Thermal Insulation

General Principles of Thermal Insulation

House showing the General Principles of Thermal InsulationAll thermal insulation materials work on a single basic principle: heat moves from warmer to colder areas. Therefore, on cold days, heat from inside a building seeks to get outside. And on warmer days, the heat from outside the building seeks to get inside.

Insulation is the material which slows this process. Rigid phenolic, rigid urethane and extruded polystyrene insulation materials possess tiny pockets of trapped gas. These pockets resist the transfer of heat. They will not stop the loss or gain of heat completely and buildings, no matter how well insulated, will need a continual input of heat to maintain desired temperature levels. The input needed however, will be much smaller in a well insulated building than in an uninsulated one.

Heat Conduction

Conduction is the process by which heat flows by molecular transportation along or through a material or from one material to another. The material receiving the heat being in contact with that from which it receives it.

Conduction takes place in solids, liquids and gases and from one to another. The rate at which conduction occurs varies considerably according to the substance and its state.

In solids, metals are good conductors, gold, silver and copper being amongst the best. The range continues downwards through minerals such as concrete and masonry, to wood, and then to the lowest conductors such as thermal insulating materials. Liquids are generally bad conductors but this is sometimes obscured by heat transfer taking place by convection. Gases (e.g. air) are even worse conductors than liquids but again they suffer from being prone to convection.

Requirements of an Insulant

In order to perform effectively as an insulant a material must restrict heat flow by any, and preferably, all three methods of heat transfer. Most insulants adequately reduce conduction and convection elements by the cellular structure of the material.

Building Regulations and minimum ‘U’ Values

Current building regulations require insulated buildings to have a ‘U’ value below 0.35 W/m²K although it has been suggested that these may be improved upon again very shortly.

Below, you will find the most popularly specified insulants along with their current ‘K’ values.

Insulant Thickness K Value
EPS Standard SD/FRA Grade 10mm – 400mm 0.036 – 0.038 W/mK
EPS EpsiTherm Grey 10mm – 400mm 0.032 W/mK
EPS Neopor® Grey 10mm – 400mm 0.030 W/mK
Polyisocyanurate (PIR) 0mm – 79mm 0.028 W/mK
Polyisocyanurate (PIR) 80mm – 119mm 0.027 W/mK
Mineral Wool 20mm – 280mm 0.036 – 0.038 W/mK
Lamella 20mm – 300mm 0.040 W/mK
Phenolic 0mm – 24mm 0.023 W/mK
Phenolic 25mm – 44mm 0.021 W/mK
Phenolic 45mm – 130mm 0.020 W/mK