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 (4 in) R-value | Fire class | Suitable for |
|---|---|---|---|---|
| EPS (expanded polystyrene) | 0.036 | 2.78 m²K/W (R-16) | E | ETICS, back-ventilated |
| Mineral wool (stone wool) | 0.033 | 3.03 m²K/W (R-17) | A1 | ETICS, back-ventilated |
| PIR (polyisocyanurate) | 0.022 | 4.55 m²K/W (R-26) | E–C | Space-constrained details |
| Graphite EPS (grey EPS) | 0.031 | 3.23 m²K/W (R-18) | E | ETICS, good value |
| Phenolic foam | 0.020 | 5.00 m²K/W (R-28) | 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 (8–10 in) mineral wool or 150 mm (6 in) PIR |
| Floor exposed to outside | 0.10 | 300–350 mm (12–14 in) mineral wool |
| Roof / loft | 0.13 | 300–400 mm (12–16 in) 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 (4½ in) | ~70 mm (2¾ in) |
| 0.30 W/m²K (100 mm / 4 in insulation) | ~75 mm (3 in) | ~50 mm (2 in) |
| 0.22 W/m²K (150 mm / 6 in insulation) | ~30 mm (1¼ in) | ~20 mm (¾ in) |
| 0.20 W/m²K (200 mm / 8 in insulation) | ~15 mm (⅝ in) | ~10 mm (⅜ in) |
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, or 1 in), then cladding.
For ETICS: see the separate render guide for base coat, mesh and finish coat.
Regulatory requirements
For new buildings, local energy regulations set minimum insulation levels. Always check local building rules and available grant programmes for retrofit insulation work.
Exterior insulation increases the building's external dimensions. An increase of more than 0.5 m (20 in) from the original facade may require a dispensation from the local development plan. Check with your municipality.
Common mistakes
- ✗Boards too thin — 50 mm (2 in) EPS rarely delivers enough improvement; aim for at least 100 mm (4 in) 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 →Tools you'll need
The kit this job actually calls for, and whether it's worth buying or renting for one weekend.
- Notched trowel
For the adhesive bed behind the boards.
Buy 8–25 € - Insulation board rasp
Flattens board joints before the base coat.
Buy 10–25 € - SDS rotary hammer
Mechanical fixings go into masonry — an SDS is not optional.
Buy 80–250 €Rent 15–30 €/day - Hand saw
Or a long blade for cutting board.
Buy 12–30 € - Plasterer's trowel
Lays on and flattens render and finish coats.
Buy 25–70 € - Snips
Cuts reinforcing mesh, beads and metal profiles.
Buy 10–25 € - Paddle mixer
Mixes adhesive, grout, render and levelling compound lump-free.
Buy 50–150 €Rent 12–25 €/day - Scaffold tower
Rent. You need both hands and a stable platform.
Buy 400–900 €Rent 60–120 €/dayrent this one - Safety glasses, gloves and dust mask
Silica dust from tile, block and render is the real hazard on these jobs — an FFP3 mask, not a paper one.
Buy 15–40 €
Prices are indicative for DIY-grade tools and vary a lot by brand and region — treat them as a budgeting starting point, not a quote. Rental rates are per day from a typical tool rental shop.
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.
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Get exact quantities and costs for your own measurements, build an itemized shopping list so you don’t overbuy, and use the PDF to collect quotes from contractors.