Solid Wall Insulation: Internal vs External, and What to Check

Should you insulate a solid wall internally or externally?

External wall insulation performs better and costs more: expect £6,000–£22,000 depending on property type, against roughly £60–£100/m² for internal insulation, or about £7,500 for a typical three-bed. Both routes have to reach the same target of 0.30 W/m²K under Table 4.3 of Approved Document L Volume 1. The decision is usually settled by three things that have nothing to do with thermal performance: whether you can change the look of the outside, whether you can afford to lose floor area, and whether you can move out of the rooms while the work happens.

What this means for your project

A solid wall is a single leaf of masonry with no cavity to fill. Most houses built before about 1920 have them, and the wall is normally the single biggest fabric heat loss in the building. There is no cheap fix here. Where a cavity wall can be filled for a few hundred pounds and loft insulation topped up for less than a thousand, solid wall insulation is a five-figure job that touches every external surface of the house.

That is worth saying plainly, because it changes the order you should do things in. If your loft is still at 100mm and your draughts have never been sealed, those come first. Solid wall insulation is the last and most expensive step of a fabric upgrade, not the first, and any installer who leads with it before asking what else you have done is selling a product rather than solving a problem.

The regulatory position is more specific than most quotes admit. Approved Document L treats an existing wall as a "thermal element", and adding insulation to its inner or outer face counts as renovating that element. Once you renovate more than half the surface of an individual element, or carry out a major renovation touching more than a quarter of the whole external envelope, the whole element has to be brought up to standard. That standard is 0.30 W/m²K, not the 0.18 figure that applies to a new wall in an extension, and not the cavity figure of 0.55.

There is a genuine relaxation buried in the same section, and installers use it more often than they explain it. If reaching 0.30 is not technically or functionally feasible, or would not pay back within 15 years, the element only has to be upgraded to the lowest U-value that is feasible and does pay back inside 15 years. Paragraph 4.13 puts a floor under that relaxation, though: generally an upgraded thermal element should still not end up worse than 0.70 W/m²K, and anything below even that is only contemplated where the work complies with Part C on interstitial and surface condensation. So the relaxation is a legitimate route, not a licence to fit a thinner board because it is cheaper. If your quote is working to something other than 0.30, ask which limb of that test it is relying on and get the answer in writing.

Scope note: the Approved Document L quoted here is the England edition, and its cover page says so in terms, "for use in England". Wales has its own Approved Document L published by the Welsh Government, with an updated version taking effect on 4 March 2027. Scotland works to Section 6 of the Building Standards Technical Handbook and Northern Ireland to Technical Booklet F. So if you are getting quotes in Scotland, Wales or Northern Ireland the paperwork will look different even though the physics does not.

Internal against external, line by line

The floor area point is not trivial and it is the one homeowners consistently underestimate. Take a 4m by 4m bedroom with two external walls. That is 8 linear metres of wall. At 90mm of insulated board you lose 0.72m², which is 4.5% of the room, and it comes off exactly where the wardrobe was going. Approved Document L's own note allows a lesser standard where the loss would exceed 5%, which tells you how close a normal room is to the limit.

Cost by property type for external systems, supplied and fitted:

Those bands are indicative installer ranges collected from UK quotes, not a published government figure: there is no official cost table for solid wall insulation. Treat them as a sanity check on your own three quotes, and expect regional variation.

They also move with the number of exposed elevations, the height, the amount of detailing around bays and porches, and access. On a two-storey house the scaffolding is a real line item and should be priced separately rather than buried in a rate per square metre. Internal work is normally quoted on the area of wall actually treated, which is why it is the cheaper route on a house where only two or three rooms are cold.

The quality problem nobody selling you insulation will mention

In January 2025 the government published guidance titled what to do if you have poor quality wall insulation, covering solid wall insulation fitted under ECO4 from April 2022 onwards and under the Great British Insulation Scheme. The problems it describes are poor installation or failure to meet building regulations, leading to damp or mould.

Two details matter enormously if you are choosing between the two systems. First, every property that had external wall insulation fitted under those schemes is being offered a home check by a qualified professional. Second, the guidance is explicit that for internal wall insulation "no action should be required", because the audit found fewer issues with it. That is the opposite of what most comparison pages imply, and it is worth holding onto: external insulation is the better system in theory and was the one that went wrong at scale in practice.

The Warm Homes Plan, published on 21 January 2026, went further. It records that solid wall insulation "has also been found to be much more likely to be non-compliant with relevant regulations than other measures, and can cause problems including damp and mould in homes if left unresolved", and confirms that the Energy Company Obligation, "which was the source of the majority of poor-quality installations, will be coming to an end".

Where remediation is needed, you should not be asked to pay any money for it. Ofgem administers the schemes and writes to affected households, TrustMark handles quality assurance, and there is a freephone line on 0808 169 4447 for checking whether someone contacting you about it is genuine. That last point exists because the story has attracted people cold-calling homeowners about insulation they may not even have.

The uncomfortable corollary: none of that applies to a privately funded job. If you pay for external wall insulation yourself, there is no scheme administrator, no Ofgem letter and no free remediation. Your protection is the contract, the system guarantee and whether the installer is still trading in ten years. That is exactly why the checks below are worth doing before you sign rather than after.

Who signs it off

Adding insulation to an internal or external wall is controlled work, and it has to be notified. There is more than one lawful route to sign-off, and your quote should say which one it is using.

A quote with no line for either self-certification or a building control application is a quote missing its building regs, and the compliance certificate is the document a buyer's solicitor will ask for. If you are unsure which route your installer is using, how building control and approved inspectors work sets out the difference. Retrofitting the paperwork afterwards is possible through a regularisation application but it costs more and it is not guaranteed.

Planning is a separate question and only really bites on external systems. Adding 100mm of insulation and render to the outside of a house materially changes its appearance, so it is development. For most houses it falls within permitted development, but in England Schedule 2, Part 1 of the General Permitted Development Order 2015 requires at paragraph A.3(a) that the materials used in exterior work are of similar appearance to those used already, and at paragraph A.2(a) it removes cladding any part of the exterior with stone, artificial stone, pebble dash, render, timber, plastic or tiles altogether on article 2(3) land, which Part 1 of Schedule 1 to the Order defines as a conservation area, an area of outstanding natural beauty (now branded a National Landscape), an area specified under section 41(3) of the Wildlife and Countryside Act 1981, the Broads, a National Park or a World Heritage Site. Wales runs a separate permitted development order, so check the Welsh rules if the house is there. Rendering over a brick terrace in a conservation area is not a grey area. Ask your local planning authority before the deposit goes down, not after the scaffolding is up.

What goes wrong

Insulating a wall that is already wet. This is the most expensive mistake on the list because it is invisible until the insulation traps it. Failed pointing, a blocked gutter, a bridged or missing damp-proof course, spalled brick after frost: all of these need fixing first. A solid wall gets rid of absorbed rain by drying to both faces. Cover one face and you halve the drying, so any existing moisture problem gets worse. If there is damp on the wall now, get the cause diagnosed properly before you insulate, and read the real cost of damp proofing before accepting a chemical injection quote from the same firm selling the insulation. The same logic is why a condensation risk assessment belongs in the quote: insulating the inside face moves the dew point into the construction, and Approved Document L points at Part C and the condensation standards BS 5250 and BS EN ISO 13788, along with BRE's BR 262. A quote naming a board thickness but nothing about vapour control has skipped the part of the design that decides whether your walls rot. On solid brick or stone with lime mortar, ask whether a vapour-open build-up beats a foil-faced laminate.

Bridging the damp-proof course with external insulation. External systems must stop above the DPC line, or be detailed with a proprietary base track and a drainage path, so that ground moisture cannot track up behind the insulation. Insulation carried straight down past the DPC into the soil is a defect, not a detail, and it is one of the faults the scheme home checks are looking for. Look at where the render stops when you view the installer's previous jobs. The same 100mm also pushes the finish towards the boundary line: on a terrace or semi the flank wall often sits on or very near it, which needs the neighbour's written agreement, and where it is a party wall the Party Wall etc. Act 1996 may be engaged in England and Wales. Neither issue is the installer's problem once the render is on, so settle it before work starts.

Cold bridges at the parts nobody insulates. Window and door reveals, cills, the eaves, intermediate floor joists built into the wall, and the returns where a party wall meets the external wall. Internal insulation is much harder here, because the floor and party wall junctions cannot be insulated through without opening up the structure. Uninsulated reveals around a newly insulated wall create a cold strip where condensation and black mould appear within a winter. A competent internal system insulates the reveals and returns the insulation along the party wall by a sensible distance. Approved Document L's own guidance recognises that returning insulation at party walls can raise acoustic issues and that a target of 0.35 or above may be more appropriate in those circumstances, which is a design decision, not something to be discovered on site.

Mixing components from different systems. External wall insulation is certified as a complete system: adhesive, board, mechanical fixings, mesh, basecoat, primer and topcoat from one manufacturer, tested together. Substituting a cheaper render or a different fixing voids the certificate and usually the guarantee with it. Ask for the certificate number for the exact system on your house and check that the products listed on the quote match it. An installer who cannot answer that is assembling something rather than installing something.

Everything that has to be moved and put back. Rainwater goods on extended brackets, meter boxes, external lights, satellite dishes, soil pipes, the boiler flue, and every extract terminal now buried 100mm deeper than it was. The flue extension is Gas Safe work and is frequently missing from the price. On internal jobs the equivalent list is skirtings, architraves, radiators, sockets and switches moved forward, curtain poles, and the plastering and redecoration at the end. A quote that does not name these is not cheaper, it is incomplete, and the difference will arrive as a variation.

Before you sign

The quotes for this job vary more than almost anything else a homeowner buys, and the spread is rarely about the insulation. It is about whether the reveals, the DPC line, the flue, the scaffolding, the building control notification and the making good are in the price or waiting outside it. Upload the quote and we check the specification against the 0.30 W/m²K target, flag where the notification route is missing, and list the detailing items that have been left out. It usually takes about ten minutes, and it is a lot cheaper than finding out in February.