General Principles of Thermal Insulation

General
All 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 a material which slows this process down. Rigid phenolic, polyisocyanurate and polyurethane insulation materials have tiny pockets of trapped gas. It is these pockets that resist the transfer of heat although they will not stop heat / loss or gain completely.

Buildings, no matter how well insulated, will need a continual input of heat to maintain desired temperature levels. The input needed will be much smaller in a well insulated building than in an uninsulated one - but it will still be needed.

Heat Transfer
Before dealing with the principles of insulation it is necessary to have an understanding of the mechanism of heat transfer. When a hot surface is surrounded by an area that is colder, heat will be transferred and the process will continue until both are at the same temperature. The heat transfer takes place by one or more of three methods:

• conduction;
• convection; and
• radiation.

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 the heat comes. 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 with 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 are prone to convection.

Convection
Convection occurs in liquids and gases. For any solid to lose or gain heat by convection it must be in contact with the fluid. Convection cannot occur in a vacuum. Convection results from a change in density in parts of the fluid, the density change being brought about by a change in temperature. The process of convection that takes place solely through density change is known as ‘natural convection’. Where the displacement fluid is accelerated by wind or artificial means, the process is called ‘forced convection’. With forced convection, the rate of heat transfer is increased - substantially so in many cases.

Convection in Gases
If a hot body is surrounded by cooler air, heat is conducted to the air in immediate contact with the body. This air then becomes less dense than the colder air further away. The warmer lighter air is thus displaced upwards and is replaced by colder heavier air, which in turn receives heat and is similarly displaced. A continuous flow of air or convection around the hot body removing heat from it is thus developed. This process is similar but reversed if warm air surrounds a colder body; the air becoming colder on transfer of the heat to the body, and the air becomes displaced downwards.

Convection in Liquids
Similar convection processes occur in liquids, though at a slower rate according to the viscosity of the liquid. It cannot be assumed however, that convection in a liquid results in the colder component sinking and - the warmer rising. It depends on the liquid and the temperatures concerned. Water achieves its greatest density at approximately 4°C. Hence in a column of water, initially at 4°C, any part to which heat is applied will rise to the top, alternatively if any part is cooled below 4°C, it too will rise to the top and the relatively warmer water sinks to the bottom. It is always the top of a pond or the water in a storage vessel which freezes first.

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. The radiation component is reduced by absorption into the body of the insulant and is further reduced by the application of a bright foil outer facing to the product.

Radiation
The process by which heat is emitted from a body and transmitted across space as energy is called radiation. Heat radiation is a form of wave energy in space similar to radio and light waves. Radiation does not require any intermediate medium such as air for its transfer; it can readily take place across a vacuum. All bodies emit radiant energy. The rate of emission is governed by:

• the temperature difference between radiating and receiving surfaces;
• the distance between the surfaces; and
• the emissivity of the surfaces - dull matt surfaces are good emitters / receivers, bright reflective surfaces are poor.

The same applies to pipes, tanks and vessels containing hot (or cold) fluids. If there is no heat input to compensate for the loss through the insulation, the temperature of the fluid will fall. A well insulated vessel will maintain the heat of the contents for a longer period of time but it will never keep the temperature stable on its own.

Thermal insulation does not generate heat, it is a common misconception that thermal insulation automatically warms the building in which it is installed. If no heat is supplied to that building, the building will remain cold. Any temperature rise that may occur will be as a result of better utilisation of internal fortuitous or incidental heat gains.

Conduction Inhibition
To reduce heat transfer by conduction, an insulant should have a small ratio of solid volume to void. Additionally, a thin wall matrix, a discontinuous matrix or a matrix of elements with minimum point contacts are all beneficial at reducing conducted heat flow. A reduction in the conduction across the voids can be achieved by the use of inert gases rather than still air.

Convection Inhibition
To reduce heat transfer by convection, an insulant should have a structure of a cellular nature or with a high void content. Small cells or voids inhibit convection within them and are thus less prone to excite or agitate neighbouring cells.

Radiation Inhibition
Radiation transfer is largely eliminated when an insulant is placed in close contact with a hot surface. Radiation may penetrate an open cell material but is rapidly absorbed within the immediate matrix and the energy changed to conductive or convective heat flow. Radiation is also inhibited by the use of bright aluminium foil either in the form of multi-corrugated sheets or as an outer facing on conventional insulants.

Density Effects
Most materials achieve their insulating properties by virtue of the high void content of their structure. The voids inhibit convective heat transfer because of their small size. A reduction in void size reduces convection but does increase the volume of the material needed to form the closer matrix, this therefore results in an increase in product density. Further increases in density continue to inhibit convective heat transfer but, ultimately, the additional benefit is offset by the increasing conductive transfer through the matrix material and any further increase in density causes deterioration in thermal conductivity. Most traditional insulants are manufactured in the low to medium density range and each particular product family will have its own specific relationship between conductivity and density.

Temperature Effects
Thermal conductivity increases with temperature. The insulating medium, the air or gas within the voids, becomes more excited as its temperature is raised. This excitement enhances convection within or between the voids and so increases heat flow. This increase in thermal conductivity is generally continuous for air filled products and can be mathematically modelled. Those insulants which employ 'inert gases' as their insulating medium may show sharp changes in thermal conductivity, these changes may occur because of gas condensation but this tends to be at sub zero temperatures.

Surface Emissivity
The effects of surface emissivity are exaggerated in high temperature applications, and particular attention should be paid to the selection of the type of surface of the insulation system. Low emissivity surfaces, such as bright polished aluminium, reduce heat loss by inhibiting the radiation of heat from the surface to the surrounding ambient space however, by holding back the heat being transmitted through the insulation, a dam effect is created and the surface temperature rises. This temperature rise can be considerable and, if insulation is being used to achieve a specified temperature, the use of a low emissivity system could well necessitate an increased thickness of insulation. For example a hot surface at 550°C insulated with a 50 mm product of thermal conductivity 0.055 and ambient temperature of 20°C would give a surface temperature of approximately 98°C, 78°C and 68°C when the outer surface is of low (polished aluminium), medium (galvanised steel) or high (plain or matt) emissivity respectively.