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Heat flows naturally from a warmer to a cooler space. In the winter, this heat flow moves directly from all heated living spaces to adjacent unheated rooms, or to the outdoors; or indirectly through interior ceilings, walls, and floors--wherever there is a difference in temperature. During the warmer months, heat flows from outdoors to the house interior.

To maintain comfort, the heat lost in winter must be replaced by your heating system and the heat gained in summer can be removed by your air conditioner. Insulating ceilings, walls, and floors decreases this heat flow by providing an effective resistance to the flow of heat.

Insulation is rated in terms of thermal resistance, called R-value, which indicates the resistance to heat flow. The higher the R-value, the greater the insulating effectiveness. The R-value of thermal insulation depends on the type of material, its thickness, and density. In calculating the R-value of a multi-layered installation, the R-value of the individual layers are added. Installing more insulation in your home increases R-value and the resistance to heat flow.

The effectiveness of an insulated wall or ceiling also depends on how and where the insulation is installed. For example, insulation which is compressed will not give you its full rated R-value. Also, the overall R-value of a wall or ceiling will be somewhat different from the R-value of the insulation itself because some heat flows around the insulation through the studs and joists. That is, the overall R-value of a wall with insulation between timber studs is less than the R-value of the insulation itself because the wood provides a thermal bridge around the insulation.

The thermal bridging through metal framing is much greater than that through timber-framed walls; sometimes a metal wal's overall R-value can be as low as half the insulations R-value. At Bond Homes, we build timber framed walls which are less thermally conductive and provide a better level of overall insulation.

Heat movement explained.

Heat moves in three ways: radiation, conduction and convection.

Radiation is heat transfer through electromagnetic light waves that we can't see. You feel radiant heat from a burning fire or from sunlight streaming into a window that strikes your body. The radiation from the sun is short-wave radiation which penetrates the glass window and warms the interior surfaces and furnishings in your home. These surfaces and furnishings then release the aquired heat in the form of long-wave radiation that cannot penetrate the glass to escape outside.

This effect is commonly known as the 'greenhouse' or 'glasshouse' effect.

Conduction heat transfer happens when heat moves through an object. The heat excites the molecules in a heated object and these molecules excite the ones next to them. The closer packed the molecules are in an object, the more rapid the heat transfer.

Gases, such as air, do not conduct heat very well. Solids, (particularly metal) conduct heat much more readily. With conductive heat transfer, heat inside the home warms the bottom layer of plasterboard ceiling which transfers heat to the next layer and so on.

Convection heat transfer happens when a fluid such as air or water gets heated by a hot object that touches the air or water. Ducted central heating is a great example of convection heat as the air moving over the heat exchanger gets warm and then passes that warmth to objects it touches once it is in the room. You feel convective drafts in a room in winter as well. If you stand in front of a large pane of glass that is not insulated glass, you can feel a cold draft. That is actually air that has lost or transferred its heat against the glass.

The resultant cooling effect makes the air at the window to drop causing a draught at floor level.

Insulation stops these different movements of heat transfer. The insulation absorbs the heat and slows its movement. It is important to realise that insulation does not block the movement of heat, it simply slows it down. This resistance to heat flow or movement is the R-value as explained above.

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