Heat Transmission

Heat moves from hot to cold via some combination of three different methods and at a rate that is a property of the materials involved, and that is also partially determined by a materials ability to absorb heat.

In this image, heat moves from atom to atom via conduction, examples of which are shown as yellow arrows. Heat will also move from the warm interior, thru the walls to the colder outside (not shown). Air heated by the stove, rises via convection (upward orange arrow), which is displaced by heavier cooler air (for example, coming off the window - downward orange arrow).  Radiant heat is coming off the woodstove and also via sunshine thru the window, warming Henry and Henrietta,  (red arrows).  If the sun were not shining, Henry and Henrietta would be radiating heat from their bodies out the windows (dashed red arrow).
(Note: in real life woodstoves need a bigger surround, and crows don't read the paper).

Conduction: when you put your finger in cold water, or touch cold metal, you are losing heat by conduction.  Conduction is what happens when heat moves thru a material or from one material to another.  It is the primary method of heat moving thru walls, ceilings and floors.  Conduction is not affected by direction: it will move down just as well as up.

Metals and stones are very good conductors, wood & sheetrock are mediocre conductors, while fiberglass batting, shredded paper and Styrofoam are poor ones.  The amount of heat moved is determined by it's U value (or its corresponding R value, which is just the inverse of U, eg R=1/U).  You can look up the U (or R) value of a material in many published tables.

Radiation: when you stand in front of a fire, or in the sunshine, you are gaining heat by  absorbing radiation.  Likewise, when you sit near a cold window, you are radiating heat out that window. The term radiation is used for a wide variety of similar effects.  In this case, we are not referring to the high energy emitted by radioactive materials, or any of the other high energy sources like x-rays or cosmic rays, or even the less dangerous ultraviolet rays, but rather their much lower energy cousin in the infrared band, whose energy level is lower than that of visible light.

Radiant heat is supposedly accomplished mainly by radiation, but in fact heats the floor (or whatever surface it is placed in), which in turn heats the air by conduction and convection.  It does raise the mean radiant temperature of the room, theoretically reducing the temperature the air needs to be to be comfortable (see the comfort section), but this reduction disappears as soon as the heat cycles off (for a complete discussion, see the hvac section.)

A more technical definition... useful for understanding radiant barriers, solar collectors, low-e windows and radiant heat.

Materials are neither perfect absorbers of radiation, nor perfect emitters.  A material's coefficient of absorption is the percentage of the radiation hitting an object that will actually be absorbed (the rest is either reflected, like a mirror, or transmitted, like glass). Most common materials between 20 and 90% of the radiant energy that falls on them, with the largest difference typically due to color rather than material.  Shiny metals are the exception, absorbing more in the range of 5 to 15%.  Materials with high absorption rates make good solar collectors, while those with lower rates are useful in hot climates to keep roofs cool.

A material's coefficient of emission says what percentage of a materials heat that could be radiated, actually is emitted.   An object with an emissivity of .2 will radiate 1/3 the energy of one whose emissivity is .6, assuming they are the same temperature.  Most common objects have an emissivity of 80 to 90%, except shiny metals whose emissivity is generally low.   When a material emits less than it absorbs, its temperature will rise until the two are balanced. When temperature is balanced (ie thermal equilibrium), the energy absorbed is equal to the energy emitted.  This implies that the emissivity plus reflection will always equal 100% (ie anything that wasn't absorbed and then radiated must have been reflected). 

Note: for emissivity and absorption, the only radiation considered is that in the vicinity of the visible light spectrum (ie like the sun).  Technically, the actual spectrum is compared to a theoretical black body, whose spectrum is similar to the sun, but not identical...and which is beyond the scope of this discussion.

A transparent material (Iike glass) is not necessarily transparent to all wavelengths of radiation, and in fact glass does block more long wave infrared than visible light.  Low-E coatings placed on glass further reflect these long wave  radiation, limiting heat loss thru the glass.

Convection: when hot air rises off a pot of boiling water, or hot pavement, this is called convection.  Convection is the movement of a hot gas or a hot liquid toward a colder one, although the movement is actually due to gravity, not heat transfer.  What happens is that warm air (or water) is less dense than cold, so the cold sinks and the hot air rises.  In convection, heat only moves up because it is carried by the lighter air.  A pot of water is heated by convection more than it is heated by conduction. Convection is responsible for the common misconception that heat rises.

In the common forced hot air heating systems, a fan moves hot air to an outlet placed low on a wall, and convection causes that air to move upward to the rest of the room.  When the air coming into a room is significantly warmer than the room, it can float to the ceiling an collect there, causing great temperature stratification, especially with tall ceilings.  In this case, the air near the ceiling can be 20 degrees warmer than the air near the floor.

Convection is generally a very rapid way to move heat.

Specific heat:   energy absorbed by a material raises its temperature.  The specific heat  of a material is how much energy is required to raise the temperature by one degree., or more accurately, how much energy is required to raise a unit weight (eg a pound or kilogram) by one degree.  By the pound, metals store very little, wood and stone store at least twice as much, and air stores even more.  But because a cubic foot of air weights very little, while a cubic foot of stone or water is quite heavy, by unit volume, water stores the most, followed by stone, then metals, with air a distant last place.

For the purposes of thermal mass, we want materials with high absorption rates (so they gather the sun's energy), and high specific heat (so they also store it).

Evaporation although this is not a heat movement method (the heat actually moves by conduction and convection), the process of evaporation of water absorbs a large quantity of heat, and hence moves much more heat than normal conduction & convection of air would have.  This is why sweating is so effective, and the principle on which swamp coolers (or any evaporative cooler) is based on.

Properties of Materials

The following are some example properties of materials.  When a range is given its because the value varies with exactly what kind of material (ie hardwood .vs. softwood), and in the case of air or water, the R value is dependent on there being no convection.