Determining the thickness of insulation for walls

The procedure is the same as for roofs. Select the desired U-value, decide on the position of the insulation layer and method of fixing, and use Table 4.5 to obtain AU, the correction to the U-value. The design U-value is found from the desired U-value using:

^design U desired AU

Table 4.8 Base thickness of insulation for walls.

Thermal conductivity of the insulation material, W/mK

Design 0.020 0.025 0.030 0.035 0.040 0.045 0.050

W/m2K Base thickness of insulation layer, mm

Thermal conductivity of the insulation material, W/mK

Design 0.020 0.025 0.030 0.035 0.040 0.045 0.050

W/m2K Base thickness of insulation layer, mm

0.20

97

121

145

169

193

217

242

0.25

77

96

115

134

153

172

192

0.30

63

79

95

111

127

142

158

0.35

54

67

81

94

107

121

134

0.40

47

58

70

82

93

105

117

0.45

41

51

62

72

82

92

103

Table 4.9 Reduction in base thickness of insulation for wall components.

Thermal conductivity of the insulation material, W/mK

0.020 0.025

0.030

0.035

0.040

0.045

0.050

Concrete blockwork density

Reduction (mm) in the base thickness of the insulation,

kg/m3

for each 100 mm thickness of the concrete blockwork

Inner leaf

600

9

11

13

15

17

20

22

800

7

9

10

12

14

16

17

1000

5

6

8

9

10

11

13

1200

4

5

6

7

8

9

10

1400

3

4

5

6

7

8

8

1600

3

3

4

5

6

6

7

1800

2

2

3

3

4

4

4

2000

2

2

2

3

3

3

4

2400

1

1

2

2

2

2

3

Outer leaf or single leaf wall

600

8

11

13

15

17

19

21

800

7

9

10

12

14

15

17

1000

5

6

7

8

10

11

12

1200

4

5

6

7

8

9

10

1400

3

4

5

6

6

7

8

1600

3

3

4

5

5

6

7

1800

2

2

3

3

3

4

4

2000

1

2

2

3

3

3

4

2400

1

1

2

2

2

2

3

Various materials and

Reduction in the base thickness of the insulation

components

mm

Cavity, 25 mm or more

4

5

5

6

7

8

9

Outer leaf brickwork

3

3

4

5

5

6

6

13 mm plaster

1

1

1

1

1

1

1

13 mm lightweight plaster

2

2

2

3

3

4

4

9.5mm plasterboard

1

2

2

2

3

3

3

12.5 mm plasterboard

2

2

2

3

3

4

4

Airspace behind

2

3

4

4

5

5

6

plasterboard drylining

9 mm sheathing ply

1

2

2

2

3

3

3

20 mm cement render

1

1

1

1

2

2

2

13 mm tile hanging

0

0

0

1

1

1

1

Then use Table 4.8 to find the base thickness of insulation, followed by Table 4.9 for any allowable reduction in the base thickness, and hence obtain the minimum thickness.

For timber framed walls, where the timber frame contains its own integral insulation, there is an additional and often significant reduction in the base thickness of the separate insulation layer. However, this reduction depends on the proportion of the area of the frame that is timber. The Approved Document gives a table for one common case when the proportion of timber is 15% of the wall area, corresponding to 38 mm wide studs at 600 mm centres, with additional timbers at junctions and around openings. This table is given here as Table 4.10. For other area proportions, or frames with insulation material of different thermal conductivity, it is necessary to use an acceptable calculation procedure, such as that given in Appendix B of the Approved Documents (see Chapter 5), to determine either the U-value or the necessary minimum thickness of an insulation layer.

Table 4.10 Reduction in base thickness of insulation for insulated timber frame walls.

Thermal conductivity of insulation within frame W/mK

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

Reduction (mm) in the base thickness of insulation, for each 100 mm thickness of the timber frame

0.035 0.040

39 49 59 69 79 89 99 36 45 55 64 73 82 91

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