Roof U-value and R′ calculator

Mc 001-2022 recommends R′ ≥ 6.67 m²K/W for the roofs of new residential nZEB buildings, i.e. U′ ≤ 0.15 W/(m²K). For a mansard with 8 × 18 cm rafters at 80 cm filled with Unifit 035, at least 9 cm more wool is needed under the rafters; for renovation (U′ 0.20), 3 cm is enough. The usual 18 + 5 cm build-up has U′ 0.175. For an attic, the same value takes 28 cm of Classic 044 wool on the slab, and for a flat roof 22 cm of EPS A150.

Glass wool is very light, so it lets the day's heat wave through almost undamped: for a mansard with 18 + 10 cm of Unifit 035, the layers under the ventilated gap have a time shift of 2.1 hours and a decrement factor of 0.96. With wood fibre of the same thickness, the time shift rises to 9.9 hours and the decrement falls to 0.35. The tile colour matters too: for 120 m² in Bucharest, the heat entering through the roof in the hot season is 229 kWh with red clay tiles and 327 kWh with anthracite clay tiles. Roof windows and night ventilation often matter more than the opaque roof.

It depends on the layer over the rafters. In Brașov, with normal indoor humidity, a vapour-open underlay laid directly on the rafters (s_d 0.02 m) gives no condensation even without a vapour control layer. With board sheathing (s_d ≈ 1.2 m) and no vapour control, 6,083 g/m² builds up, which does not dry out within a year; with an s_d 2 m membrane, 379 g/m², dry by June. With bitumen felt on the boards (s_d of tens of metres), the 2 m membrane leaves 1,230 g/m², does not dry out within a year, and a 100 m polyethylene barrier 13 g/m², dry by July. The rule: the outer layers must be more vapour-open than the inner ones. Airtightness matters just as much, because warm air escaping through gaps carries more water than diffusion.

Timber (λ 0.13) conducts heat almost four times better than wool. With 18 cm of wool only between 8 cm rafters at 80 cm (10 % timber), U′ is 0.230, against 0.185 without timber: 24 % more. With 10 cm rafters at 60 cm, U′ rises to 0.260. A 5 cm layer under the rafters covers the timber and cuts the difference to 20 % (U′ 0.175). The calculation follows the EN ISO 6946 combined method, the mean of the upper and lower limits.

No. Under EN ISO 6946, a well-ventilated air layer (openings of at least 1500 mm² per metre) and everything above it are left out of U; the outer face of the underlay is treated as a surface in still air, with the same resistance as inside (0.10 m²K/W). The tiles and the ventilated gap protect against rain and sun and help the roof dry out, but they add no thermal insulation.

With 25 cm of Unifit 039 on the slab, U′ is 0.147. With 5 cm joists at 60 cm and an 18 mm OSB walkway, U′ rises to 0.167. The OSB (μ ≈ 200, s_d ≈ 3.6 m) sits on the cold side of the wool, so vapour can stop under it: in Brașov the calculation gives 16 g/m², dry by April; with an s_d 2 m membrane on the slab, 5 g/m², dry by March.

At 20 cm, EPS A150 polystyrene (λ 0.034) gives U′ 0.162 and rigid stone-wool roof boards (λ 0.039) 0.184. Both sit under the bitumen waterproofing (s_d ≈ 400 m), whose underside stays close to the outdoor temperature however thick the insulation is. So condensation depends on the vapour barrier, not on thickness: with an aluminium-faced bitumen vapour barrier (s_d ≈ 1500 m), the calculation for Bucharest gives no condensation; with a plain bitumen barrier (s_d ≈ 200 m), 5 g/m², dry by June (in Brașov, 8 g/m², does not dry out within a year); without a barrier, 47 g/m², dry by July for polystyrene and 80 g/m², dry by August for stone wool. Stone wool is non-combustible (A1) and, being heavier, delays summer heat longer (time shift 13.7 h against 9.3 h).

For 120 m² of roof in Bucharest (2005–2023 typical meteorological year, 172 days averaging 12 °C or less, 2,579 degree-days): the old mansard with 16 cm of wool loses about 2,107 kWh in the cold season, the mansard with 18 + 10 cm 1,062 kWh, the attic with 30 cm of Classic 044 1,027 kWh, and the flat roof with 20 cm of polystyrene 1,200 kWh. In summer more heat enters through the roof than through the walls, because the sun is almost overhead: 229 kWh for the mansard with red tiles, 401 kWh for the flat roof with black bitumen and 111 kWh for the same flat roof with a white membrane.