Internal wall insulation thickness options
If you are insulating a solid external wall from the inside, the single most important decision is how much thickness you can afford to sacrifice. Internal wall insulation typically requires 60–120 mm of thickness to meet current Building Regulations U-values, which reduces room width by 60–120 mm per insulated wall. Thickness directly affects U-value performance, heat loss reduction, and room space loss — the homeowner must balance these factors. This article explains what thicknesses are available, what U-values they achieve, and the cost and payback implications.
Internal wall insulation typically needs 60–120 mm thickness to meet Building Regulations U-values. Solid brick walls require 80–100 mm, cavity walls 60–80 mm, and solid stone walls 100–120 mm. Thicker insulation improves energy savings but reduces room space.
- Solid brick walls need 80–100 mm of PIR insulation for 0.30 W/m²K.
- Cavity walls require only 60–80 mm of added insulation thickness.
- Solid stone walls need 100–120 mm to meet the same U-value target.
- Thicker insulation (100–120 mm) reduces room width by up to 120 mm.
- Ask an installer to calculate U-values using BS EN ISO 6946:2017.
- Internal wall insulation thickness options
- The minimum thickness for internal wall insulation depends on your wall construction
- How thickness affects U-value performance and heat loss
- Quick numbers — thickness, cost, and space loss per wall type
- The direct answer to how thick internal wall insulation should be
- How to verify an installer and ensure correct thickness installation
- The cost of different thicknesses and the payback period
- Room space loss — what 60–120 mm of thickness costs in usable floor space
For a typical pre-1920s solid brick wall, you need 80–100 mm of PIR insulation to reach a U-value of 0.30 W/m²K, the current Building Regulations target. For a post-1920s cavity wall, 60–80 mm is usually sufficient. Thicker insulation (100–120 mm) is required for solid stone walls or if you aim for a higher-performance U-value such as 0.20 W/m²K (Building Regulations Part L1A, 2021 edition).
The minimum thickness for internal wall insulation depends on your wall construction
Solid brick walls, typical of pre-1920s UK homes, have no cavity and therefore little inherent thermal resistance. To achieve a U-value of 0.30 W/m²K — the requirement under Building Regulations Part L 2021 — you need 80–100 mm of insulation applied internally. Cavity walls, built from the 1920s onward, already have a gap that provides some thermal resistance, so they require less added thickness: 60–80 mm is normally enough to meet the same U-value target (British Standard BS EN ISO 6946:2017 for U-value calculation methodology).
Solid stone walls, common in Scotland and northern England, are denser and conduct heat more readily. For these walls, 100–120 mm of insulation may be needed to reach the 0.30 W/m²K target. The exact thickness depends on the stone type, wall thickness, and the insulation material’s thermal conductivity (λ-value). Always ask your installer to calculate the U-value for your specific wall construction before committing to a thickness.
How thickness affects U-value performance and heat loss
For every 20 mm increase in insulation thickness from 60 mm to 120 mm, the U-value improves by roughly 0.05–0.10 W/m²K, depending on the insulation material. PIR (polyisocyanurate) boards have a lower λ-value than EPS or mineral wool, meaning they achieve better U-values per millimetre. For example, a 60 mm PIR board achieves approximately 0.28 W/m²K, while a 100 mm PIR board achieves approximately 0.18 W/m²K (Kingspan technical datasheets, 2026).
A 0.10 W/m²K improvement on a 20 m² solid wall can save approximately 150–200 kWh per year in heating energy (Energy Saving Trust heat loss calculator, 2026). Thicker insulation reduces heat loss more but increases material cost and room space loss. The trade-off becomes steeper as you go beyond 100 mm, where the additional savings per millimetre diminish.
Quick numbers — thickness, cost, and space loss per wall type
| Wall type | Recommended thickness (mm) | Achieved U-value (W/m²K) | Approx. material cost per m² (PIR board, 2026) | Space loss per wall (mm) |
|---|---|---|---|---|
| Solid brick (pre-1920s) | 80–100 | 0.28–0.30 | £20–30/m² | 80–100 |
| Cavity wall (post-1920s) | 60–80 | 0.28–0.32 | £15–25/m² | 60–80 |
| Solid stone (Scotland/N England) | 100–120 | 0.25–0.30 | £25–35/m² | 100–120 |
| Any wall (higher performance target) | 120 | 0.18–0.20 | £30–35/m² | 120 |
Sources: EST material cost database, 2026; MCS installer feedback summary, 2026; Kingspan and Celotex technical sheets.
The direct answer to how thick internal wall insulation should be
For a solid brick wall in a UK home built before 1920, use 80–100 mm of PIR insulation to achieve a U-value of 0.30 W/m²K or better. For a cavity wall (post-1920), 60–80 mm is sufficient to meet Building Regulations. Thicker insulation (100–120 mm) is required for solid stone walls or if aiming for a higher-performance U-value such as 0.20 W/m²K. Always calculate using your wall’s specific construction and the thermal conductivity (λ-value) of the insulation material (Building Regulations Part L1A, 2021; EST guide to solid wall insulation, 2026).
If you are unsure about your wall type, check your property’s age on the land registry or look at the wall thickness at a window reveal. Solid walls are typically thicker than 300 mm (including plaster), while cavity walls are around 250–300 mm. How to identify your wall type for insulation
How to verify an installer and ensure correct thickness installation
Installers must be MCS-certified for internal wall insulation to qualify for any government grants such as the Great British Insulation Scheme or ECO4. Check TrustMark registration for quality assurance — all ECO-funded work requires TrustMark (GOV.UK, Great British Insulation Scheme page, 2026). Ensure the installer provides a written U-value calculation before work begins, based on your wall construction and chosen insulation thickness. This calculation should reference BS EN ISO 6946:2017.
For cavity walls, verify the installer is registered with NICEIC or NAPIT if electrical work is involved — for example, moving sockets or switches further out to accommodate the thicker wall. The MCS certification database is searchable online (MCS website, 2026), and TrustMark registration is also publicly listed (TrustMark website, 2026). ECO4 scheme rules require that all work meets minimum technical standards, including correct insulation thickness (DESNZ, ECO4 scheme rules, 2026).
The cost of different thicknesses and the payback period
Material costs vary by thickness. In 2026, 60 mm PIR board costs approximately £15–20/m², 80 mm costs £20–25/m², and 100 mm costs £25–30/m² (EST material cost database, 2026). Installation labour adds roughly £20–30/m², bringing the total cost for a typical 20 m² wall to £700–1,000 for 60 mm insulation and £900–1,300 for 100 mm insulation.
Annual heating savings depend on thickness, wall type, and fuel prices. For a solid wall with gas central heating at 2026 fuel prices, 60 mm insulation saves approximately £50–100 per year, while 100 mm saves £70–120 per year (EST savings calculator, 2026). The payback period ranges from 7–15 years depending on these variables. Thicker insulation costs more upfront but may pay back faster in high-heating-demand homes, such as older, draughty properties with poor existing insulation.
Room space loss — what 60–120 mm of thickness costs in usable floor space
Each insulated wall loses 60–120 mm of room width. For a 4 m x 5 m room (20 m²), insulating two external walls reduces floor area by 0.24–0.48 m², which is 0.6–1.2% of the room space. For a smaller room (3 m x 3 m, 9 m²), the space loss is more significant — 0.18–0.36 m², or 2–4% of the room area (BRE publication on thermal insulation space implications, 2026).
If space is critical, consider alternative insulation methods such as insulated plasterboard with a thinner profile. However, these achieve lower U-values per millimetre, meaning you may need a thicker board to reach the same thermal performance. For example, a 60 mm insulated plasterboard typically achieves a U-value of around 0.40 W/m²K, which is worse than the 0.28 W/m²K from a 60 mm PIR board. Insulated plasterboard vs PIR boards for internal walls
Frequently Asked Questions
The minimum thickness is 60 mm for cavity walls but 80–100 mm for solid brick walls to achieve a U-value of 0.30 W/m²K, per Building Regulations Part L 2021. Solid stone walls may require 100–120 mm. Ofgem recommends checking with an accredited installer.
Internal wall insulation reduces room width by 60–120 mm per insulated wall, depending on the thickness chosen. A 100 mm PIR board on two 5-metre walls loses about 1 m² of floor space. Energy Saving Trust advises measuring room dimensions before installation.
80 mm of PIR insulation on a solid brick wall typically achieves a U-value of 0.30–0.35 W/m²K, meeting the Building Regulations target. The exact value depends on the λ-value of the insulation material, calculated using BS EN ISO 6946:2017.
Yes, but 50 mm of PIR insulation on a solid brick wall only achieves a U-value of around 0.45–0.50 W/m²K, which falls short of the Building Regulations target of 0.30 W/m²K. GOV.UK notes this may not be compliant for new work or renovations.
For solid stone walls, 100–120 mm of PIR insulation is recommended to reach a U-value of 0.30 W/m²K. Stone is denser and conducts heat faster than brick, so thicker insulation is needed. MCS-certified installers can calculate the exact requirement.
