10 Questions to Ask an External Wall Insulation Installer Before You Sign
What to ask, what a good answer sounds like and which answers should end the conversation
Last updated: 27 October 2026. By Darren Smith, Ace Rendering Specialists. Rendering and insulating homes across Kent and South East London since 1999.
Ask an external wall insulation installer ten things before you sign: who does the work, which certified system they use, the target U-value, how damp risk is detailed, who notifies Building Control, what guarantee backs the job, and what the quote and payment terms say. A clear written answer to each question separates a reliable installer from a risky one.
In October 2025 the National Audit Office reported that an estimated 98% of external wall insulation fitted under the ECO4 and Great British Insulation Scheme programmes needed work to correct major issues. That is a finding about government schemes, not about every installer. It does show why a homeowner should check before paying a deposit.
Quick answer: the 10 questions at a glance
Are you TrustMark registered, and can I see the certificate?
Will your own employees do the work, or subcontractors?
Which complete system will you fit, and is it independently certified?
What U-value will my wall reach, and who calculated it?
How will you stop damp getting in at sills, reveals and the base of the wall?
What will you check on my existing walls before quoting?
Who tells Building Control, and what certificate do I receive?
What guarantee do I get, and who pays if you stop trading?
What does the written quote cover, and how is payment staged?
When will you start, and can I speak to a recent customer nearby?
Our recommendation: if an installer cannot answer questions 3, 4, 7 and 8 in writing, keep looking. Those four cover the system, the performance, the legal sign-off and the safety net.
What is external wall insulation?
External wall insulation (EWI) is a layer of insulation boards fixed to the outside of a wall and covered with a reinforced basecoat and a finish such as silicone render. It reduces heat loss through solid walls. It also changes how the house looks and how the wall handles moisture, so installation quality matters more than the brand name on the board.
For a full explanation of the process and benefits, see our guide to external wall insulation in Kent. This post is about choosing who fits it.
Why is installer choice such a big deal for EWI right now?
Installer choice matters because EWI hides its workmanship behind a finished coat. Mistakes at the sills, reveals or base of the wall can let water in months or years later. The 2025 audit of publicly funded installations put that risk into the news.
The key facts, from the NAO press release of 14 October 2025:
An estimated 22,000 to 23,000 homes (98%) with EWI fitted under ECO4 and GBIS needed work to correct major issues.
Between 900 and 2,000 EWI homes (6%) faced an immediate health and safety risk.
Some audits were carried out on 3% to 4% of installations rather than the 7% to 10% the standard stipulated.
According to Gareth Davies, head of the NAO, in October 2025, there were "clear failures in the design and set-up of ECO and in the consumer protection system" that "have led to poor-quality installations".
The government's Warm Homes Plan, announced in January 2026, replaces ECO and GBIS and promises stricter consumer protection, according to Which?. A privately funded job sits outside those schemes, so the checking is down to you.
1. Are you TrustMark registered, and can I see the certificate?
Summary: verify membership yourself, not from a logo on a van.
TrustMark is the government-endorsed quality scheme for work in and around the home. Ask for the registration number and look it up on the TrustMark website. Also check independent review profiles such as Checkatrade and the Which? Trusted Traders directory.
Good answer: a registration number and a certificate you can check the same day.
Red flag: "We are about to join" or a badge with no number.
Ace Rendering Specialists is TrustMark registered, holds a 10/10 Checkatrade rating from over 300 verified reviews, and is endorsed by Which? Trusted Traders.
2. Will your own employees do the work, or subcontractors?
Summary: the person who quotes should be accountable for the person on the scaffold.
EWI has several trades in one job: boards, mesh, basecoat, finish and detailing. When different crews handle each stage, responsibility for a leak can get lost between them. Ask who will be on site, whether they are employees, and who supervises.
Good answer: "Our own in-house team, supervised by a named person."
Red flag: vague talk of "our lads and a few contractors".
Ace uses its own in-house team and no subcontractors on every job.
3. Which complete system will you fit, and is it independently certified?
Summary: an EWI system is boards, adhesive, fixings, mesh, basecoat and finish designed to work together.
A system is the complete, tested set of components. Mixing parts from different suppliers can undermine the performance and any certification. Ask for the system name, the manufacturer and a copy of its third-party certificate, such as a BBA certificate where one exists. Ask which board material and thickness are proposed for your wall, and why.
Good answer: a named system with certificate and datasheet.
Red flag: "It's all the same stuff", or a refusal to name the manufacturer.
Ace works with manufacturers including Johnstone's, K-Rend, Weber and Parex.
4. What U-value will my wall reach, and who calculated it?
Summary: insulation without a calculation is a guess.
A U-value measures how quickly heat passes through a wall. Lower is better. For a renovated wall with external insulation, Approved Document L sets a target of 0.30 W/m²K, as we explain in our guide to the building regulations for rendering. Ask for the before and after figures, the board thickness used in the calculation and the name of the person who produced it.
Good answer: a written calculation tied to your wall build-up.
Red flag: a thickness plucked from the air, or "it meets regs" with nothing to show.
5. How will you stop damp getting in at sills, reveals and the base of the wall?
Summary: most water problems start at the edges, not on the open wall.
Water finds the weak points: window sills, reveals, the base of the wall, pipes, vents and the top of the wall under the eaves. Ask how each will be detailed. Look for extended sills with drips, proper beads and trims, a base track set above ground level, and sealed penetrations. Ace's EWI process includes sealing, detailing, reveal beads and window trims, and sills may need extending or replacing.
Good answer: a specific method for each detail, ideally shown on a previous job.
Red flag: "We just seal it with mastic."
6. What will you check on my existing walls before quoting?
Summary: insulating a damp or failing wall locks the problem in.
A proper survey looks at the condition of the existing render or brickwork, signs of damp, cracks, gutters and downpipes, and whether the wall suits EWI at all. Our guide to signs of failing render lists what we look for. Survey notes should name any repairs needed first.
Good answer: a site visit, measurements and repairs listed in the quote.
Red flag: a price over the phone with no visit.
7. Who tells Building Control, and what certificate do I receive?
Summary: the paperwork is what a buyer's solicitor will ask for later.
EWI is a renovation of a thermal element, so Building Regulations normally apply. Ask who submits the notice, whether the fee is in the quote, and which completion or compliance certificate you will get. Planning permission is a separate check. Conservation areas, listed buildings and Article 4 directions can restrict EWI, as set out in our post on planning permission to render in Kent.
Good answer: a named route, a named person and a certificate at handover.
Red flag: "You won't need anything."
8. What guarantee do I get, and who pays if you stop trading?
Summary: a guarantee is only as strong as the entity behind it.
Ask for the guarantee in writing: its length, what it covers (materials, workmanship or both) and what exclusions apply. Then ask what happens if the company closes. An insurance-backed guarantee is designed to keep protecting you in that case, while a workmanship promise from the installer alone is not.
Good answer: a written guarantee, with the insurer or guarantor named.
Red flag: "Lifetime guarantee" with no document.
Ace offers insurance-backed guarantees.
9. What does the written quote cover, and how is payment staged?
Summary: the quote should leave nothing to argue about later.
A good quote lists scaffolding, preparation, boards and thickness, finish and colour, sills and trims, making good, waste removal, Building Control fees and start and finish dates. Payment should follow progress rather than demand most of the price up front.
Your cancellation rights also matter. Citizens Advice says 14 days is the minimum cooling-off period for work agreed at your home, and starts the day after you approve the work. If you ask for work to begin inside that window, you may have to pay for what has been done if you cancel.
Budget context: a standard semi-detached re-render typically costs £3,000 to £8,000 with Ace, and EWI costs more because of the boards, fixings and extra detailing. Which? notes that a private external solid wall insulation job can cost up to £11,000. Your own figure depends on size, access and system.
Good answer: an itemised, dated quote and a sensible deposit.
Red flag: high-pressure "today only" prices or a large cash deposit.
10. When will you start, and can I speak to a recent customer nearby?
Summary: timing affects quality, and local references prove it.
Render and EWI finishes need suitable weather. Weber, a major render manufacturer, says not to apply render below 5°C, and that "frost and render don't mix". Installers who work through winter should be tenting scaffolding, heating or postponing. Ask what happens if the forecast turns cold, and how long the job will take. Ace states that a standard residential EWI installation typically takes one to three weeks.
Then ask for local examples. Our case studies include EWI projects in Sidcup, Barnehurst and Beckenham.
Good answer: a realistic programme, a weather plan and two or three references.
Red flag: "We can start tomorrow" in January, with no weather plan.
Good answer or red flag? A summary table
Topic
A good installer says
Walk away if you hear
Accreditation
Registration number and certificate
"Joining soon"
Workforce
Own employees, named supervisor
Vague subcontracting
System
Named, certified complete system
Mixed components, no datasheet
Performance
Written U-value calculation
No figures
Detailing
Specific method per junction
"Just mastic"
Survey
Site visit and repairs listed
Phone price only
Compliance
Building Control route and certificate
"No paperwork needed"
Guarantee
Written, insurance-backed
Verbal "lifetime"
Quote
Itemised, staged payment
Big deposit, pressure
Timing
Weather plan and references
Start now, no plan
Who is EWI best for, and who should think twice?
EWI suits solid-wall homes, typically built before the 1920s, and homes needing re-rendering anyway, because scaffolding and wall preparation are already paid for. It is less suitable where the added depth would hide period detail, where space at eaves or boundaries is tight, or where planning restrictions apply. A spray cork coating or a standard silicone re-render may be a better fit for some walls. A surveyor who recommends the cheaper option when it suits your house is a good sign.
Next step: get your answers in writing
Use the ten questions above with any installer, including us. If you are weighing up external wall insulation from Ace Rendering Specialists, we offer a free survey across Kent and South East London. Book a free survey or call 01622 947 167.
Frequently asked questions
How do I choose an external wall insulation installer?
Check TrustMark registration, ask for a named certified system, a written U-value calculation, a Building Control route, an insurance-backed guarantee and an itemised quote. Prefer a firm that uses its own employees and can show finished local jobs. Compare at least three written quotes.
Is external wall insulation worth it?
It can be, for solid-wall homes that lose a lot of heat, particularly when the wall needs re-rendering anyway. Savings depend on the property, existing insulation and heating system. Ask the installer for a calculation specific to your home rather than a generic figure.
Were there problems with EWI fitted under ECO4 and GBIS?
Yes. The National Audit Office reported in October 2025 that an estimated 98% of EWI installed under the two schemes needed work to correct major issues. Which? says Ofgem is writing to affected homes, and that faulty work should be fixed at no cost to the homeowner.
Can I get a grant for external wall insulation in 2026?
Possibly. The Warm Homes Plan replaces ECO and GBIS, and Which? reports a Warm Homes Local Grant, usually for households with an income of £36,000 or less, where local council funding exists. Check eligibility with your council before you commit.
Do I need Building Regulations approval for external wall insulation?
Usually yes. EWI renovates a thermal element, so it normally needs Building Control involvement and a completion certificate. Confirm in writing who notifies Building Control before you accept a quote.
How long does external wall insulation take?
Ace states a standard residential installation typically takes one to three weeks, depending on property size, elevations and access. Cold or wet weather can extend this.
What should an EWI guarantee cover?
It should state its length, cover both materials and workmanship, name who stands behind it, and say what happens if the installer stops trading.