Exterior Insulation
External wall insulation is the most effective way to reduce heat loss without losing interior floor area. Learn about EPS, mineral wool and PIR, U-values, fixings and building regulations.
Insulation types for external use
| Type | λ (W/mK) | 100 mm R-value | Fire class | Suitable for |
|---|---|---|---|---|
| EPS (expanded polystyrene) | 0.036 | 2.78 m²K/W | E | ETICS, back-ventilated |
| Mineral wool (stone wool) | 0.033 | 3.03 m²K/W | A1 | ETICS, back-ventilated |
| PIR (polyisocyanurate) | 0.022 | 4.55 m²K/W | E–C | Space-constrained details |
| Graphite EPS (grey EPS) | 0.031 | 3.23 m²K/W | E | ETICS, good value |
| Phenolic foam | 0.020 | 5.00 m²K/W | B | Very restricted spaces |
U-value requirements
| Element | Recommended max U (W/m²K) | Typical min. insulation |
|---|---|---|
| External wall | 0.18 | 200–250 mm mineral wool or 150 mm PIR |
| Floor exposed to outside | 0.10 | 300–350 mm mineral wool |
| Roof / loft | 0.13 | 300–400 mm mineral wool |
| Glazing | 0.80 | Triple glazing with argon fill |
| External doors | 0.80 | Insulated steel/timber door |
Wall build-up — cross section
Required additional insulation thickness
To reach the recommended target of 0.18 W/m²K (mineral wool λ = 0.033):
| Existing wall (U-value) | Min. extra mineral wool | Min. extra PIR |
|---|---|---|
| 0.40 W/m²K (old log/timber) | ~110 mm | ~70 mm |
| 0.30 W/m²K (100 mm insulation) | ~75 mm | ~50 mm |
| 0.22 W/m²K (150 mm insulation) | ~30 mm | ~20 mm |
| 0.20 W/m²K (200 mm insulation) | ~15 mm | ~10 mm |
Installation steps
Check for moisture damage, rot and loose render. Repair before insulating — moisture trapped behind new insulation causes serious damage.
Calculate the existing U-value and determine required additional thickness from the table above.
Mechanical fixings: Standard for back-ventilated cladding. 6 fixings/m² on field areas, 8–10 in wind-exposed zones (zone 3+). Use approved Ø8 mm fixings penetrating at least 50 mm into studs.
Adhesive + fixings (ETICS): Resin-based adhesive on 40–60% of board area, always supplemented with mechanical fixings per EAD 040083.
Start from the bottom. Stagger joints (brick-bond pattern). Avoid continuous vertical joints. Keep boards tight at windows and corners.
EPS/PIR: cut with a knife or hot wire. Mineral wool: use an insulation knife. Use a rasp/float to level the surface.
Brackets, fixings and penetrations create thermal bridges. Use thermal-break fixings where available. Pay careful attention to corners — apply extra insulation layers at all corners.
For back-ventilated systems: fit wind barrier after insulation, then battens for the air gap (minimum 25 mm), then cladding.
For ETICS: see the separate render guide for base coat, mesh and finish coat.
Regulatory requirements
Common mistakes
- ✗Boards too thin — 50 mm EPS rarely delivers enough improvement; aim for at least 100 mm to approach current building code targets
- ✗Open joints between boards — air leakage and thermal bridging through the insulation layer, especially at corners
- ✗Reinforcing mesh too close to the surface — the mesh must have at least 5 mm of render cover to prevent corrosion
- ✗Window reveals not adjusted — the new insulation makes windows appear recessed and requires new reveals or outer trims
- ✗Starting work before applying for energy grants — most schemes require applications to be submitted before work begins
- ✗Skipping the priming step — the render coat does not bond properly to EPS without an approved primer or adhesive
- ✗Incorrect anchor spacing — too few fixings causes boards to bow and detach over time
Calculate exterior insulation
Use the calculator to estimate m² of insulation boards, fixing quantities and total cost.
Open insulation calculator →Frequently asked questions
What U-value do exterior walls need to meet?
Most EU/EEA countries require U ≤ 0.18–0.25 W/m²K for new-build exterior walls. In the US, IECC climate zone 5+ requires effective R-20 (U ≈ 0.24). Check your local energy code.
What is the difference between EPS, mineral wool and PIR for exterior insulation?
EPS (R-3.8/in, λ 0.036) is the most common and economical. Mineral wool (R-4.2/in, λ 0.033) is non-combustible — required where fire class A2 is needed. PIR (R-6.5/in, λ 0.022) gives best performance per inch for space-constrained retrofits.
How many fixings per square metre / square foot do I need?
Standard: 6 fixings/m² (0.55/ft²) on field areas. Wind-exposed zones or upper floors require 8–10 fixings/m² (0.75–0.93/ft²). Always follow the system manufacturer's fixing pattern table.
References
- → Rockwool — external wall insulation (ETICS) technical documentation
- → Isover — external insulation technical guides and U-value calculation tools
- → SINTEF Byggforskserien — exterior insulation and moisture management
- → Local building authority — U-value targets for renovation and energy requirements in your jurisdiction
Insulation thickness, fixing counts and U-values are indicative. Actual system design must be verified against the specific product's ETA and local building regulations. Last reviewed: May 2026