Solid Wall Insulation – Complete Guide & Materials (UK 2025)
Improve energy efficiency, reduce bills and protect your property. This definitive guide covers internal and external solid wall insulation, costs & savings, moisture control, step-by-step installation, and the complete range of EWI materials we supply for homeowners and contractors.
- Expert advice
- Full EWI range
- Fast UK delivery
- For homeowners & trade
What is solid wall insulation?
Solid wall insulation (SWI) upgrades external walls that are built without a cavity (typically older UK homes, pre‑1920s). Because there’s no air gap between inner and outer layers, these walls lose heat quickly. Insulating them either internally (from inside the rooms) or externally (as an EWI system on the facade) improves thermal performance, reduces drafts and condensation risk, and enhances comfort all year round.
Unlike cavity wall insulation, which fills the gap between two leaves, SWI adds a continuous thermal layer. With external systems, that layer also protects the fabric from weather, reducing freeze–thaw stress and helping the building last longer. Penguin BM supplies full EWI systems (adhesives, insulation boards, mesh, basecoat and renders) for contractors and homeowners.
- Quick facts
Best for: solid brick/stone walls without a cavity.
Routes: Internal (IWI) or External (EWI).
Key gains: lower bills, warmer rooms, improved façade protection (with EWI).
Disruption: IWI = indoors; EWI = outdoors (no loss of floor space).
| Feature | Solid wall | Cavity wall |
|---|---|---|
Construction |
Single solid leaf (brick/stone) |
Two leaves with an air gap |
Heat loss |
Higher without insulation |
Moderate; cavity helps |
Insulation approach |
Internal or external layer |
Inject material into cavity |
Disruption |
IWI: indoors; EWI: outdoors |
Low; small exterior holes |
How to identify solid walls
- Age of property: pre‑1920s homes are likely to have solid brick or stone walls.
- Wall thickness: solid brick walls are typically ~225mm thick (one brick). Cavity walls are ~300mm or more.
- Brick pattern: alternating headers and stretchers (Flemish bond) often indicate a solid wall.
- Survey: a professional survey confirms wall type and suitability for insulation.
Internal vs external solid wall insulation
- What it is
EWI adds insulation to the outer face of the building, then covers it with mesh, basecoat and a decorative render. It creates a continuous thermal blanket and protects brickwork from weathering.
- No loss of internal floor area
- Improved weather protection and façade refresh
- Ideal for occupied homes (work happens outside)
- Pros & cons
Pros
- Highest energy savings potential
- Enhances building durability
- Minimal indoor disruption
Cons
- Higher upfront cost than IWI
- Alters external appearance (planning may apply)
- Access needed around the building
- What it is
IWI adds insulated lining to the inside face of external walls (e.g., insulated plasterboard on battens or adhesive). It’s often quicker and cheaper but reduces room dimensions.
- Lower upfront cost
- Suited to single rooms or staged projects
- No change to external appearance
- Pros & cons
Pros
- Lower cost than EWI
- Works room-by-room
- No external scaffolding required
Cons
- Loss of floor space
- Thermal bridges at partitions if not detailed
- Greater indoor disruption during works
Benefits of solid wall insulation
- Lower energy bills
A continuous thermal layer reduces heat loss and demand for heating.
- Comfort all year
Warmer in winter, less overheating risk in summer.
- Damp risk reduction
- Greener home
Cut carbon emissions and improve your EPC rating.
Costs and savings
Project costs vary with wall area, access, render finish and insulation thickness. External systems typically cost more upfront than internal linings but deliver higher savings and added weather protection. Use our calculator for a personalised bill of quantities and price estimate.
- Costs influenced by thickness (e.g., 50–100 mm), scaffolding and detailing
- Energy savings depend on baseline performance and heating fuel
- Finishes range from acrylic to silicone renders; pricing differs
- Wall area (m²)
- Number of storeys & access
- Openings (reveals, sills, eaves)
- Existing substrate condition
- Insulation type & thickness
- Render system & colour
- Fire & moisture detailing
- Location & logistics
- Moisture-safe checklist
- Pre-insulation damp survey
- Ventilation strategy confirmed
- Thermal bridge details at reveals
- Breathable or VCL layers as designed
- Manufacturer-approved components
Managing moisture and ventilation
Successful SWI requires moisture-aware design. Before installation, address rising or penetrating damp and ensure appropriate ventilation. External systems add weather protection; internal systems demand careful placement of the vapour control layer and airtightness at junctions. Use breathable finishes where appropriate and follow manufacturer guidance.
- Repair sources of damp before insulating (gutters, flashings, pointing).
- Provide ventilation (extract fans in kitchens/bathrooms, background vents).
- Specify compatible primers, basecoats and renders to manage vapour.
Installation process – step by step
Wall assessment, damp checks, substrate repairs, fixings plan.
Apply adhesive; fix EPS/phenolic/mineral wool boards as specified.
Prime and apply decorative render (acrylic/silicone/silicate).
Our products for solid wall insulation
High-performance EPS for external systems; multiple thicknesses for UK retrofits.
Foam & cement-based adhesives for secure, fast installation.
Silicone, silicate and acrylic finishes; wide colour range and textures.
Solid wall insulation vs cavity wall insulation
| Feature | Solid wall insulation | Cavity wall insulation |
|---|---|---|
Suitable homes |
Older, pre‑1920s solid brick/stone |
Post‑1920s with cavity |
Installation |
Internal lining or external system |
Injected beads/wool/foam into cavity |
Upfront cost |
Higher (especially EWI) |
Lower |
Energy savings |
High |
Moderate |
Disruption |
IWI: indoor works; EWI: outside |
Low; quick drilling & filling |
Solid wall insulation – FAQs
What’s the difference between solid and cavity wall insulation?
Solid wall insulation adds an internal lining or an external system to walls with no cavity. Cavity wall insulation fills the gap between two wall leaves using beads, wool or foam. SWI suits older solid walls; cavity insulation suits properties with a built-in gap.
Do I need planning permission for EWI?
How long does solid wall insulation last?
Properly installed systems can last decades. External systems also protect masonry from weather, improving fabric durability.
Is internal or external insulation better?
EWI usually yields greater savings and fabric protection, with minimal indoor disruption. IWI is cheaper upfront and works room‑by‑room. The best option depends on budget, access, planning and project goals.
How do I estimate costs?
Costs depend on wall area, access, thickness and finish. Use our free EWI calculator for a tailored estimate and bill of quantities.
Ready to insulate your solid walls?
Get a quick estimate and a materials list with our free calculator, or talk to our team for expert advice and UK-wide delivery.
