Table 8: Insulation of roofs
(a) Pitched roofs
Lofts: An uninsulated loft space would have a Uvalue in the region of 2.5 W/m2K.With 100mm of insulation (λ =
0.040 W/mK) this improves to 0.4 W/m2K. Insulate (or top up) with two layers, covering the joists, to achieve a
Uvalue of 0.16 W/m2K.
Thickns of loft insulation / Insl. conductivity (λ) / Thickness required (W/mK) (mm)
0.030 / 100 + 124
0.040 / 100 + 150
0.045 / 100 + 180
Thicknesses given are based on new insulation of known conductivity Sloping ceilings: insulate between and under
the rafters to achieve a Uvalue of 0.16 W/m2K where possible. If space is limited, particularly where rafters are
less than 185mm deep, the best practicable level of insulation should be installed.
Thickness of sloped ceiling insulation
Rafter depth Practically achievable
(mm) Uvalue (W/m2K)
Less than 135 / 0.25
135185 / 0.20
Greater than 185 / 0.16
(b) Flat roofs
To improve an uninsulated roof comprising a weatherproof membrane over a timber deck (Uvalue about 2.57
W/m2K):
 apply insulation with an Rvalue of 3.7 m2K/W over the deck – this gives a final Uvalue of 0.25 W/m2K.
Achieving the required Rvalue (R = d/λ) Insulation conductivity (λ) Thickness (d) required
(W/mK) (mm)
0.020 / 74
0.025 / 93
0.030 / 111
0.035 / 130
0.040 / 148
Table 9: Ground Floors
(a) Solid floors
To improve a solid concrete floor slab (Uvalue between 0.450.7 W/m2K):
 apply insulation with an Rvalue of 2.5 m2K/W under or over new concrete slab, to give a new Uvalue of
0.200.25 W/m2K (depending on floor geometry)
(b) Suspended floors
To improve energy performance of a floor composed of timber floorboards on joists (Uvalue 0.450.7 W/m2K):
 apply insulation with an Rvalue of 3.75 m2K/W between joists tight against the underside of the floor
deck.
This will give a final Uvalue of 0.200.25 W/m2K, (depending on floor geometry).
Achieving the required Rvalue (R = d/λ) Insulation conductivity (λ) Thickness (d) required
(W/mK) (mm)
Solid floors Suspended floors
0.020 / 50 / 75
0.025 / 63 / 94
0.030 / 75 / 115
0.035 / 88 / 133
0.040 / 100 / 150
