IMPORT TERMS AND SYMBOLS
Thermal Conductivity (K)
This defines a materialís ability to transmit heat, being measurated in watts per square metre of surface area for a temperature gradient of 1 kelvin (K) per unit thickness of one metre. It is expressed as : W/m 2.K
Thermal Conductance
This defines a materialís ability to transmit heat measured in watts per square metre of surface area for a temperature gradient of one Kelvin ( K ) in terms of a SPECIFIC THICKNESS expressed in metre.
It is expressed as : K/m 2 .K.
Thermal Conductance
This defines a materialís ability to transmit heat measured in watts per square metre of surface area for a temperature gradient of one Kelvin ( K ) in terms of a SPECIFIC THICKNESS expressed in metre.
It is expressed as : K/m 2 .K.
Thermal Resistance
It is both appropriate and convenient to measure insulation materials performance directly in terms of materialís thermal resistance, which is obtained by dividing thickness (T) expressed in metres by thermal conductivity (K) expressed in W/m.K:
R = T/K
It is expressed as: m2.K/W
Thermal Transmittance
This defines the ability of an element of structure, consisting of given thicknesses of material, air spaces, etc. to transmit heat under steady state conditions.
It is a measure of the quantity of heat that will flow through unit area in unit time per unit difference of temperature of the environments between which the structure intervences.
This value can be calculated by adding the reciprocal of the respective thermal resistance (R) of the component materials and the internal and external surfaces resistances: U=1/R 1+1/R 2+1/R 3 .
It is expressed as: W/m 2.Kances:
Thermal Resistance(R value) Calculation
For example, the following table shows the THERMAL RESISTANCE(R) OF 12 & 48kg/m 3 insulation for each standard thickness at 25 0C (77 0F), according to S.I and IMPERIAL Units Systems.
Thickness
in mm
Thermal conductivity in W/m.K R in m2.K/W
for 12kg/m3 for 48kg/m3 for 12kg/m3 for 48kg/m3
25 0.044 0.033 0.568 0.758
50 0.044 0.033 1.136 1.515
75 0.044 0.033 1.705 2.273
100 0.044 0.033 2.273 3.030
 
Thickness
in inches
Thermal conductivity in Btu.in/ft 2.h.f R in Ft 2.h.F./Btu
for 0.75lb/ft 3 for 3 lb/ft 3 for 0.75lb/ft 2 forr 3 lb/ft 3
1 0.31 0.23 3.226 4.348
2 0.31 0.23 6.452 8.696
3 0.31 0.23 9.677 13.043
4 0.31 0.23 12.903 17.39
Thermal Transmittance (u value) Calculation
U=1/Sum of the different elements' R value of the structure
RSO = External Surface Resistance
RSI = Internal Surface Resistance
International System Imperial System
Element of Structure K value in W/m.K R value in
m 2.K/W
K value in Btu-
in/Ft 2.hF
R value in
ft 2.hF/Btu
Facing bricks 5.5cm 0.700 0.079 4.85 0.446
48kg/m 3 0.036 1.389 0.25 7.875
5cm at 40 0C        
Block work 15cm 0.900 0.167 6.24 0.946
Plaster 1.5cm 0.58 0.026 4.0 0.148
RSO- 0.049 - 0.278  
RSI- 0.123 - 0.689  
R   1.833 10.391
U Value 0.546 W/m 2.K 0.096 BTU/ft 2h.F
RSO and RSI depends on some climatic conditions (wind speeds for example) the thermal exchange ways (from the top to bottom, bottom to the top) if the wall to be