Convection is the transfer of heat energy through a material by the bodily movement of particles and will occur in fluids (liquids and gases).

Convection arises when a fluid is warmed, and thus expanded. The expanded fluid is less dense and therefore rises and is replaced by cooler fluid which then undergoes the same process. This is called a convection current.

Convection can be natural or forced. Natural convection is when the fluid movement is caused by the fluid itself, whilst forced convection uses external means (such as a fan) to drive the fluid movement.

The rate of heat transfer due to convection is given by:

Where:

            the relative velocity of the fluid;

            the temperature difference between the surface and the environment;

            the direction of heat flow;

            the surface size and orientation;

            the fluid properties (density, viscosity, heat capacity etc);

            surface roughness.

The resistance of convective heat transfer is given by:

Where:

To lower the heat flow due to convective heat transfer you can reduce the area in contact with the fluid, or decrease the convective heat transfer coefficient.

Convective heat transfer only occurs when there is a high enough temperature gradient across the gas/fluid and there is enough space to overcome the drag of the surfaces.

Convective losses can be significant.  If you consider a window frame for example there are often lots of cavities within it in which convection will occur due to the temperature differences across them.  The larger the cavity the greater the heat loss by convection since there will be comparatively less drag due to surfaces restricting the convection currents.  This is one of the reasons why cavities in frames are split up into smaller sizes.

Convective heat flows are also very significant in double glazed units.  The width of the air cavity is very important - if it is too small then conduction and losses are high, and if it is too large then the increase in convection may be larger then the decrease in conduction.  The optimum width will depend on the gas that is used to fill the cavity.  For air it is approximately 16 mm whereas for krypton it is around 10 mm.

The conductive heat transfer coefficient varies with the orientation of the surface and with the heat flow direction.

Hence for the same double glazing unit there will be more heat loss through it when it is used as over-head glazing compared to being used horizontally (opening or fixed light).