Thermal Conductivity (k-values) Units = W/mK
This is a measure of the rate at which heat is conducted through a particular material under specified conditions. This figure has been calculated for a vast variety of materials from those common to the construction industry to those that are probably never used in the building industry. These have been calculated using a standardised technique with standard materials and are for a one metre thick block of material. In lists of these figures, a range of different values will be seen for similar materials due to the variations created my moisture levels and air levels within the material. The largest selection of materials will probably be found in the CIBSE Guide A (Environmental Design) pp3-44 – 3-52.
Thermal Resistivity (r-values) Units = mK/W
This is merely the reciprocal of thermal conductivity and so may be calculated easily.
r = 1/k
Because the values for k are for a one metre thick piece of material, these values are also for a one metre thick piece of material.
An R-value will takes into account the thickness of the material, thus allowing for more accurate comparisons between materials carrying out the same job. Steel, wood and concrete may all be used for the frame of the building however each has a radically different thickness and r-value. To allow for a more accurate comparison in the insulation properties of each material, an R-value should be calculated
Thermal Resistance (R-value) Units = m2K/W
This is a measure of the opposition to heat transfer offered by a particular component in a building element. R-values are created by dividing the thickness of the material (metres) by the k-value for a particular material.
R = d/k (or R = d x r)
Thermal Resistance provides a specific result for a material of known thickness and can therefore allow for almost any material on sites insulation properties to be examined.
Thermal Transmittance (U-values) Units = W/m2K
This is a measure of the overall rate of heat transfer, by all mechanisms under standard conditions, through a particular section of construction. This measure takes into account the thickness of each material involved and is calculated from R-values of each material as well as constants accounting for surface transmittance (Rsi and Rso, inner and outer surfaces respectively) and also for a small standard air gap (Rso). Each of these has a standard value assigned which in reality may vary slightly but for the purposes of this work will be ignored.
U = 1/Rsi + R1 + R2 +….. + Ra + Rso
Standard U-values have been calculated for specific types of walls which have been modelled for. Where non-standard wall types are used these pre-determined values are of no use. The tool on this web page may be used to estimate the U-value for a far larger variety of wall types because it will allow for you to select the variety of materials and then to input their thickness.
This is the portion of a structure whose higher thermal conductivity lowers the overall thermal insulation of the structure. The bridge creates an area where heat loss is far greater in one area than it is for the general building structure thus creating a number of problems.
These areas with poorer insulation will lower the average U-value for the building.
This will also create cold spots on the walls inside the building where condensation will form more rapidly resulting in damp patches and in the long term in the possible rotting of the building structure.
Calculating floor U-value is done using tables and involves the comparison between the perimeter and the floor area. The calculations are complex and as they have to take into account both downward and lateral movement of heat. As during a refurbishment it is unlikely that the foundations will be altered, we have ignored this section. Even so floors may be a major source of heat loss and where possible insulation should be added or foundations widened.