Cavity walls: extract, refill or EWI?

How to make the right call and avoid site pauses

Getting cavity wall decisions right at the start saves time, money and headaches. The quickest way to stall a programme is to choose a measure before you know what sits inside the cavity or what condition the ties and substrate are in.

This guide shows a practical route from first survey to a clear decision: refill, extract then refill, or move to external wall insulation (EWI).

A great example of EWI on our Newport City Homes project.

Step 1: borescope first

Open up the evidence before you open up the wall.

What to record

• Insulation present, type and condition

• Damp staining, debris, mortar snots and cavity bridges

• Cavity width and continuity

• Signs of ingress at openings and roof abutments

• Exposure zone and orientation of each elevation

Take dated photos and short notes for every elevation. Store them in the property evidence pack.

Step 2: check wall ties and structure

Wall tie condition can make or break your choice.

Assess

• Tie type and spacing

• Corrosion or failure

• Cracks in outer leaf, bulging or movement

• Lintels, sills and damp proof courses

If ties are failing, programme replacement before any insulation decision. There is no point refilling a cavity that needs structural work.

When refill is OK

Cavity refill can be a good solution where the fabric and exposure allow.

Refill is usually suitable when

• Cavity is clear, continuous and dry

• Ties are sound or programmed for replacement first

• No systemic rain penetration risk for the exposure zone

• Openings and penetrations can be detailed to limit bypass and moisture tracking

• There is no persistent condensation or mould history

Good practice

• Specify certified materials for the wall type

• Drill pattern, density and QA to be recorded

• Reinstate and seal openings properly

• Add background and extract ventilation if airtightness is expected to improve

When to extract first

Extraction is often needed where old insulation has failed or become saturated.

Choose extraction if

• Old fibre has slumped or is saturated

• Debris or mortar snots bridge the cavity

• Uncontrolled bypass around openings was found

• You plan EWI but need a clean, known substrate

• Ties need replacement and the cavity must be clear

After extraction, re assess. Some homes will suit a controlled refill. Others will be better served by EWI.

When EWI is the better answer

EWI can solve thermal bridges and rain penetration issues that cavity refill cannot touch.

Favour EWI where

• Exposure is high with a history of penetrating damp

• Cavity is narrow, irregular or interrupted

• Multiple junctions create repeating cold bridges

• Aesthetic refresh is acceptable or planned

• You want a robust fabric first uplift with continuous insulation

Plan the details early

• Extend eaves and adjust canopies where needed

• Design cills, meter boxes, SVPs, RWPs and vents

• Confirm substrate prep, fixings and fire performance

• Coordinate with window reveals and frame positions

• Define a ventilation strategy that meets Part F

EWI substrate preparation checklist

Before boards go on, make the base right.

• Cavity cleared of loose material after any extraction

• Tie replacement complete and signed off

• Masonry sound, cleaned and primed to system guidance

• Services relocated or spaced off to suit insulation thickness

• Starter tracks level and set to the agreed datum

• Fire stopping, closers and cavity barriers specified for the system

Programme impacts to plan in

The choice you make will shape time, access and cost.

Refill

• Fast on site once surveys are complete

• Limited finishing trades

• Performance depends on cavity quality and junctions

Extract then refill

• Adds time for extraction and re survey

• Better control of outcome than blind topping up

EWI

• Longer lead time for design, details and permissions

• More visible works and resident management

• Strong thermal performance with fewer cold bridges when designed well

Common pitfalls to avoid

• Topping up failed insulation without extraction or tie checks

• Window replacements done without future reveal design

• Re-roofing without coordinating eaves ventilation and planned insulation

• Missing evidence photos of cavities, ties, and junction detailing

• Treating damp symptoms rather than fixing rain ingress or ventilation

Example decision paths we see

• 1960s cavity terrace in moderate exposure

  Borescope shows partial, dry fill and sound ties. Refill with QA, add dMEV to wet rooms, loft top up and basic airtightness works. Good uplift without moisture risk.

• 1950s semi in very severe exposure

  Saturated fibre and corroded ties. Extract, replace ties, move to EWI with extended eaves and designed reveals. Ventilation commissioned at handover.

• Low rise block

  Mixed cavity conditions, complex junctions, and resident complaints about damp. Extraction on worst elevations, EWI to problem facades, and a block wide ventilation upgrade.

Build the evidence pack as you go

• Borescope photos and notes per elevation

• Tie survey and any replacement sign off

• Extraction logs where relevant

• Refill certificates and drill pattern plans

• EWI design, fixings pull out tests and key install photos

• Ventilation commissioning sheets and resident handover note

Quick flow to brief your team

Borescope → Tie check → Decide: refill, extract or EWI → Design details and permissions → Install → Verify and commission

How Target Green can help

We run cavity investigations, coordinate tie surveys, and design moisture safe specifications for refill, extraction and EWI. Our team manages the details that keep programmes moving and reduces the risk of costly pauses.

Get our cavity decision tree and book a short review. We will help you choose the right route for each property, then deliver it with clear evidence and compliant outcomes.