Insulation - Principles and Calculations


1 Heat Transfer

2 Thermal Insulation

1 Heat Transfer

Heat is transferred because a material will attempt to achieve thermal equilibrium with its surroundings. Heat flow will occur within a material (solid, liquid or gas) or between materials until the temperature of each is equal. Heat transfer will occur through three mechanisms which might operate alone or in combination. These are conduction, convection and radiation.

Conduction: This is where heat energy is transferred because of the physical contact between molecules within a material (or between materials that are touching each other). The direction of flow will be from the warm area to the cool area. Thermal conductivity is the rate of heat flow - a factor which is determined by the ability of the molecules to conduct heat. The human body is sensitive to this heat flow rather than to temperature. If a person stands in bare feet on a concrete floor, and then a wooden one, the body will sense the different rates of heat flow. That is, heat will be transferred from the body to the concrete more quickly than to the wooden floor because concrete is a better conductor. The concrete floor will be less thermally comfortable than the wooden one, although, in fact, a ground bearing concrete floor usually offers better thermal insulation.

Convection: Convection refers to heat being transferred by the movement of a fluid. In buildings the fluid in question is usually air or water. When it comes into contact with a warmer or colder surface the air or water will either absorb heat (from a colder surface) or lose heat (to a warmer surface). If it becomes colder the fluid will sink because of its increased density and vice versa.

Radiation: Heat can also be transferred through the air (or rather through space) from one body to another by radiation. Heat is radiated to and absorbed from the surfaces that surround a body without heating the air. When radiant energy hits a body some of the energy is reflected and some is absorbed (the respective amounts will vary between different materials and, for instance, colour will play a significant role in determining the amount reflected).

In a typical small detached house with one-brick solid walls, timber ground floor, uninsulated roof and single glazing approximately 25% of the heat goes through the roof, 30% through the walls, 15% through the windows, 15% through the floor, and 15% through ventilation and draughts.

Buildings and People

Buildings will lose heat by all three means. Heat will be conducted from the interior to the exterior through walls, floors and the roof. Convection can transfer heat from a warm interior cavity skin to the cooler outer skin and the wind will convect heat away from exterior surfaces of the building. The exterior surfaces of the building will radiate heat to its surroundings. Human beings will also lose and gain heat by these mechanisms. They will lose heat by conduction when they stand on a cold floor, by convection because of draughts and by radiation to cold surfaces such as windows or uninsulated walls.

Thermal Mass

This refers to the amount of heat a material can store. A so-called heavyweight building will have a high thermal mass. High thermal mass can be useful in smoothing out fluctuations in temperature. Rooms with a high mass will heat up slowly and cool down slowly. Thermal mass can be useful in storing solar energy coming in through southerly orientated windows.

2 Thermal Insulation

Thermal insulation will lower energy use by reducing the amount of heat lost through an element of the house. The effectiveness of the insulation in restricting heat loss will be related to the thickness and the conductivity of the material. Conductivity depends upon:

a) the temperature of the insulation material. In normal conditions, this does not have a significant influence

b) the moisture content of the material. The higher the moisture content, the lower will be the material's resistance to heat-flow. For some insulation materials the moisture content will strongly influence thermal insulation properties, and considerable attention should be paid to this point, particularly where the insulation is exposed to the danger of condensation or direct rain penetration

c) the structure of a material. Insulation materials are generally constructed so that they trap still air in small pockets within the material. Air is a poor conductor of heat and the pockets are so small that the air cannot move; which means that heat transfer by convection is minimised. Although thermal resistance will increase with the thickness of the insulation, after a certain point cost effectiveness (at least in simple pay back terms) decreases as the thickness increases. For example doubling the thickness of existing roof insulation will not save double the amount of energy. In practice the thickness of insulation will also be determined by construction factors, an obvious example being the width of the cavity in masonry walls.

Thermal Insulation Materials

These are produced from three main sources:

Insulation materials come in different forms - the main ones being:

Effectiveness of Insulation Materials

Some insulation materials will have a higher insulation value than others, for example, polyurethane foam is a better insulator than expanded polystyrene and most mineral quilts. However, there may be reasons why the better insulating material is not always specified, for example:

  1. cost
  2. the material may not be available in the form required.
  3. the material may be inappropriate for the position on the building that is being insulated. It will sometimes have to resist loads (on floors and flat roofs, for example) or be stable under conditions of heat from the sun (external insulation, flat roofs) or resistant to water absorption (inverted flat roofs), or it may not be suitable for walls over a particular exposure rating.
  4. some insulation materials can settle with age or become displaced. Others, however, are flexible and resilient. They fit well into awkward spaces and therefore reduce the potential for thermal bridging and heat loss.
  5. the fire resisting properties and combustion characteristics of the insulation material should be taken into account. This includes the possibility of toxic gases being released in a fire
  6. there may be adverse reactions between insulation materials and other materials. For example, polystyrene insulation should not be allowed to come into contact with PVC materials (dpcs, cavity trays or electric cables) as it may embrittle them. Also some remedial works such as chemical damp proof courses are incompatible with some insulation materials.
  7. the risk factor in installation - for example, foamed insulants rely upon two components - a resin and a hardener being mixed together correctly.

Thermal Insulation and Heating

Generally the insulation standards to be applied to the structure should be decided upon before designing the heating system. This is because the insulation standard may affect both the choice of heating system and the required capacity of the heat source (i.e. the boiler). In existing dwellings the standards to be achieved in insulating the fabric will affect the payback period calculations of a new boiler. So, in a well insulated house the advantages of a more efficient boiler may not be very significant. Although there are risks associated with insulation, (examined later in this guide), unlike a boiler, no servicing is required.

Source of Information

There are plenty of sources for information on insulation. However, be cautious about any claims regarding savings and installation costs. In our experience claimed savings do not always materialise and installation costs are often far higher than anticipated. It's also worth remembering that some insulation techniques may have unexpected consequences on the building fabric and structure - many of these are explained in the following pages. Note there is much information on insulation under the appropriate main topic headings, Walls, Floors, and Roof Coverings (from Home page).     

The Energy Savings Trust www.est.org.uk and the National Energy Foundation http://www.nef.org.uk are useful sources of further information. The EST produces some excellent booklets on all aspects of insulation and heating.      

©2009 University of the West of England, Bristol
except where acknowledged