contain asbestos, and drilling or sanding them releases fibres. Do not disturb it — have it sampled first. This is not a job for better PPE.
- Stop and check first. Paint under later coats in older housing can be lead-based, and sanding or heat-stripping it produces lead dust that is especially harmful to children and to anyone pregnant. Test before you disturb it — this is a job for a tested, controlled method, not a better mask.
Tools and safety for this job
To drill the fill holes and blow the fill into the cavity or dense-pack the stud bays. Generic types, no brands, no prices.
- Cutting or grinding concrete, masonry, screed, tile or fibre-cement board releases respirable silica: cut wet or extract at the tool, and wear a P2/N95 respirator at minimum — a nuisance dust mask does not filter it.
- Mineral and glass wool irritate skin, eyes and airways: cover your arms, wear a P2/N95 respirator while cutting, and rinse in cold water — hot water drives fibres in.
- Saws, grinders and breakers run above 85 dB, where hearing damage accumulates and does not recover: defenders or plugs for every cut, not just the long ones.
- Breakers and grinders cause permanent nerve damage: limit continuous trigger time, keep hands warm, and stop if fingers tingle or blanch.
- Boards, blocks and bagged material cause most lasting back injuries on small sites: two people or a lifter for full sheets, and never a bag on one shoulder up a ladder.
- Most fatal falls on small jobs are from under three metres: use a tower or a ladder tied at the top, and never work off the top two rungs.
- Nailing, chiselling and cutting all throw fragments: glasses to EN 166 or ANSI Z87.1, and goggles rather than glasses overhead.
Blow the fill into the cavity or dense-pack the stud bays: Filling a cavity is the work of an installer approved under the system's certificate, which sets the product, its density, the drilling pattern and the walls it covers; this site does not certify an installation, never judges a wall suitable and gives the bags only to discuss and order against. Fill that reaches a flue or blocks a vent is what the check after filling looks for, and an open-flued boiler, fire or stove in the house is then spillage-tested by the engineer its fuel needs, listed as its own item.
Show the 7 tools this job needsHide tools
Essential
Insulation blower and hose
SDS rotary hammer
Recommended
Cordless drill/driver
Dust extractor or vacuum with a fine filter
Dust sheets and masking
Extension ladder
Hole saw set
Also needed as materials: bagged loose-fill, drill and driver bits, masking tape, plastic sheeting, SDS bits and chisels.
what each insulation tool is for, and the spec that decides which to buy where one does.