Thermal conductivity ('k value') is a property of a material that determines how much heat is conducted through it for a given temperature difference. However, with a fibrous or porous insulation, which will include both solid material and gas, heat transfer is by a combination of conduction, convection and thermal radiation. 'Thermal conductivity' is still normally the quoted property for such insulation products although strictly it is an 'effective thermal conductivity'.
Units: W/(m.K)
The reciprocal: Thermal resistivity
Measurement techniques: Guarded Hot-plate, Heat-Flow Meter or Hot-Wire

Thermal resistance ('R value') is a property of an object that determines the resistance to heat transfer through it a for a given temperature difference. As the 'thermal resistance' refers to an object, as opposed to a particular material, it can be used to describe the insulating effect of products that are constructed of layers of different materials. The resistance of an object to heat flow will be greater if it is thick and made from low thermal conductivity materials.
Units: (m².K)/W
The reciprocal: Thermal conductance
Measurement techniques: Guarded Hot-plate or Heat-Flow Meter

Thermal transmittance ('U value') is a property of a structure that determines how much heat is transferred from the air space on the hot side of a structure, to the air space on the cold side. Whereas 'thermal conductivity' and 'thermal resistance' refer to heat flow through homogeneous materials, 'thermal transmittance' takes into account all three modes of heat transfer through structures that may have many materials and complex heat flows. It also accounts for the heat transfer between the surrounding air and the structure's outer surfaces. 'Thermal Transmittance' would be used to describe heat transfer through building components such as windows, doors, walls and roofs.
Units: W/(m².K)
Measurement technique: Hot Box

Details of Thermal Measurement Services can be found at Thermal Performance.