EXTERNAL WALL INSULATION MATERIALS
Calculate my external wall insulation project
Enter the surface area of the house and the number of linear metres, then deduct your windows and doors: the Centrale ITE calculator draws up the complete list of materials for your rendered external wall insulation (EWI), with the exact quantities to order, from the starter track to the bucket of render.
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How much material per m² for rendered external wall insulation?
For 1 m² of rendered EPS external wall insulation, allow approximately 1.5 boards, 8 kg of mortar, 7 anchors, 1.1 m² of mesh, 0.14 l of primer and 2.5 kg of finishing render.
Material quantities per m² of rendered EWI
| Product | EPS | Rock wool | Source |
|---|---|---|---|
| Boards 1200×600 | 1.39 + waste | 1.39 + waste | Manufacturer format |
| Bonding mortar | 4 kg | 4 kg | A 96 data sheet |
| Reinforced base coat | 4 kg | 6.3 kg | Fassatherm Technical Assessments |
| Anchors | ≈ 7 | ≈ 7 | ETA 17/0450, DTA |
| Glass fibre mesh | 1.1 m² | 1.1 m² | 10 cm overlap |
| FX 526 primer | 0.14 l | 0.14 l | FX 526 data sheet |
| 1.5 mm render | 2.55 kg | 2.55 kg | RX 561 data sheet |
Rock wool requires more base coat, because its fibrous surface absorbs more mortar and its Technical Assessment (Avis Technique) requires two passes. Wood fibre follows its own system: 8 fixings per m² and a lime-based base render suited to a breathable insulation material.
How to measure your façade before calculating
Measure the length of each wall to be insulated at the foot of the façade to get the linear metres, multiply by the height up to the underside of the roof, add the gables, then count your windows and doors.
A laser measure is all you need. Add up the lengths of the façades concerned: this is also the length of starter track you need. For a gable, measure the width and the height from the eaves to the tip; its area is half the product of the two. Enter the area including openings: the calculator itself deducts 1.5 m² per window and 1.9 m² per door. Count a bay window as two windows, and a garage door as three doors.
What insulation thickness do you need to reach R 3.7?
For a thermal resistance R of 3.7 m².K/W, you need 120 mm of graphite EPS TH32 (R 3.75), 140 mm of white EPS TH38 (R 3.70) or 140 mm of Re Coat+ rock wool (R 4.00). R 3.7 is the threshold commonly required for external wall renovation grants in France.
The calculator displays the ACERMI-certified R-value of the chosen thickness and warns you if it falls below this threshold. To compare insulation materials on other criteria (fire, acoustics, summer comfort), see our guide How to choose your external insulation.
Which accessories should you not forget in your EWI order?
The most frequently forgotten items are starter tracks, corner beads, drip profiles for lintels, shutter hinge blocks and protective consumables for windows and doors. The starter track, chosen to match the insulation thickness, protects the bottom of the system.
Mesh-backed PVC corner beads reinforce every external corner and every opening reveal. The drip profile prevents run-off under lintels. High-density EPS hinge blocks let you refit shutters without creating a thermal bridge. The calculator adds these items automatically. For installation details, read the steps of an EWI project and which fixing to use for which substrate.
Homeowner or professional: what to do with your list?
Whether you are a homeowner or a professional, you can add the entire list to your basket at Centrale ITE trade prices.
Centrale ITE is the platform specialising in the sale of external wall insulation (EWI) materials, created by Sun Distribution, a distributor of insulation products since 2012. Our advisers review the lists generated by the calculator free of charge, check that the products are compatible with your substrate and adjust the anchoring pattern according to the wind exposure in your region, in France, Belgium or Switzerland.
Frequently asked questions about calculating an EWI project
How many bags of adhesive do you need for 100 m² of EWI?
For 100 m² of EPS external wall insulation, allow approximately 800 kg of A 96 mortar, i.e. 32 to 34 bags of 25 kg: 4 kg/m² for bonding and 4 kg/m² for the reinforced base coat. For rock wool, allow around 42 bags instead.
How many anchors per m² for external wall insulation?
On the main wall area, allow 5 anchors per 1200×600 mm board, i.e. approximately 7 per m². This number increases at the edges of the façade and in areas highly exposed to wind. A pull-out test on your substrate determines the final anchoring pattern.
What waste margin should you allow for insulation boards?
Allow 5% for a rectangular façade with few openings, and 8 to 10% for an irregular façade with gables, many windows or offsets. EPS offcuts are often used to insulate window reveals and small areas.
Which anchor length should you choose for your insulation thickness?
The anchor must pass through the insulation and the adhesive, then anchor into the load-bearing wall. Allow the insulation thickness plus approximately 50 mm: a 200 mm LFM 10 is suitable for 140 mm of EPS on concrete block. On a thick existing render, add its thickness.
Is the calculator suitable for rock wool and wood fibre?
Yes. It adapts the board format, the quantity of base coat, the number of fixings and the base render to each insulation material. Gutex wood fibre uses a 1200×400 format and a lime-based base render, compatible with its breathable nature.
Are the calculated quantities guaranteed?
They are indicative and are based on the consumption rates given in the products' technical data sheets and Technical Assessments. Wall flatness, wind exposure and the installer's skill all affect actual consumption. A Centrale ITE adviser can review your list before you order.
