Thermal conductivity (K):
- It explains the how readily het can be conducted through a particular materials. Its unit is watt/m 0k.
Thermal resistance (R):
- It is the insulating ability of the material. It is an inherent property of material and depends upon composition, formation, size, shape and thickness. Its unit in [watt/m 0k]-1
Equation for heat loss and gain across the structures can be calculated by using the formula:
Q = K. A/ X (T1 – T2) watt
Where, Q=Heat loss or gain across the structure in watt
K =Thermal conductivity of material used in the structure watt/m 0k.
A=Area of surface in m2
X=Thickness of material in m.
T1= Inside temperature of structure in 0k.
T2=Outside temperature of structure in 0k.
T=Difference temperature of outside and inside.
The values of K varies for the construction of material.
Table: value of K
|
Construction material |
K, watt/m.0k |
|
Concrete or cement mortar |
0.4 |
|
Wood |
0.1 |
|
Compressed Straw |
0.06 |
|
Loose Straw |
0.045 |
|
Mud / Plaster of mud |
0.45 |
|
GI Sheet/ CGI Sheet |
73 |
|
Iron |
73 |
|
Aluminium |
204 |
|
Saw dust |
0.04 |
|
Brick |
0.45 |
|
Asbestos |
0. 151 |
|
Glass plate |
270.7 |
|
Air |
0.022-0.068 |
- As the construction materials shall used different materials.
- Hence, materials heat conductivity and heat resistance is in different degree while competing the insulation effects of given structure it is necessary to consider overall thermal resistance of material used in construction.
Now, Q= (T1 – T2) / R
R=X/AK
Where, R is the thermal resistance and its unit is [watt/m 0k]-1.
For the more than one material, thermal resistance is:
R0 = R1 + R2 + R3 +…………….+ Rn
= X1 /A1K1 + X2/A2K2 + X3/A3K3 + ………….. + Xn/AnKn
Where, X1, X2, X3 = Thickness of different materials
A1, A2, A3 = Area of different materials
K1, K2, K3 = Thermal conductivity of different materials.
Air film presents in inside and outside of wall should be consider. The film of still air has good insulating effect. For air film the thermal resistance could be competed as :
Rf = 1/ AKf
Where, Rf = resistance offer by film air.
A= surface area in sq. m.
Kf = conduction of air film.
Now overall, R0 = Rfoutside + R1 + R2 + R3 +…………….+ Rn + Rfinside
=1/AKfo + X1 /A1K1 + X2/A2K2 + X3/A3K3 + ………….. + Xn/AnKn + 1/AKfi
Where, Kfo= thermal conductivity of outside air film.
Kfi = thermal conductivity of inside film.