Determining the thickness of insulation for roofs

Figure 4.1 illustrates three common roof types, and Table 4.6 gives the base thickness of insulation for these roofs.

Because the base thickness may not be the minimum, the reductions in heat flow due to other elements of the construction must be considered, and further corrections may be made to reduce the base thickness to give the minimum thickness. The amounts by which the base thickness may be reduced are given in Table 4.7.

MMMMMiWiiMWM

11

111

(b) Insulation laid between and over joists or rafters

(b) Insulation laid between and over joists or rafters

Fig. 4.1 Three common roof types.

(c) Continuous layer of insulation over a structural base

Fig. 4.1 Three common roof types.

Table 4.6 Base thickness of insulation for roofs.

Design

Thermal conductivity of the insulation material, W/mK

0.020

0.025

0.030

0.035

0.040

0.045

0.050

U-value

W/m2K

Base thickness of insulation layer,

mm

Insulation laid between ceiling joists or rafters

0.15

371

464

557

649

742

835

928

0.20

180

224

269

314

359

404

449

0.25

118

148

178

207

237

266

296

0.30

92

110

132

154

176

198

220

0.35

77

91

105

122

140

157

175

0.40

67

78

90

101

116

130

145

Insulation laid between and over joists or rafters

0.15

161

188

217

247

277

307

338

0.20

128

147

167

188

210

232

255

0.25

108

122

137

153

170

187

205

0.30

92

105

117

130

143

157

172

0.35

77

91

103

113

124

136

148

0.40

67

78

90

101

110

120

130

Continuous layer of insulation

0.15

131

163

196

228

261

294

326

0.20

97

122

146

170

194

219

243

0.25

77

97

116

135

154

174

193

0.30

64

80

96

112

128

144

160

0.35

54

68

82

95

109

122

136

0.40

47

59

71

83

94

106

118

Table 4.7 Reduction in base thickness of insulation for roof components.

Concrete

Thermal conductivity of the insulation material, W/mK 0.020 0.025 0.030 0.035 0.040 0.045 0.050

slab density kg/m3

Reduction (mm) in the base thickness of the insulation, for each 100 mm thickness of the concrete slab

600

10

13

15

18

20

23

25

800

7

9

11

13

14

16

18

1100

5

6

8

9

10

11

13

1300

4

5

6

7

8

9

10

1700

2

2

3

3

4

4

5

2100

1

2

2

2

3

3

3

Various materials and

Reduction in the base thickness of the

insulation

components

mm

10 mm plasterboard

1

2

2

2

3

3

3

13 mm plasterboard

2

2

2

3

3

4

4

13 mm sarking board

2

2

3

3

4

4

5

12 mm calcium

1

2

2

2

3

3

4

silicate liner board

Roof space (pitched)

4

5

6

7

8

9

10

Roof space (flat)

3

4

5

6

6

7

8

19 mm roof tiles

0

1

1

1

1

1

1

19 mm asphalt

1

1

1

1

2

2

2

(or 3 layers of felt)

50 mm screed

2

3

4

4

5

5

6

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