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Table 8: Insulation of roofs

(a) Pitched roofs

Lofts: An uninsulated loft space would have a U-value 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 U-value 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 U-value 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) U-value (W/m2K)

Less than 135 / 0.25

135-185 / 0.20

Greater than 185 / 0.16

(b) Flat roofs

To improve an uninsulated roof comprising a weatherproof membrane over a timber deck (U-value about 2.57 W/m2K):

- apply insulation with an R-value of 3.7 m2K/W over the deck – this gives a final U-value of 0.25 W/m2K.

Achieving the required R-value (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 (U-value between 0.45-0.7 W/m2K):

- apply insulation with an R-value of 2.5 m2K/W under or over new concrete slab, to give a new U-value of 0.20-0.25 W/m2K (depending on floor geometry)

(b) Suspended floors

To improve energy performance of a floor composed of timber floorboards on joists (U-value 0.45-0.7 W/m2K):

- apply insulation with an R-value of 3.75 m2K/W between joists tight against the underside of the floor deck.

This will give a final U-value of 0.20-0.25 W/m2K, (depending on floor geometry).

Achieving the required R-value (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


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