IMPORT TERMS AND SYMBOLS 
Thermal Conductivity (K) 
This defines a material’s 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 material’s 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 material’s 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 material’s 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 