R & U value
R and U values are used to indicate the design thermal performance of a building material or assembly. R values represent the resistance of heat flow through a building material. The higher the R value, the greater the resistance and the insulating value.
U values on the other hand are the direct opposite. U values represent the amount of heat flow that transmits through a building assembly which is built-up of various materials and the surface air film resistances. The lower the U value is, the slower the rate of heat flow and the better the insulating quality.
Thermal conductivity of basic materials
All building material have individual thermal conductivity values, it is expressed in W/mK and is defined as the quantity of heat transmitted under steady-state conditions through unit area of the material of unit thickness in unit time when unit temperature difference exists between its opposite surfaces.
The lower the thermal conductivity value of a material is, the better the insulating properties. Insulation material such as stone wool is one of the most ideal insulation materials with its low thermal conductivity values that contribute significantly to the overall U value performance.
Thermal conductivity (k or lambda) of a material are measured using EN, ASTM or other international or local standards. It is by far the most important aspect of an insulating material. From this k value, thermal resistance (R) of a material and thermal transmittance (U) of a building assembly are measured.
R Value (Thermal resistance)
Thermal resistance or R value of a material is governed by the thickness and thermal conductivity of that material. It is important to note that the thermal resistance value will increase with the thickness of the material (provided thermal conductivity remains constant). Thermal resistance is expressed in m2K/W.
R = d / k
R = thermal resistance
d = material thickness (in meter)
k = thermal conductivity
Thermal resistance or R value of a building material also influences the construction of the roof and building. However, traditionally used building materials such as bricks, concrete, tiles, steel and wood are not very resistant towards energy or heat transfer.
On the other hand, the use of special materials for thermal insulation, such as stone wool, is 20 times better than bricks (of the same thickness) and 40 times better than concrete, in resisting heat transfer. Actual independent studies have shown that overall; the most feasible way of improving energy efficiency of buildings is to use thermal insulation.
U Value (Thermal transmittance)
Thermal transmittance or U value of a construction is defined as the quantity of heat that flows through a unit area of a building section under steady-state conditions in unit time per unit temperature difference of the air on either side of the sections. It is expressed in W/m2K.
U = 1 / Rt
Where Rt is the total thermal resistance and is given by:
Rt = Ro + d1 / k1 + d2 / k2 + ........... dn / kn + Ri
Ro = air film resistance of external surface (m2K/W)
Ri = air film resistance of internal surface (m2K/W)
k = thermal conductivity of basic material (W/mK)
d = thickness of basic material (m)
A general rule of thumb is that the lower the U value of a construction element, the better it would be in withstanding the heat.
Surface air film resistance
The transfer of heat to and from a surface of a body through air is impeded by the presence of a thin layer of relatively motionless air at the surface of the body. This offers resistance to the heat flow and results in a temperature drop across the layer of air.
Air film resistance is affected by wind velocity and therefore different resistance values for outside and inside air films are given. These are defined as follows.
Ro = outside surface air resistance (moving air)
Ri = inside surface air resistance (still air)