Insulating a loft: wool between the rafters
Between rafters the wool carries nothing, it just sits in a friction fit — so a light slab goes there, not a dense one “to be safe”. And thickness matters more than density here: 150 mm no longer passes in Tbilisi.
How many millimetres
A pitched roof in Tbilisi needs 200 mm of stone wool: 150 between the rafters and 50 across underneath them. A single 150 mm layer in 50×150 rafters is not enough: the rafters themselves take up about 8 % of the area and act as thermal bridges, and with them the slope's resistance drops below the standard. The layer under the rafters covers the timber and makes up the shortfall. In the mountain zone — Gudauri, Bakuriani — the thickness is set by the project.
Which density
Manufacturers put light slabs of 28–60 kg/m³ between rafters: they are cheaper per centimetre and sit better in a friction fit. Such wool comes from suppliers of the light grades. We usually lay our 70 kg/m³ 50 mm slab (15.95 ₾/m²) — three layers between the rafters and one layer under them, with staggered joints — or 110 kg/m³ 100 mm (31.90 ₾/m²) plus 50 mm. Both options cost the same: 3.19 ₾ per m² for each centimetre, so 200 mm is about 64 ₾/m². A dense slab is no worse thermally but heavier: 200 mm of the 70 weighs 14 kg/m², the version with the 110 — 20 kg/m², light wool — 6–8; under seismic loads that weight on the rafters is taken into account. And a rigid slab is not squeezed in like a soft mat, it is cut to fit with a gap of no more than 2 mm. We do not recommend 120 kg/m³ or denser on slopes — more money for nothing.
Cutting to the rafter spacing
The slab is 1200×600 mm. With rafters at 600 centres and 50 wide the clear gap is 550 mm: the slab is cut to 560 mm so that it sits in a friction fit, and the remainder goes to offcuts — a factor of 1.07 on the area. Together with trimming at the ridge and eaves, take 1.125 times the slope area. At 640 centres the clear gap is 590 mm — the slab fits whole. The slope area is measured along the slope, not in plan: divide the plan area by the cosine of the pitch and add the overhangs.
Build-up from inside out
- Lining (plasterboard or boards) with a 10 mm gap to the film.
- Vapour barrier on the underside of the rafters — on the warm side, 15 cm overlap, seams taped.
- A 50 mm layer across under the rafters on 50×50 battens at 600 mm.
- 150 mm between the rafters in a friction fit, layer joints staggered, joints no wider than 2 mm; until the film goes on, the slabs are held with a zigzag of cord.
- A diffusion-open roofing membrane over the wool, 15 cm overlap; we do not offer the facade wind barrier here — a roof needs a roofing membrane.
- Counter-battens — a ventilation gap of at least 50 mm; intake at the eaves, outlet at the ridge.
- Battens and the roof covering. We do not sell films, tapes, battens or nails — they come with the roofing.
Example for 100 m² of slopes
Rafters 50×150 at 600, 200 mm in total: 110 kg/m³ slab 100 mm — 112.5 m², 53 packs — 3,652 ₾; 70 kg/m³ slab 50 mm in two layers — 225 m², 53 packs — 3,652 ₾. About 7,300 ₾ in total, 73 ₾ per m² of slope including cutting. Doing it all with the 70 in four layers costs the same. The same thickness in a 120 kg/m³ slab comes out 30 % dearer with no gain.
Mistakes
- 50 mm of wool “just so there is some”: heat loss twice the standard, condensation on the film.
- 150 mm between the rafters only, with no layer under them.
- A vapour barrier on the outside of the insulation, or a membrane with no ventilation gap above it.
- Insulating before the roof covering and membrane are in place.
- Squeezing slabs into the gap: compressed wool loses thickness and warmth.
- A chimney through the wool without a fire break — clearances to timber and films are set by the project.