Insulation - Principles and Calculations

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:

Vegetation (i.e. organic fibres)
Minerals (often volcanic in origin)
Coal or Oil (in the form of cellular plastics)

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

Batts and rigid boards
Quilts
Loose fibres or granules
Foam
An integral part of a construction material eg blockwork or plasterboard.

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:

cost
the material may not be available in the form required.
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.
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.
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
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.
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.