Temperature is a force. It is the wind. It is a wind which passes through solid objects at different rates or speeds, depending on the physical properties of the material it is passing thru. You can measure this rate by walking barefoot from carpeting to hardwood to tile flooring. The temperatures will all be the same, but will seem very different. The tile has the highest rate and will seem the coldest, while the carpeting has the lowest rate and will seem the warmest. The "wind" speed is also determined by difference in temperature. The greater the difference, the higher the speed. A temperature difference of 40 degrees is twice as fast as a temperature difference of 20 degrees.

Insulation is resistance. It resists the pressure of the temperature's wind. It slows the wind down. R-value is the measurement of a material's ability to slow the temperature's wind. The higher the r-value, the slower the wind speed thru that substance. It takes the wind twice as long to penetrate a R20 as an R10.

A house has what is known as an insulation "envelope". It is the total combined "wrapper" that a house presents to the outside climate. They are not necessarily outside surfaces, but are all of the temperature surfaces between the inside and the outside, and include doors, windows, and wall and ceiling insulation.

There are basically two types of insulation. Rigid insulation comes in sheets, and is installed below a slab or on surfaces. Cavity insulation comes loose, in batts, or as a mixture which can be pumped into the empty spots created by the frame's structure. Whatever the type used, the term R-value always refers to the same resistance to temperature penetration. 

Because the rates and speeds of temperature differ from one geographic area to another, the requirements for home insulation will also vary. The correct amount of insulation for a house is dictated by the severity of the outdoor temperatures that it will be subjected to. Most new homes are calculated as a package, with all insulating factors mixed in together. The r-values of the doors, the windows, and the rigid or cavity insulation is blended into a final composite for the house. This gives the house an overall value which must meet the standards for it's specific area.

The most recent Energy Codes include the efficiency of the heating and cooling system as a factor when designing the insulation envelope. The higher the efficiency of the equipment, the less r-value required of the other components. A home with a furnace that is 95% efficient can have less attic insulation, or a lower grade of insulated window than a home with an 80% furnace. This places conservation on a sliding scale, where the efficiency of the comfort system can substitute for the r-value of the envelope. A homeowner/builder has the option to spend more on the windows, doors, or insulation; or on the heating equipment itself. But codes only dictate minimum standards, and there is no penalty to improving upon these terms. It is therefore to one's benefit to use higher efficiencies in conjunction with higher r-values. One thing the codes overlook, however; is the actual physical location of the heating and/or cooling equipment. A furnace or air handler installed outside of the insulation envelope, (above the attic insulation, or below the floor insulation) can have it's efficiency reduced by up to 25%. A 12 SEER air conditioner operating in a 140 degree attic can be producing at a 10 SEER level. It then becomes the responsibility of the architect or contractor to spend an extra effort on the equipment location to derive the most benefit from the conservation envelope.