Heat Energy Transfer

Conduction

is direct heat flow through matter (molecular motion). It results from contact of a warm area of an object with a cooler area or with another object. The greatest flow of heat possible between materials is where there is direct conduction between solids. Heat is always conducted from warm to cold and always moves via the shortest and easiest route. In general the denser a substance the better conductor it is. Because air has low density the percentage of heat transferred by conduction through air is comparatively small. A less dense mass has less flow of heat by conduction thereby resulting in a better insulating material.

Convection

is the transport of heat within a gas or liquid, caused by flow of the material itself (mass motion). Natural convection heat flow is largely upward and somewhat sideways. This is called “free convection”. A warm stove loses heat by conduction to the cooler air in contact with it. This added heat activates (warms) the molecules of the air which expand, becoming less dense, and rise. Cooler, heavier air rushes in from the side and below to replace it. Convection when mechanically induced by a fan is called, “forced convection”.

Radiation

is the transmission of electromagnetic rays (infrared) through space. Infrared rays occur between light and radar waves. Radiation refers only to infrared rays. All objects radiate infrared rays from their surfaces in all directions in a straight line until they are reflected or absorbed by another object. Traveling at the speed of light these rays are invisible and they have no temperature only energy. Heating an object excites the surface molecules causing them to give off infrared radiation. When these infrared rays strike the surface of another object the rays are absorbed and heat is produced in the object. This heat spreads throughout the mass by conduction. The heated object then transmits infrared rays from exposed surfaces by radiation as they are exposed directly to an air space. Radiation, which accounts for 50% to 80% of all heat transfer, will pass through air with ease just as radiation travels the many million miles that separate the earth from the sun.

Water vapor

is the gas phase of water and as a gas it will expand or contract to fill any space it may be in. In a given space with air at a given temperature, there is a limited amount of vapor that can be suspended and any excess will turn into water. The point just before condensation commences is called 100% saturation. Vapor laws: (1) The warmer the temperature the more vapor the air can hold.(2) A larger space holds more vapor. (3) The more vapor in a given space, the greater its density. (4) Greater vapor density flows to areas of lower vapor density. (5) Permeability of insulation is a prerequisite for vapor transmission, the less permeable means less vapor transfer.

R factor

or resistance to heat flow is a measure of insulation’s ability to slow conductive heat flow however doesn’t consider radiation or convection loss. EPS foam becomes a conductor of heat and loses its insulating efficiency when the relative humidity increases because of moisture absorption. Insulations with mass normally contain at least the average amount of moisture in the air and must be completely dried out before achieving lab rated R values. The R value of EPS foam spa covers is reduced by over 35% with typical ambient moisture content (from R12 rated to R8 effectual). As EPS spa covers continue to gain water weight from radiant energy they increase energy costs exponentially.

Summary

In both winter and summer, 65% to 80% of heat is lost through radiation. Aluminum has low thermal emissivity (0.05%) and when coupled with the low thermal conductivity of air it is possible to practically eliminate heat transfer by radiation and convection. Aluminum insulation can reflect 95% of all radiant energy and is impervious to water vapor and convection currents. The performance of most aluminum insulation is unsurpassed for winter heat and adds efficiency for summer by minimizing convection currents. Aluminum inhibits infrared rays from penetrating its surface while reflecting energy back. There is no such thing as, “dead air space”, as far as heat transfer is concerned even with an air-tight compartment such as a Thermos bottle. Convection currents are inevitable when differences in surface temperatures exist and since air has some density, there will be some heat transfer by conduction when space is heated.