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 |
The fire-class letters describe the insulation board on its own, not the finished wall. In an ETICS (render-on-insulation) or a clad build-up, the reaction-to-fire performance is a property of the whole tested system, and what is permitted — especially combustible boards such as EPS/PIR on taller buildings or near boundaries — is set by your local fire regulations. Confirm the system's fire classification and its allowed use with the system supplier and your building authority.
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. Adding external insulation also changes how the wall dries: on solid masonry, log or other older walls that need to dry outward, or where there is existing damp or rising/penetrating moisture, get a hygrothermal (condensation/moisture) assessment first — a vapour-tight board such as EPS or PIR on the wrong wall can trap moisture inside it, and a vapour-open mineral-wool system is often safer on such walls.
Calculate the existing U-value and determine required additional thickness from the table above.
Mechanical fixings: Standard for back-ventilated cladding. Roughly 6 fixings/m² on field areas and more (e.g. 8–10) in wind-exposed zones — but the actual count comes from a wind-load calculation for your building. Crucially, the anchor type, length and embedment depend on the substrate: expansion or screw anchors set to the manufacturer's spec and tested pull-out values for masonry, concrete or AAC, or screws into the framing for a timber wall (only then do you fix “into studs”). Match the anchor to what you are actually fixing into.
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 in the wrong position — it should be embedded in the outer third of the base coat (fully covered, not touching the insulation). Glass-fibre ETICS mesh does not rust; correct embedment is about crack control and impact resistance, and the mesh must be alkali-resistant so the cement render does not attack it
- ✗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.