Essentially, insulation is the use of a material with a low overall conductance to reduce the energy flow across another material. The insulation acts to retard and/or reduce the flow of heat, thus it must have a high resistance (resistance being the inverse of conductance).
In general, apart from vacuum, the worst conductors of heat are gases, and these insulate best when convection within the gas can be suppressed. Fibrous blankets in which the gas is trapped in a mat made from a low conductivity solid - such as glass or organic fibre (wool or polyester) - are good insulators, and closed-cell foams in which the gas is trapped in bubbles in a poor conductor such as polystyrene or polyurethane are even better.
Many materials can be thought of as insulative, but basically there are three main types of insulative effect.
Resistive insulation, also called bulk insulation, insulates against the transfer to heat simply through its resistance to conduction. Because air has one of the highest resistances to conduction, the best resistive insulators are those that trap small pockets of air within themselves.
Insulators such as glass-fibre, mineral wool and expanded polystyrene work extremely well as long as the air within these pockets cannot move and thus transfer heat by convection. Some of the worst resistive insulators are, obviously, highly conductive metals such as steel and copper.
Reflective insulation works by reducing radiative heat transfer. The ability of a material to absorb or emit infra-red radiation depends on both the nature and colour of its surface. In most building materials there is a proportional relationship between the ability to emit and absorb radiation. The best absorbers and emitters are usually matte-black whilst the worst are shiny, white and reflective. Because they only reduce radiative transfer, reflective insulation is only useful on the inner or outer surfaces of a composite material or within a cavity.
Capacitive insulation has virtually no effect in steady-state heat flow, that is when temperatures are relatively constant on each side of a material. If the temperature on either side fluctuates, however, capacitive insulation effects become important. Because heat transfer is not instantaneous, diurnal variations take time to pass through building elements. For some materials like glass this is not that noticeable, however for double-brick or rammed earth walls this can take up to eight or nine hours. This delay is termed thermal lag and is measured as the time difference between peak outside temperature and the peak temperature on the inside surface of a element.
Forms of Insulation
Although insulation can be made from a variety of materials, it usually comes in five physical forms: blankets, blown-in, loose-fill, rigid foam board or reflective films. Each type is made to fit a particular part of a building.
In the form of bats or rolls, blankets are flexible products made from glass or mineral fibres. They are available in widths suited to standard spacings of wall studs, ceiling or floor joists. Continuous rolls can be hand-cut and trimmed to fit. They are available with or without vapour retarding facings. Bats with a special flame-resistant facing are available in various widths for basement walls where the insulation may be left exposed. Fibre glass is manufactured from sand and recycled glass, and rock wool is made from basaltic rock and recycled material from steel mill wastes.
Loose-fill insulation includes loose fibres. or fibre pellets that are blown into building cavities or roof-spaces using special pneumatic equipment. Another form includes fibres that are co-sprayed with an adhesive to make them resistant to settling. The blown-in material can provide additional resistance to air infiltration if the insulation is sufficiently dense. This type of insulation is usually made of fibre glass, rock wool or cellulose, although there are some local products based on sheeps wool. Cellulose is made from recycled plant material treated with fire retardant chemicals.
Foamed in Place
Polyurethane foam insulation can be applied by a professional applicator using special equipment to meter, mix, and spray the foam directly into cavities within the building. The foam then expands as it sets to fully seal the cavity. This form of insulation can therefore help to reduce air leaks.
This kind of insulation is made from fibrous materials or plastic foams and is pressed or extruded into board-like forms and moulded pipe-coverings. These provide both thermal and acoustical insulation, strength with low weight, and coverage with few heat loss paths.
Rigid foam boards are made of polyisocyanurate, extruded polystyrene (XPS or blueboard), expanded polystyrene (EPS or beadboard), or other materials. These boards are lightweight, provide structural support, and generally have a high R-values. Rigid board insulation is made to be used in confined spaces such as exterior walls, basements, foundation and stem walls, concrete slabs, and cathedral ceilings. Such boards may also be faced with a reflective foil that reduces heat flow when next to an air space.
Reflective films are usually fabricated from aluminium foils with a variety of backings such as craft paper, plastic film, polyethylene bubbles or cardboard. The resistance to heat flow depends on the heat flow direction, with this type of insulation being most effective in reducing downward heat flow. Reflective systems are typically located between roof rafters, floor joists, or wall studs.
If a single reflective surface is used alone and faces an open area, such as a roof-space, it is often termed a radiant barrier. Radiant barriers are installed in buildings to reduce both summer heat gain and winter heat loss. They are most effective in hot climates rather than in cool climates. All radiant barriers must have a low remittance (0.1 or less) and high reflectance (0.9 or more).
Selecting an Insulation
The type of insulation you use will be determined by the nature of the spaces in the building that you plan to insulate. For example, since you cannot conveniently 'pour' insulation into an overhead space, blankets, spray or board products, or reflective systems are used between the joists of an unfinished basement ceiling. The most economical way to fill closed cavities in finished walls is with blown-in insulation applied with pneumatic equipment or with foamed-in-place polyurethane foam.
It is important to know that the different forms of insulation can be used together. For example, you can add bat or roll insulation over loose-fill insulation, or vice-versa. Usually, material of higher density (weight per unit volume) should not be placed on top of lower density insulation that is easily compressed. Doing so will reduce the thickness of the material underneath and thereby lower its R-value.
- Thermal insulation in dwellings, CBD-16