# Insulation R values and U values

The efficiency of any insulation material is measured by its R–Value or Thermal Resistance (R-2 or R2, for instance, means a thermal resistance of 2).
The greater the R-value of a product, the higher its insulating effectiveness. When dealing with contractors, always deal and think in R-values (not inches).
SI and non-SI Units
Some countries use the SI (International System of Units) units to express R-values, but many don’t. USA, UK and Canada don't use the SI units for thermal resistance, contrarily to countries like New Zealand or Australia...
Non SI units are expressed in a higher standard: the relation is approximately 1 (SI) to 5,67 (non SI).
Up R values or winter R-values define the resistance to heat flow upwards (heat escaping into outdoors through roofs or walls…).
Down R or summer R-values define the resistance to heat flow downwards (heat entering into the house through roofs or walls…).
An example: in a hot humid climate we should use insulation that prevents heat gains without restricting heat losses. Insulation should stop overheating.
U is the inverse of R.
http://en.wikipedia.org/wiki/R-value_%28insulation%29#U-value
The R-value is a measure of thermal resistance [1] used in the building and construction industry. Under uniform conditions it is the ratio of the temperature difference across an insulator and the heat flux (heat flow per unit area, \dot Q_A) through it or R = \Delta T/\dot Q_A. The bigger the number, the better the building insulation's effectiveness[2]. R-value is the reciprocal of U-value.
Around most of the world, R-values are given in SI units, typically square-metre kelvins per watt or m²·K/W (or equivalently to m²·°C/W). In the United States customary units, R-values are given in units of ft²·°F·h/Btu. It is particularly easy to confuse SI and US R-values, because R-values both in the US and elsewhere are often cited without their units, e.g. R-3.5. Usually, however, the correct units can be inferred from the context and from the magnitudes of the values.
Heat transfer through an insulating layer is analogous to electrical resistance. The heat flows can be worked out by thinking of resistance in series with a fixed potential, except the resistances are thermal resistances and the potential is the difference in temperature from one side of the material to the other. The resistance of each material to heat transfer depends on the specific thermal resistance [R-value]/[unit thickness], which is a property of the material (see table below) and the thickness of that layer. A thermal barrier that is composed of several layers will have several thermal resistors in the analogous circuit, each in series. Like resistance in electrical circuits, increasing the physical length of a resistive element (graphite, for example) increases the resistance linearly; double the thickness of a layer means half the heat flow and double the R-value; quadruple, quarters; etc. In practice, this linear relationship may be only approximate some materials[citation needed].
Increasing the thickness of an insulating layer increases the thermal resistance. For example, doubling the thickness of fibreglass batting will double its R-value, perhaps from 2.0 m²K/W for 110 mm of thickness, up to 4.0 m²K/W for 220 mm of thickness. Heat transfer through an insulating layer is analogous to adding resistance to a series circuit with a fixed voltage. However, this only holds approximately because the effective thermal conductivity of some insulating materials depends on thickness. The addition of materials to enclose the insulation such as sheetrock and siding provides additional but typically much smaller R-value.