Insulators and Insulation

This page refers to electrical insulation. For thermal insulation see insulation, and for sound insulation see sound proofing. Insulator is also a DNA-sequence that prevents eukaryotic gene regulatory proteins from influencing distant genes, see Insulator (DNA)

Definition

An Insulator is a material or object which resists the flow of electric charge.

Electrical insulator

Insulated copper lamp wire

The term electrical insulator has the same meaning as the term dielectric, but the two terms are used in different contexts.The opposite of electrical insulators are conductors and semiconductors, which permit the flow of charge. Semiconductors are strictly speaking also insulators, since they prevent the flow of electric charge at low temperatures, unless doped with atoms that release extra charges to carry the current. However, some materials (such as silicon dioxide) are very nearly perfect electrical insulators, which allows flash memory technology. A much larger class of materials, (for example rubber and many plastics) are "good enough" insulators to be used for home and office wiring (into the hundreds of volts) without noticeable loss of safety or efficiency.

High voltage insulators

High voltage insulators used for high voltage power transmission are either porcelain insulators or composite insulators. Porcelain insulators are made from clay, quartz or alumina and feldspar. Alumina insulators are used where high mechanical strength is a criterion. In recent times there is a shift towards composite insulators which have a central rod made of fibre reinforced plastic and outer weathersheds made of silicone rubber or EPDM. Glass insulators were, and in some places are still used to mount electrical power lines. Most insulator manufacturers stopped making glass insulators in the late 1960's, switching to ceramic materials. Composite insulators are less costly, light weight and have excellent hydrophobic capability and hence can be used in polluted areas.

Low voltage insulators

Insulating materials such as PVC (polyvinyl chloride) are used to minimise the possibility of a person coming into contact with a 'live' wire. Some appliances such as electric shavers and hair dryers are doubly insulated to protect the user. They can be recognised because their leads have two pins, or on 3 pin plugs the third (earth) pin is made of plastic rather than metal. In the EU, double insulated appliances all are marked with a symbol of 2 squares, one inside the other.

Double insulation requires that cables have basic and supplementary insulation, each of which is sufficient to prevent electric shock. Usually, the internal electrical components are totally enclosed in an insulated packaging which prevents any contact with live parts.

See also

• Pylon

External links

• http://www.myinsulators.com/downtownseattle/ — one person's obsession with telephone pole insulators
• Transcontinental Telegraph Insulators, 1867
• www.insulators.com
• www.insulatorscanada.com
• http://www.nia.org — National Insulator Association

This page refers to thermal insulation. For electrical insulation see insulators, and for sound insulation see sound proofing. Often the same techniques used for thermal insulation can be used for sound insulation as well.

Insulation means a barrier to the flow of energy, usually heat, but often sound, and sometimes both.

Thermal insulation is the practice of surrounding a building or other object, with a material that reduces the flow of heat, that is, it provides a thermal gradient. Convection to or from the surrounding air is addressed with a thick, low density, layer of material that inhibits air currents and minimizes solid conduction flow at the same time. A reflective barrier of metallic film or white paint is sometimes added to reduce radiation heat flow.

Insulation is used, for example, on stoves and ovens, refrigerators, freezers, water heaters, water pipes and other appliances or industrial applications where it is important to reduce heat energy losses or to prevent heating up nearby objects.

Thermal insulation of buildings is one means of achieving indoor comfort and energy-efficiency, weatherization and thermal mass are others.

Commonly-used insulation materials

for residential construction include:

• Fiberglass mats
• Polystyrene foam, usually as pre-cut boards
• Polyurethane foam, as boards or foamed in-situ
• Cellulose insulation or other blow-in fibrous materials
• Adobe bricks are made of mud, common in Southwest United States
• Vacuum insulation consisting of thin panels with extreme insulation capacities

In the 1970's, foam insulations suffered a setback when urea formaldehyde [1] was used as a blowing agent, causing indoor pollution. Subsequently, other safer blowing agents have been used.

The use of highly-compressed straw bales as insulation, though uncommon, is gaining popularity in experimental building projects and passive solar design because of its high R-value.

Silica aerogels are new very high-performance materials used in special applications.

Effectiveness of insulation

Thermal insulation usually works best on the outside of the structure, to allow the walls, floor and ceiling to take on room temperature, which prevents condensation of the airborne humidity. This also employs the thermal mass of the structure to reduce temperature fluctuations for a more comfortable room climate.

A vapour barrier is needed with good insulation because of the risk of condensation on cold parts of the structure with resulting damage, e.g. by mold. The vapour barrier is usually a sealed plastic film inside the wall and should go on the warm side of the insulation. That is, in a heated house, the vapour barrier goes close to the warm inside of the wall. In very warm climates, on a well-insulated air conditioned house, the vapour barrier should go on the outside so that the insulation is between the vapour barrier and colder, air conditioned parts of the house.

R-values

Main article: R-value

R-values are calculated from the thermal conductivity, k, and the thickness, d, of materials: R = d/k. Engineering/British units for R-value are ft²•°F•h/Btu and in metric, R = K•m²/W. The thickness of the materials must be considered when comparing R-values. For instance, the R-value of a 14" adobe brick is about R-4, while that of cellulose is about R-3/inch, so that of cellulose with equal thickness is ahout R-42. While a relatively poor insulator, adobe regulates indoor temperature by damping the daily swings in outdoor temperature.

R-values are measured at 75°F (22°C). This must also be taken into account [2] when comparing R-values [3] since some materials, especially blown fiberglass [4], suffer significant loss of insulating ability at lower temperatures. Other materials gain R-value at lower temperatures, such as polystyrene. Additionally, installation conditions [5] can significantly reduce R-value due to compression, wind washing [6] or temperature differences creating air convection bypassing the insulation through even very small air gaps. Sprayed in place foam and densely packed cellulose insulations [7] avoid air gaps.

Typical approximate R-values

• Cellulose (1") = R-3
• Fiberglass (1") = R-2.5
• Stud wall made of 2x4s with 3.2 inches of fiberglass = R-12 (wood studs conduct heat more than fiberglass)
• Polyisocyanurate board (1") = R-7
• Straw bales = R-40 to R-50 range
• Single pane glass window = R-1
• Double pane glass window = R-2
• Double pane glass window with low emissitivity coating = R-3
• Triple pane glass window = R-3

Helpful Links

• Home Insulation
• R-value Recommendations

References

• University of North Carolina at Chapel Hill
• DOE/CE 2002

See also

• Building construction
• Dewar flask
• Superinsulation