Conduction, Convection and Radiation

By Kenton Shepard

CONDUCTION

Conduction is the transfer of energy through solid matter from particle to particle. Heat is conducted through a solid material much the same way electricity is. Conduction is slowed by insulation.

CONVECTION

Convection is the transfer of heat energy through a gas or liquid by movement of currents. Heat is carried by the fluid. When warm air comes out of a register, it rises. This action is called thermal buoyancy.  As it cools, it falls. This rising and falling movement is called convection current. Convection can be interrupted by a physical barrier.

RADIATION

Radiation is electromagnetic waves which directly transport heat energy through space. Sunlight is radiated through space to our planet without the aid of fluids or solids. Through radiation, the sun transfers heat through 93 million miles of empty space! Heat energy transferred through radiation can be interrupted by a material which reflects it.

SOLAR RADIATION MOVEMENT THROUGH ATTICS

In addition to solar gain from sunlight shining through windows, sunlight striking a rooftop can send heat down into the building interior.

Depending on the roof color, a large part of the sunlight radiated to earth is absorbed by a roof as heat, which moves downward through the roof covering material into the sheathing and rafters through conduction, since these are solid materials. As the solid parts of the roof become warmer, they warm the air next to their surfaces, which is then carried to another part of the attic by convection currents, since air is considered to be a fluid.

As they warm, the roof covering and framing materials also begin to radiate heat down toward the ceiling framing members and insulation. Ceiling joists absorb this heat and conduction helps spread it throughout the length and width of each joist. Attics with trusses will also transfer heat downward through the trusses via conduction.

As radiation hits the insulation and is absorbed by the fibers on the upward-facing surface, the fibers begin to warm. As they warm, they pass the heat they contain to the microscopic air pockets nearest to them. Through conduction, convection and radiation heat moves slowly downward through the insulation and interior ceiling-covering materials.

As heat levels in the roof-covering materials increase, this process happens more quickly until the ceiling has warmed enough to begin to radiate heat into the home interior where it’s absorbed by objects, including people. Heat is distributed through these solid objects by conduction, and they in turn warm the air around their surfaces. This warm air is spread through the home by convection currents, natural air movement (windows and doors), fans and blowers.