Most insulation works by reducing the conduction. Heat does not travel through small pockets of air as well as it does through dense solid materials, so most insulants incorporate lots of air and are therefore relatively light. Some work by also reflecting the energy.
Conductivity
Most data given on thermal properties is given in terms of their Thermal conductivity k or ? Units W/mK (1 Watt =1joule/second)This represents the amount of heat or thermal energy in Watts that will pass through 1m2 (one square metre) of material 1m thick when there is a temperature difference of 1K between the faces.
(Within normal temperature ranges assume 1K is similar to 1 degree centigrade)
The higher the conductivity the faster heat will flow through the material
Sometimes the resistivity (r) is given (just to be confusing)
Resistivity is the inverse of conductivity i.e. 1/k Units mK/W (watch those units)
Resistance
Units m2K/W is the resistance to the flow of heat of a given thickness of a material.This is obtained by multiplying the thickness by the conductivity -
the higher the resistance the slower the heat transfer.
For insulation purposes the higher the resistance the better.
'U' Value; Thermal Transmittance.
Sometimes, rather misleadingly, insulation packages give 'u' values which is not strictly correct. Units w/m2 KThis is the inverse of all the resistances of all the materials and surfaces of your wall or roof (say) added up.
For example, the resistances of the external surface+slates+felt+sarking+cavity+insulation+plasterboard+ internal surface divided by 1
So in brief when comparing insulations
- Only compare 'u' values for products of the same thickness. (otherwise it is not a true comparison)
- Try and compare products of similar thickness (Multifoils are a bit of a separate case)
- Make sure you are comparing either conductivity (k) values or resistivities or Resistances
- Remember conductivities and conductance should be low
- Resistivity and Resistance should be high.