Which UK properties have cavity walls?
The short answer is: most houses built after roughly 1920 and before the mid-1990s. Before that period, the standard construction method was solid brick or stone, with two leaves of masonry laid tight against each other. A solid-wall house cannot have cavity-fill insulation installed without a different retrofit approach entirely (internal or external wall insulation), so these properties are excluded from a cavity-wall campaign without exception.
From around 1920, builders began leaving a gap between the inner and outer leaves of brickwork, both for structural reasons and to resist rain penetration. That gap (typically 50 mm to 75 mm) is the cavity. Most of these homes were built without insulation fill, which means the void still exists and can accept blown mineral wool, bead, or foam today. The 1930s semi-detached house is the canonical example: tens of thousands of them occupy streets across the East Midlands, Yorkshire, and the North West, and a large proportion remain unfilled.
By the late 1980s and through the 1990s, Part L of the Building Regulations began requiring insulation in new construction. Post-1995 new-builds are generally assumed to have been built with cavity insulation already in place, making them low-priority for a retrofit installer. There are exceptions, particularly in the early 1990s transitional period, but the diminishing returns do not justify a wide net on properties built after 1995.
Property type matters too. Detached and semi-detached houses, and traditional terraced houses, are the primary targets. Purpose-built flats constructed with cavity external walls can be targeted, though the surveying and installation process is more complex, and many block-of-flats decisions rest with a freeholder or managing agent rather than individual leaseholders. For a consumer direct-to-householder campaign, houses are the cleanest segment.
Why property age data is imperfect
Consumer prospect data carries property age derived from Valuation Office Agency (VOA) records, council tax banding files, and land registry data. These are reasonable proxies but not architectural surveys. A data-driven campaign will inevitably include some solid-wall properties miscoded to the 1920s-to-1995 band, and miss some properties where cavity fill was retrofitted in the 1980s under an earlier scheme. The industry norm is to accept that a proportion of contacts will self-select out early in the sales process when a telephone surveyor or site visit confirms the property is unsuitable. In our experience, proper property-age filtering, combined with a phone survey step before dispatch, keeps disqualification rates below 20% on a well-selected list.
What is ECO4 and why does it shape data selection?
The Energy Company Obligation scheme, in its fourth iteration (ECO4), requires the six largest UK energy suppliers to fund insulation and heating improvements for low-income and fuel-poor households. It ran from April 2022 with an original end date of March 2026, and a successor obligation is expected. For installers, ECO4 is the mechanism that turns a £3,000 to £6,000 installation job into a free offer for the householder. That is a fundamentally different sales proposition from a self-funded upgrade.
ECO4 eligibility is determined primarily by household income and benefit receipt. The key qualifying benefits include Universal Credit, Pension Credit, Child Tax Credit, Working Tax Credit, Income Support, and several disability-related benefits. Households not on qualifying benefits can still access ECO4 funding through a LA Flex route if the local authority deems them fuel-poor, typically assessed against an income threshold of around £31,000 per year gross (the figure varies by scheme iteration and local authority).
For data selection, this means layering two additional filters on top of the property criteria:
- Benefits-receipt indicator: Consumer files sourced from survey and lifestyle questionnaire data often include a self-declared field for receipt of qualifying benefits. This is not a real-time DWP data feed; it is a stated response from when the individual completed the survey, so it will have some decay. A 12-to-18-month freshness threshold is sensible.
- EPC rating: ECO4 funding is directed at properties with a low energy performance rating (band D, E, F, or G). EPC certificates are published by the Ministry of Housing, Communities and Local Government (MHCLG) and can be matched against postal addresses. Many consumer data suppliers append EPC rating or a modelled equivalent. A band-D-or-below filter, combined with the property-age band and homeowner status, narrows the total universe to approximately 400,000 to 600,000 households once TPS and MPS suppression is applied.
For installers working on a self-funded basis (where the householder pays), the ECO4 filters are irrelevant. The selection shifts to homeowners in the right property-age band who have disposable income and declared interest in energy-saving home improvements. Affluence indicators, council tax band (B or above is a common proxy for willingness to self-fund), and an age skew toward the 45-to-70 range are common additions.
Targeting criteria table: funded versus self-funded campaigns
The table below sets out the recommended selection criteria for each installer type. These are starting points; a data supplier can run counts against multiple combinations before you commit to a volume.
| Filter | ECO4-funded campaign | Self-funded campaign | Notes |
|---|---|---|---|
| Property age | 1920 to 1995 | 1930 to 1985 (tighter) | Tighter band reduces survey failures on solid-wall properties |
| Property type | Detached, semi-detached, terraced house | Detached, semi-detached | Self-funded campaigns often exclude terraced to raise average job value |
| Tenure | Owner-occupied | Owner-occupied | Homeowner status is mandatory for both routes |
| EPC band | D, E, F, or G (required) | D or E preferred; C acceptable | Band A or B properties are unlikely to benefit from cavity-fill alone |
| Benefits receipt | Universal Credit, Pension Credit, or other qualifying benefit | Not required; exclude where possible to avoid wasted calls | Self-declared; treat as indicative, not definitive |
| Income indicator | Below approximately £31,000 gross household income | Council tax band B or above; household income £25,000+ | Modelled income; verify intent through telephone survey step |
| Age | 40 to 75 (longer tenure, more likely to be the long-term resident) | 45 to 70 (disposable income peak) | Occupiers under 35 in owned property have lower average tenure |
| Declared interest | Home improvements, energy saving, or government grants | Home improvements, energy efficiency, or eco/environment | Declared interest on the source survey; strong uplift to response rate |
| Channel preference | Telephone (fastest to qualify) with postal follow-up | Telephone or direct mail, email where opted-in | All channels require appropriate consent or suppression wash |
How to combine affluence data with grant-eligibility filters
The two segments above are not mutually exclusive universes; they overlap in a specific way that creates a targeting problem for some installers. A household can be on Pension Credit (ECO4-eligible) and live in a well-maintained semi in a prosperous postcode. Equally, a household with modest income may not be on any qualifying benefit but could access funding through the LA Flex route. Rigid binary filters will miss both groups.
The practical answer is to build the selection in two phases. Phase one targets the core ECO4 segment: benefits-confirmed, EPC band D or below, property age 1920 to 1995, homeowner. That produces your highest-conversion, lowest-cost-per-installation universe. Phase two adds a modelled-income overlay, targeting households in the £18,000 to £31,000 income range who are not confirmed on benefits but are statistically probable ECO4-eligible through LA Flex. The data file carries a confidence score or decile rank on the income model; select the top three to four deciles to keep the signal-to-noise ratio acceptable.
For self-funded campaigns, the affluence story is simpler. Council tax band C or above, detached or semi-detached house in the 1930s-to-1985 build range, household head aged 50 to 69, with a declared interest in home improvements. That band represents a buyer who can write the cheque and has clear motivation. The interest declaration is the single highest-value signal: a householder who ticked "energy-saving home improvements" on a lifestyle questionnaire two years ago is four to six times more likely to agree to a survey than a cold demographic-only record.
GDPR and PECR considerations for green-energy outreach
The Information Commissioner's Office (ICO) has been alert to aggressive marketing in the green-energy and insulation sector since at least 2019, when a series of enforcement actions targeted companies operating lead-generation websites that obtained consent through ambiguous or pre-ticked boxes. The context matters here: when you buy a consumer prospect file for cavity wall insulation outreach, the compliance obligation sits with both the data supplier and you as the data user.
Which channels need what basis?
Under the Privacy and Electronic Communications Regulations (PECR), unsolicited marketing calls to individuals require TPS suppression as a minimum; calling a TPS-registered number without consent is a civil offence and the ICO has issued fines into six figures for bulk offenders in this sector. For email and SMS to consumers, PECR requires specific, granular prior consent. A consumer data file for electronic outreach must therefore be a fully opt-in consumer file under UK GDPR and PECR consent, with consent that names the marketing category (home improvements, energy saving, or equivalent) and was given to the data controller or a named third party.
Direct mail sits in a different position. The UK GDPR permits direct mail to consumers under Article 6(1)(f) legitimate interests, provided three conditions are met: a Legitimate Interests Assessment (LIA) is documented, the marketing is not overridden by the individual's interests or fundamental rights, and opt-outs are honoured promptly. A postal-led campaign to homeowners is the most straightforward channel from a legal standpoint, and in our experience it still produces strong response rates for high-value home-improvement offers because the physical piece carries credibility that a cold text message does not.
ICO scrutiny in the green-energy sector
The ICO has specifically called out the insulation and green-energy sector as an area of heightened concern. Before running a telephone or electronic campaign, confirm that your data supplier can provide a copy of the consent record for each contact, confirm the consent was obtained in the last 24 months, and that the consent language is sufficiently specific to cover cavity wall insulation marketing. Vague "home improvements" consent granted five years ago is unlikely to satisfy an ICO investigation. Review the ICO's direct marketing guidance before launching.
Suppression files: TPS, MPS, and your own do-not-contact list
A TPS wash is legally mandatory for outbound telephone campaigns to individuals. The Telephone Preference Service is maintained by the Direct Marketing Association (DMA) and must be checked within 28 days of use. The Mail Preference Service (MPS) is the postal equivalent; washing against it is not a legal requirement but is considered best practice under the DMA Code and reduces complaints significantly. For a cavity wall insulation campaign, the typical suppression package is TPS plus your own existing customer file and do-not-contact list. Corporate Telephone Preference Service (CTPS) suppression applies only to calls to business lines, which is not relevant here.
See our guide to TPS, MPS, and CTPS explained for UK marketers for a full breakdown of suppression requirements by channel.
What data fields should you specify when ordering?
The fields below are those most commonly requested for a cavity wall insulation campaign. Not every field is available on every record; a good data supplier will run a count showing coverage rates before you commit to volume.
- Full name and postal address: mandatory for all channels.
- Telephone number (landline and/or mobile): with TPS flag appended; essential for telephone outreach.
- Email address: for digital follow-up; only usable for PECR-consented records.
- Property age band: the primary structural filter (see above).
- Property type: detached, semi-detached, terraced, flat/maisonette.
- Homeowner indicator: owner-occupier vs. renting; this is a modelled field derived from mortgage propensity data and Land Registry matches.
- EPC band or modelled equivalent: D, E, F, G preferred for ECO4 campaigns.
- Benefits-receipt indicator: self-declared on source questionnaire.
- Household income band: modelled; typically presented in £5,000 or £10,000 bands.
- Age of head of household: essential for both ECO4 age-targeting and self-funded disposable-income profiling.
- Declared interest category: home improvements, energy saving, government grants, eco/environment.
- Consent date: the date on which the individual provided opt-in consent; for PECR-covered channels, reject records more than 24 months old unless refreshed.
Install rates and conversion benchmarks
Realistic conversion benchmarks are difficult to state as single figures because they vary enormously by channel, campaign quality, and whether the job is funded or self-funded. The following are broad industry ranges based on multi-installer campaign data.
For direct mail, a response rate of 1% to 3% is the norm on a cold homeowner list. That means one to three householders requesting more information per 100 pieces posted. ECO4-funded campaigns offering a free survey and free install can push this to 3% to 5%, because removing the cost objection changes the proposition entirely.
Telephone outreach to opted-in consumer numbers in this segment typically converts at 8% to 15% of answered calls to a booked survey, provided the caller opens with a property-eligibility check (year built, property type) before moving to the offer. Calling without a pre-screen step inflates survey-fail rates and wastes both caller time and consumer goodwill. Of booked surveys, roughly 60% to 70% result in a confirmed installation in well-run operations, though this depends heavily on the surveyor's ability to confirm the property is technically suitable.
The combined funnel from initial mailing to installed job is therefore roughly: 1,000 records mailed, 20 to 50 responses, 15 to 40 surveys booked, 10 to 28 installations completed. At an average self-funded job value of £800 to £1,500 (or an ECO4 installer margin of £400 to £900 per job depending on the funding agreement), that makes the economics of a well-targeted list very attractive against the data cost.
These numbers assume the list has been correctly filtered for property age and homeowner status before dispatch. Sending to an unfiltered homeowner file without the property-age overlay typically halves the survey conversion rate, as a significant portion of contacts live in post-1995 or pre-1920 properties where the installer cannot offer cavity fill.
For more on reaching homeowners at scale, see our article on UK homeowner data for targeted campaigns. Solar panel installers targeting a similar audience may also be interested in our forthcoming guide on solar panel prospect data for UK installers.