Definitions:
R-value is the numerical reciprocal of U-value.

That is:
R = 1/U and U = 1/R

For Example:
If the U-value is 0.1, then the R-value is 10 (R=1/0.1)

If the R-value is 12, then the U-value is 0.83 (U=1/12)

One Btu is the amount of energy required to heat one pound of water

from 58.5 to 59.5 degrees Farenheit.

This is roughly the equivalent to the energy in one kitchen match.
(Of course the metric system u-value is different.

To convert, multiply our standard u-value by 5.6783)

R-value is a numerical expression of a material's resistance to heat transfer. It is the numerical reciprocal of U-value and to understand these terms, it is necessary to first explain the U-value.
The U-value of a material is the number of British Thermal Units (Btus) that transfer through a 1 square foot area in one hour when the temperature difference between the two sides of the structure is 1 degree Farenheit. The smaller the U-value, the better the insulation properties of the material.
We can use U-values to help calculate a building's energy loss. U-values can be used in multiplication and division. Complex computer programs assist the professional energy rater in this process but the fundamentals are simple.
An example:

The reason that we use two different values, U and R, to express the same thing is that they allow us to perform different types of calculations. As we just saw, U values can be used in multiplication and division problems. But U-values cannot be added or subtracted, R-values can!

The insulation value of a window is always expressed as a U-value.
Insulation materials are usually given an R-value. Manufacturers of traditional insulation materials have done an excellent job of marketing “"R-value" as the only number to care about when choosing insulation. In fact, they have been so effective that their efforts are mirrored in building codes throughout the country.
But there is much more to the efficiency of an insulation system than its claimed R-value. In fact, official research has shown that up to fifty percent of the energy loss in a typical building has nothing to do with “"R-value".
Let us investigate a little further.

Foam insulation addresses all of these elements of concern. But for now, let us examine the six factors in a little more detail.

Conduction
Conduction is the transfer of heat within a material and R-value is a numeric expression of a given material's resistance to this transfer. The higher the R-value, the greater the resistance to heat transfer. To be affected by conductive heat from one to another, materials must touch. Think of a steak on a barbeque grill, the sear marks are created by conductive heat from the steel grate that the meat is sitting on.

Convection
Convection is basically currents of gasses (air) or liquids. Convection currents can transfer heat from one object to another and can occur within closed spaces. As air is heated, it rises so if one surface of a wall cavity is warmer than the other, a natural convective current will transport heat from one surface to the other until the temperatures are equalized. Convective heat transfer happens without materials touching one another. It is (primarily) the convective heat rising from the gas flames on the grill that heats the grate.

Radiation
Radiative heat transfer happens through electro-magnetic waves as one material releases energy (heat) to warm another. If materials are the same temperature, there is no radiation. Close the grill and on a cold day you will feel the radiation heat.

Air Infiltration
Most of us have experienced air infiltration first hand. Bring your hand close to an electrical receptacle box on an outside wall on a windy day and chances are that you will feel the draft long before you touch the wall. Clearly, infiltration must be considered when evaluating insulation. (Ex-filtration from living spaces into attics is a major source of energy loss and attic problems).

Air Intrusion
Gaps and cracks in the sheathing of a building allow the wind to penetrate into the wall cavity. If the inside drywall is glued to the studs and is without openings, there will be no infiltration into the living area. But, the intrusion of air into the cavity creates currents that transfer heat.

Moisture
Air infiltration and air intrusion account for almost all of the moisture that penetrates into an undamaged wall system. It has been determined that during a normal heating season, as much as 30 quarts of water can be collected in a wall through a 1 square inch hole in a 4’" x 8’" area of drywall. In contrast, diffusion would generate only 1/3 of a quart of accumulation. Water is an excellent conductor of heat so the wetter insulation becomes, the less effective it is.

Conclusions
Up to fifty percent of the energy loss in a building is caused by factors not influenced by the level of r-value.  Consequently, an effective insulation material must deal with all of the six factors that affect heat transfer. Spray foam insulation is that material:

• The R-value of foam is as high or higher than traditional types of insulation and it controls conductive heat loss.
• Foam insulation does not allow airflow within itself so it blocks convective currents.
• The cells in foam are tiny so there is very little temperature difference from one cell wall to the next - without temperature difference, there can be no radiant heat loss.
• Foam completely fills and seals any opening in the wall sheathing. There can be no air infiltration or intrusion.
• Air infiltration accounts for the vast majority of moisture in a wall system - without air infiltration, no moisture problem.

There is more to an effective insulation system than piling on "R"s. 

But can there be such a thing as too much R-value?
The short answer is no. But the more thoughtful answer is that the appropriate level of R-value depends on where you are building and on the level of return that you are seeking on your investment in insulation.

As we know from the discussion of the relationship between U- and R-value, we must double the R-value to cut conductive heat loss in half.  This is clearly a worthwhile thing to do when we first add insulation but there is a rapid drop-off in return on investment as insulation thickness increases. At R=12 we eliminate about 92% of the conductive energy loss. To reach 96%, i.e. cut the remaining loss in half, we must double the R-value to 24. Is a savings of 4% worth the increased cost associated with doubling the thickness of your insulation? The answer depends on your climate and the cost of energy but chances are that you are better off spending your money on a system that both air seals and provides r-value.

Spray foam insulation is all we do and we do it WELL!