Architects, contractors and builders use various "factors" to express the insulation value of a material or a composite structure: U, C and R. Material suppliers often speak of products having a particular K factor.
The purpose of this document is to explain what these factors mean, how they differ from each other, and how they relate to each other.

K, Factor

K Factor is a rate at which heat flows through a material. Values for insulation are normally based on one-inch thickness of one homogeneous material, and are expressed in BTU/ft2/F/hr./inch.
The lower the K factor, the better the insulation value of the material. Some examples are:

These values are shown as a range because the K Factor does change with age. For example, urethane foam exhibits a K of .11 or .12 when new. In two to six months, the K increases to .15 or .16, then remains stable.

C, Factor

C Factor is also a rate of heat through a material, but could be for any given thickness, not just at 1 inch. C Factor at 1 inch would be equal to the K factor. C factor of the same material at three inches thickness is 1/3 of the K factor; at two inches, it is 1/2 the K factor.

U, Factor

U factor is the overall coefficient of heat transfer (conductivity) for all the elements of construction, as well as the environmental factors. An example would be the U factor of a composite structure such as qypsum wallboard, fiberglass core and exterior wood sheathing. The U factor should not be used with a single material, only with combinations.
A U factor is determined by adding the C Factors of the various individual materials making up a composite structure. U=C1+C2+C3, etc. If the materials are not in close contact, the C factor of an air space must be included in the calculation.  Units are expressed as BTU/ft2/f/hr. The smaller the U factor, the better the insulation value of the composite structure.

R, Factor

R factor is a measure of the resistance to heat flow. R can be determined for a single material at a specific thickness. As the thickness of the insulation increases, the resistance to heat flow increases.

R can be determined in TWO ways:
1.) R = one divided by the C factor of the material.
2.) R = the thickness divided by the K factor.
Example:
With rigid urethane foam at 3-inch thickness R = 3 / .15 = 20
At two inches thickness  R = 2 / .15 = 13
The higher the R, the better the insulation.

Approximate R values are presented for common insulation materials:

From this table, it can be seen that rigid urethane foam is 60% better than fiberglass and 350% better than Vermiculite when all are the same thickness.
K factors  or C factors  can be obtained from material suppliers, lumber yards, and Engineering handbooks.