Stone wool density: which goes where
Density does not make wool warmer — it governs strength. Buying “denser to be safe” means overpaying, while a light slab under render gives you a wavy facade within a year.
Warmth does not depend on density
The thermal conductivity of slabs from 70 to 190 kg/m³ is almost the same: 0.038–0.043 W/(m·K). Insulation holds heat with the air between the fibres, not with mass. Density decides how much the slab takes in compression and in tension: under render the adhesive and wind pull on it, on a roof people walk on it, under a screed it carries the floor.
Rendered facade: from 120 kg/m³
A wet facade with a base coat and render needs a slab of 120 kg/m³ or more with a delamination strength TR of at least 15 kPa. For a house up to 10 m, 90–110 kg/m³ is acceptable if the slab's data sheet confirms TR 15. The most common mistake is a 70–80 kg/m³ slab under the base coat: it gets pressed in and the facade goes wavy. We stock the 120 kg/m³ slab in four thicknesses — 30, 50, 80 and 100 mm; 50 mm costs 20.65 ₾/m², 100 mm 41.90 ₾/m².
Ventilated facade: 70–90 kg/m³
Behind cladding with an air gap nothing presses on the slab; it only has to keep its shape and be wind-protected. Use 70–90 kg/m³ (15.95–19.35 ₾/m² at 50 mm) and cover it on the outside with a non-combustible membrane. There is no reason to pay for 120 here.
Flat roof: two layers of different density
The bottom layer is a slab with a compressive strength of at least 30 kPa at 10 % deformation; the top layer, which is walked on and carries the membrane, at least 60 kPa — usually 140–170 kg/m³. Upstands and fillets at the parapet from 150 kg/m³. Two layers come out about 20 % cheaper than one thick strong slab. Compressive strength is stated in the slab's data sheet, not implied by density: for the bottom layer we request the data sheet from the supplier before ordering.
Under a screed: 110–140 kg/m³
A floating floor against impact noise — a 110 kg/m³ slab, with 120 as the fallback. Under underfloor heating and heavy loads — 140. Slabs of 70 and 100 kg/m³ are not laid under a screed, and XPS is no good for sound insulation: it is rigid and transmits noise.
Pitched roofs, lofts, frames and partitions: light wool
Between rafters and in framed partitions manufacturers use light slabs of 28–60 kg/m³: the wool carries nothing there, and a light slab sits better in a friction fit. We usually put our 70 kg/m³ slab there in two layers with staggered joints — it holds by friction and carries nothing, so it works just as well. If the client wants a light 28–60 kg/m³ slab specifically, it is bought separately: we do not stock it.
Short table
- Wet facade under render — 120 kg/m³ (90–110 up to 10 m with TR 15).
- Ventilated facade — 70–90 kg/m³ plus a membrane.
- Flat roof — bottom from 30 kPa, top from 60 kPa (140–170 kg/m³), upstands from 150 kg/m³.
- Floating floor — 110–140 kg/m³.
- Pitched roofs and partitions — light wool 28–60 kg/m³, or our 70 kg/m³ in two layers.
- Plinth and anything in the ground — not wool but XPS.
Thickness is calculated separately: for Tbilisi 100 mm on a concrete or brick facade, 150 mm on a roof. Density does not change the thickness.