How much stone wool a facade needs

A simple calculation a foreman does on site in five minutes, and two mistakes that later force you to buy wool one pack at a time.

Step 1. Wall area without openings

Multiply the perimeter of the house by the height to the eaves. Subtract windows and doors, but only if they take up more than 10% of the wall: reveals are insulated with a strip anyway, and on small openings the saving is eaten up by cutting. A 10×12 m house 6 m high gives 264 m² of walls; minus eight windows of 2 m² and two doors of 2 m², that leaves 244 m².

Step 2. Reserve for cutting

Add 5% to the net area on a plain facade and 7–10% if there are many corners, bay windows and reveals. Slabs are cut on site, and offcuts smaller than a third of a slab will not be used anywhere. For our house we take 7%: 244 × 1.07 = 261 m².

Step 3. Conversion to packs

Wool is sold in packs, and the pack area depends on the slab thickness: at 100 mm a pack holds 2.16 m², at 80 mm 2.88 m², at 50 mm 4.32 m². For 261 m² at 100 mm you need 261 / 2.16 = 120.8, that is 121 packs. The shop basket rounds up to whole packs by itself, and the insulation calculator does the whole calculation from the area and shows the cost straight away.

Which density to take

Under render you need a slab of 120 kg/m³ or more; 90–100 kg/m³ is fine only for low-rise houses (up to 10 m). For a ventilated facade 70–80 kg/m³ is enough. Density below the norm under a wet facade is the most expensive mistake: the slab is crushed under the base coat, and within a year the facade goes wavy.

What to order with the wool

Disc dowels at 6 per m² in the main wall field (8 in the edge zone, 12 above 25 m), which is about 1600 pieces for our house, and fibreglass mesh with a reserve for the overlap (at least 10 cm). Adhesive and the base coat for insulation are chosen to suit the system you pick — ask the manager. How to choose the dowel length is covered in a separate guide.

Updated 31 Oct 2026