Insulation dowels: length and consumption

A short dowel means a slab that comes away from the wall after one winter. A long one means extra money and extra work. The formula fits on one line.

The length formula

Dowel length = insulation thickness + 10 mm adhesive layer + anchorage into the base. Anchorage is at least 50 mm into concrete and solid brick, 70 mm into hollow brick and 90 mm into aerated concrete. For 100 mm of wool on a brick wall that gives 100 + 10 + 50 = 160 mm, and we take the nearest size up: IZL-T 8L×170. For 50 mm on concrete, 110 mm is enough. If the catalogue has no suitable size (the longest IZL-T is 260 mm), the manager makes the selection.

Consumption

In the main facade field, 6 dowels per m²; in the edge zone (1–2 m from the building edge), at corners and around openings, 8 per m², because the wind load is higher there; above 25 m the number doubles to 12 per m². The scheme is simple: one dowel in the centre of the slab and one at each joint at the corners, which for a 600×1200 mm slab comes to exactly 6 per m². The design makes the final decision on the number and layout. A 250 m² house needs about 1500 dowels in the field, and they are sold in packs of 400–1000.

IZL-T or IZO

IZL-T is a dowel with a galvanized steel nail and a polyamide thermal head: the thermal head prevents a cold bridge, and the dowel is driven in with a hammer. IZO, with a plastic expanding nail, is cheaper and suits polystyrene and wool on concrete and solid brick. For a ventilated facade, for heavy slabs above 150 kg/m³ and for tall buildings, the design specifies the right dowel — ask the manager.

Retaining washers

With wool, retaining washers are added — 60 mm pressure discs that spread the pressure of the nail and keep it from punching through the soft slab. They are not needed on polystyrene.

Check before you order

Drill one hole and look at the drilling dust: grey and dusty is concrete, red is brick, white and light is aerated concrete. The anchorage and the dowel type depend on it. The dowel calculator on the site works out the length and quantity from the insulation thickness and the wall material.

Updated 31 Oct 2026