Thermal resistance of plane airspaces (°F · ft2 · hr)/Btu

Abstracted from ASHRAE (1989), with permission

The effective emittance is E-1 = 1/e1 + 1/e2 - 1 where e1 and e2 are the airspace boundary surface emittances

Position of airspace

Direction of heat flow

Airspace

1.5 inch airspace

3.5 inch airspace

Effective emittance E Effective emittance E

Mean temp., °F

Temp. diff., °F

0.03

0.05

0.2

0.5

0.82

0.03

0.05

0.2

0.5

0.82

Horizontal

Up

90

10

2.55

2.41

1.71

1.08

0.77

2.84

2.66

1.83

1.13

0.80

50

30

1.87

1.81

1.45

1.04

0.80

2.09

2.01

1.58

1.10

0.84

50

10

2.50

2.40

1.81

1.21

0.89

2.8

2.66

1.95

1.28

0.93

0

20

2.01

1.95

1.63

1.23

0.97

2.25

2.18

1.79

1.32

1.03

0

10

2.43

2.35

1.90

1.38

1.06

2.71

2.62

2.07

1.47

1.12

-50

20

1.94

1.91

1.68

1.36

1.13

2.19

2.14

1.86

1.47

1.20

-50

10

2.37

2.31

1.99

1.55

1.26

2.65

2.58

2.18

1.67

1.33

Down

90

10

6.09

5.35

2.79

1.43

0.94

10.07

8.19

3.41

1.57

1.00

50

30

6.27

5.63

3.18

1.70

1.14

9.60

8.17

3.86

1.88

1.22

150

10

6.61

5.9

3.27

1.73

1.15

11.15

9.27

4.09

1.93

1.24

0

20

7.03

6.43

3.91

2.19

1.49

10.90

9.52

4.87

2.47

1.62

0

10

7.31

6.66

4.00

2.22

1.51

11.97

10.32

5.08

2.52

1.64

-50

20

7.73

7.20

4.77

2.85

1.99

11.64

10.49

6.02

3.25

2.18

-50

10

8.09

7.52

4.91

2.89

2.01

12.98

11.56

6.36

3.34

2.22

Position of airspace

Direction of heat flow

Airspace

1.5 inch airspace

3.5 inch airspace

Effective emittance E Effective emittance E

Mean temp., °F

Temp. diff., °F

0.03

0.05

0.2

0.5

0.82

0.03

0.05

0.2

0.5

0.82

45° slope

Up

90

10

2.92

2.73

1.86

1.14

0.80

3.18

2.96

1.97

1.18

0.82

50

30

2.14

2.06

1.61

1.12

0.84

2.26

2.17

1.67

1.15

0.86

50

10

2.88

2.74

1.99

1.29

0.94

3.12

2.95

2.10

1.34

0.96

0

20

2.30

2.23

1.82

1.34

1.04

2.42

2.35

1.90

1.38

1.06

0

10

2.79

2.69

2.12

1.49

1.13

2.98

2.87

2.23

1.54

1.16

-50

20

2.22

2.17

1.88

1.49

1.21

2.34

2.29

1.97

1.54

1.25

-50

10

2.71

2.64

2.23

1.69

1.35

2.87

2.79

2.33

1.75

1.39

Down

90

10

5.07

4.55

2.56

1.36

0.91

4.81

4.33

2.49

1.34

0.90

50

30

3.58

3.36

2.31

1.42

1.00

3.51

3.30

2.28

1.40

1.00

50

10

5.10

4.66

2.85

1.60

1.09

4.74

4.36

2.73

1.57

1.08

0

20

3.85

3.66

2.68

1.74

1.27

3.81

3.63

2.66

1.74

1.27

0

10

4.92

4.62

3.16

1.94

1.37

4.59

4.32

3.02

1.88

1.34

-50

20

3.62

3.50

2.80

2.01

1.54

3.77

3.64

2.90

2.05

1.57

-50

10

4.67

4.47

3.40

2.29

1.70

4.50

4.32

3.31

2.25

1.68

Position of airspace

Direction of heat flow

Airspace

1.5 inch airspace

3.5 inch airspace

Effective emittance E Effective emittance E

Mean temp., °F

Temp. diff., °F

0.03

0.05

0.2

0.5

0.82

0.03

0.05

0.2

0.5

0.82

Vertical

Horizontal

90

10

3.99

3.66

2.25

1.27

0.87

3.69

3.40

2.15

1.24

0.85

50

30

2.58

2.46

1.84

1.23

0.90

2.67

2.55

1.89

1.25

0.91

50

10

3.79

3.55

2.39

1.45

1.02

3.63

3.40

2.32

1.42

1.01

0

20

2.76

2.66

2.10

1.48

1.12

2.88

2.78

2.17

1.51

1.14

0

10

3.51

3.35

2.51

1.67

1.23

3.49

3.33

2.50

1.67

1.23

-50

20

2.64

2.58

2.18

1.66

1.33

2.82

2.75

2.30

1.73

1.37

-50

10

3.31

3.21

2.62

1.91

1.48

3.40

3.30

2.67

1.94

1.50