Fixing insulation to concrete, brick and aerated block
The same dowel holds in concrete and pulls out of aerated block together with the slab. The difference is the embedment and what you drill with. Substrate by substrate.
The general rule
A disc dowel holds not with its disc but with the expansion zone in the load-bearing layer of the wall. So its length adds up four parts: insulation thickness + 10 mm of adhesive + the old render, if the wall is rendered, + embedment in the bearing layer. Embedment depends on the substrate: concrete and solid brick 50 mm, hollow brick and clay block 70 mm, aerated and foam block 90 mm. Round up to the nearest size. Consumption: 6 per m² in the field, 8 in the edge zone at the corners (1–2 m, by building width), twice as many above 25 m. The formula with examples is in the dowel-length guide.
Concrete and solid brick
The easiest substrate. Wool 100 mm — IZL-T 8L×170, 50 mm — 8L×110, 150 mm — 8L×210. Drill with a hammer drill and an Ø8 bit, the hole 10 mm deeper than the dowel, blow out the dust. In solid brick never into a vertical joint — keep 30 mm from it. For EPS and XPS on these substrates up to 60 mm thick, IZO 10×100 or 10×120 with a plastic nail is fine, 4 per m².
Hollow brick and clay block
The main mistake is hammer drilling: the impact breaks the webs of the block and the expansion zone ends up in a void. Drill without hammer, embedment 70 mm: wool 100 mm — IZL-T 8L×190, 50 mm — 8L×130. A steel-nail dowel is mandatory on wool: its expansion zone is longer and grips several webs. If the facade is old and you do not know what is under the render, tap it and make test holes in two or three places.
Aerated and foam block
Embedment 90 mm and no hammer, otherwise the hole widens into a funnel: wool 100 mm — IZL-T 8L×210, 50 mm — 8L×150, 120 mm — already IZL-T 10L×260. For aerated concrete there are also screw-in dowels with 110 mm embedment, which block makers recommend. We stock a 12/18×130 anchor screw with a disc washer, but it has no data sheet with a load, so on a facade that matters we fit IZL-T with the longer embedment, and screw-in dowels are taken by data sheet from the system supplier. Prime aerated block before the adhesive: adhesive does not hold on a dusty surface.
Old render and weak substrates
Render does not count as a bearing layer: add its thickness to the length and count the embedment from the brick or concrete behind it. If the render is flaking, strip it to the substrate. Cinder block, pumice block and masonry without a data sheet — only after a pull-out test: fit two or three dowels and try to pull them; if they come easily, go to a longer embedment or a chemical anchor.
Consumption for a 100 m² facade
- Concrete, wool 100 mm: 640 IZL-T 8L×170 dowels (80 % field at 6, 20 % edge zone at 8) — 2 boxes of 500.
- Hollow brick, wool 100 mm: the same count, but 8L×190 and no hammer.
- Aerated block, wool 100 mm: 8L×210; plus primer and adhesive per the system data sheet.
- EPS 100 mm on concrete: 4 per m² in the field, 8 at the edges — about 480 of 8L×170; IZO only up to 60 mm.
The starter profile at the plinth is fixed with SM-L 6×60 or 6×80 nail plugs at no more than 30 cm — into concrete and brick; on hollow and aerated block the profile needs 90–110 mm of embedment, and we use the same IZL-T for it. The calculator counts dowels from the area and picks the length by substrate and thickness; dowels are sold by the box.