Summary

Solid-wall properties — the pre-1920s Victorian terraces, Edwardian semis, and inter-war housing that make up a large share of the UK's least thermally efficient housing stock — cannot use cavity wall insulation, because there's no cavity. The two remaining options are to insulate the wall from the outside (EWI) or from the inside (IWI), and this decision is one of the highest-value conversations a tradesperson can have with a solid-wall homeowner. Get it wrong and the consequences range from a rejected planning application to genuine structural moisture damage; get it right and the property gains a transformed EPC rating and comfort level for decades.

This is deliberately a head-to-head decision article rather than a technique guide. For the detailed installation specification of each system individually — including insulant selection, fixing details, fire performance rules, and condensation risk assessment methodology — see the dedicated articles external wall insulation and internal wall insulation, and for detailed EWI cost breakdowns see external wall insulation pricing guide. This article focuses specifically on which system to recommend, and why, for a given property and client.

The headline physics: EWI wraps the whole building in a continuous insulation layer, keeping the masonry warm and on the "inside" of the thermal envelope — this is close to a strictly-better outcome from a building physics perspective, because it removes cold bridging at floor slabs, joists, and internal walls almost entirely, and because it doesn't move the dewpoint into the wall structure. IWI insulates room by room from the inside, which is cheaper and less disruptive to arrange, but leaves the masonry cold and pushes the dewpoint toward or into the wall — creating the interstitial condensation risk that dominates the IWI conversation. Cost, planning, and disruption are the practical factors that decide most real jobs; condensation risk is the technical factor that decides whether IWI can be done safely at all on a given wall.

Key Facts

Quick Reference Table

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Factor External Wall Insulation (EWI) Internal Wall Insulation (IWI)
Typical full-house cost £6,000-18,000 Lower per m², but room-by-room labour adds up
Cold bridging Largely eliminated Persists at floor/partition junctions
Condensation risk (PAS 2035:2023) Medium Risk Medium (breathable) to High Risk (vapour-closed)
Internal floor area lost None ~0.1-0.2m² per metre of wall treated
Occupant disruption None (external work) Room must be vacated, 2-4 days/room
Changes external appearance Yes No
Planning permission needed Often yes in Conservation Areas/listed buildings Almost never
Guarantee SWIGA insurance-backed, 25 years typical Installer warranty only, no industry scheme
Best for Unlisted properties, exposed elevations, whole-house upgrades Conservation Areas, listed buildings, budget-constrained or room-by-room projects

Detailed Guidance

Choose EWI when the building can take an external change

EWI is the stronger recommendation whenever the client can accept a change in external appearance and the property isn't in a planning-restricted area. It solves the whole-building thermal problem in one pass, doesn't disrupt the occupants, doesn't reduce living space, and doesn't create the interstitial condensation risk that dominates the IWI conversation. For a semi-detached or terraced house outside a Conservation Area where the client is staying long-term and can fund the higher upfront cost, EWI is close to a default recommendation on technical merit alone.

EWI is also the stronger choice where the existing wall has known damp issues from failed pointing or render — because it stays on the warm side of the masonry, EWI reduces (rather than risks worsening) condensation, and can be combined with masonry repair as part of the same scaffold-up project. See external wall insulation for full system detailing including window/door reveal treatment and fire performance rules above 11m and 18m.

Choose IWI when EWI is planning-restricted or the budget won't stretch

IWI is the practical answer whenever EWI is off the table for planning reasons — Conservation Areas, Article 4 Direction areas, listed buildings, or any property where the external character must be preserved. It's also the right recommendation where the client's budget genuinely can't reach EWI's typical £6,000-18,000, or where the client only wants to treat one or two rooms rather than the whole house (a home office extension, a particularly cold bedroom).

The trade-off the client must understand clearly is the condensation risk. IWI on solid masonry moves the dewpoint toward the cold masonry face; without correct vapour control layer detailing (continuous VCL on the warm side, taped joints, no unprotected service penetrations) and, ideally, a proper Glaser or WUFI condensation risk assessment, IWI can cause hidden interstitial condensation that rots joist ends and damages the masonry from within — a failure mode that is invisible until it's advanced. See internal wall insulation for the full risk detail and the choice between vapour-closed (PIR) and vapour-open (wood fibre, cork) systems.

The Conservation Area / listed building default

This is the single most common real-world reason IWI is chosen over EWI, and it's worth stating plainly to clients: if the property is listed or in a Conservation Area, EWI will very likely be refused or require expensive matching-materials finishes (lime render, brick slips) that erode much of its cost advantage — and Listed Building Consent is always required for any external alteration on a listed building, with a real risk of refusal on heritage grounds. In these cases IWI, ideally using a breathable system (wood fibre, cork, hemp) in line with Historic England guidance, is usually the only realistic route to improving thermal performance without a planning fight. See conservation areas and listed buildings.

Whole-house vs room-by-room — a genuine hybrid option

These two approaches aren't always mutually exclusive on the same property. A common and pragmatic specification on a semi-detached solid-wall house: EWI to the rear and side elevations (where planning sensitivity is lower and appearance change is less visible from the street), combined with IWI to the front elevation (where the street-facing appearance must be preserved, particularly in a Conservation Area). This hybrid approach is increasingly used by Retrofit Coordinators on PAS 2035 funded schemes and is worth proposing where a client's planning position is genuinely mixed by elevation.

Cost — the comparison customers usually get told incompletely

Per m², IWI materials and labour are usually cheaper than EWI — no scaffolding, no render system, no mechanical fixings rated for wind load. But the full-project comparison needs to include everything IWI disrupts that EWI doesn't: decoration reinstatement in every room, skirting and architrave replacement, socket and switch relocation, and the lost use of each room for several days. On a whole-house basis, the total cost gap between EWI and IWI is often smaller in practice than the headline per-m² figures suggest, particularly once decoration and disruption costs are properly itemised for IWI. Always quote IWI as a full room-reinstatement job, not just an insulation-fixing job — under-pricing the finishing work is a common IWI quoting error.

Which improves EPC rating more?

Both, similarly, when correctly specified to the same U-value target (0.30 W/m²K for existing dwellings under Part L). The EPC calculation doesn't distinguish between EWI and IWI achieving the same U-value — the rating improvement is driven by the resulting wall U-value, not the insulation location. The practical difference is that EWI more reliably achieves the target U-value across the whole wall (because it's continuous and eliminates cold bridging at junctions), whereas IWI's performance can be undermined at the untreated cold-bridge junctions even where the field-of-wall U-value looks identical on paper.

Frequently Asked Questions

Can I mix EWI and IWI on the same house?

Yes, and it's a genuinely useful specification where planning constraints differ by elevation — EWI to the rear/side, IWI to the street-facing front elevation in a Conservation Area, for example. The main technical care point is detailing the junction between the two systems at the corner of the building to avoid creating a new cold bridge or condensation risk at the transition.

Is IWI really more risky than EWI?

In terms of interstitial condensation, yes, materially — this is why PAS 2035:2023 classifies IWI on solid walls as a higher risk measure than EWI and requires more stringent design oversight for funded work. The risk is manageable with correct system selection (vapour-closed with continuous VCL, or vapour-open breathable) and detailing, particularly at joist ends, but it is a real risk that EWI largely avoids by keeping the masonry warm.

Does EWI always need planning permission?

No — in most cases, on an unlisted building outside a Conservation Area, AONB, National Park, or World Heritage Site, EWI falls under Permitted Development and doesn't need planning permission. It's the sensitive-area and listed-building cases where consent becomes a real obstacle, which is why checking the property's planning status is the first step in this decision, before quoting either system.

Which is better for stopping damp and mould?

Both can resolve surface condensation and associated mould, by raising the internal wall surface temperature above the dewpoint of room air. EWI does this more reliably and with lower risk, because the masonry itself stays warm. IWI can achieve the same surface-temperature improvement but must be detailed correctly to avoid creating a new, hidden condensation problem within the wall structure — neither system fixes penetrating damp or rising damp, which must be resolved first regardless of which insulation route is chosen.

How much does floor area loss from IWI actually matter?

For most rooms it's a modest but real reduction — roughly 0.1-0.2m² per metre of treated wall, or about 5-6% of a typical box room's floor area. In small rooms, box rooms, or narrow hallways, this can be a genuine deciding factor against IWI, particularly where the client is already tight on space. In larger rooms it's rarely decisive on its own.

Regulations & Standards