How does insulation work? The control of heat flow.

We’ve decided to go back to basics and explain just how insulation works. This week we are looking at how thermal insulation helps to control heat flow.

Heat moves from warmer to colder areas. This is what causes buildings to get colder in winter and hotter in summer. It happens through one or more of the three heat transfer mechanisms:

• Conduction
• Convection
• Radiation

Thermal insulation works by restricting how heat moves via conduction, convection and/or radiation.

Conduction

Conduction is how heat moves along or through a material. The ability of a material to conduct heat depends on the material itself. For example, metals are a good conductor of heat. Think how hot your seatbelt buckle can get during the summer months! A low lambda value (which measures how well an insulation conducts heat) is important for insulation materials to reduce heat loss through conduction. The lower the lambda value of an insulation, the better it can resist heat transfer through conduction.

Convection

You know how we say “heat rises”? Well this is applicable here. When the molecules that make up a gas or liquid heat up, their density will change. So warmer air will become less dense and rise. This is called ‘natural convection’, but it can also be sped up by wind or artificial means known as ‘forced convection’. Closed cell insulation with small cell sizes inhibits convection within the cell, making it less prone to affecting neighbouring cells.

Radiation

Radiation is how heat transfers across space from one body to another as energy. The rate of heat transfer through radiation is controlled by:

• The difference in temperature of the surface that is radiating heat and the surface that is receiving heat
• The distance between these surfaces
• The emissivity of the surfaces (so how shiny a surface is). A material with a low emissivity reflects heat as radiation. An example would be a low–emissivity foil facing on an insulation board.

Now you know how insulation helps to control heat flow, you can probably also see how a good insulation can reduce energy usage and keep your bills as low as possible.

But what exactly is a ‘good insulation’? Well, if you install a low-lambda, closed-cell, foil-faced insulation in a previously uninsulated attic space, for example, you will significantly reduce the amount of heat lost through conduction, convection and radiation.

Over the next few weeks in our series on how insulation works, we’ll look in more detail at how the effectiveness of thermal insulation is measured and the importance of the whole building envelope.

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