Insulation value: R-value, u-value, UP and Down r-value, Si and Non-SI

R–Value stands for Thermal Resistance (R-2 or R2 means a thermal resistance of 2) and measures the insulation value or effectiveness of a material. The higher the R-value of a product the higher its insulating effectiveness.

When dealing with insulation contractors, deal in insulation value (R-values, not inches).

See: U-values, SI and Non-SI R-values, Up and down R-values

R-value depends on ..

The final insulation value (R-value) of a wall, a attic, a room, a house... depends on the type of materials, their thickness and number of layers, the quality of the instalation and also on issues like thermal bridging.

Required R-values

Different rooms and different climates require different R-values. Features like building orientation to the sun and design options, or the type of cooling and heating system have also a huge impact on the required insulation values.

R-value and Types of Materials..

R-values vary with the type of material, its thickness, density and installation. Here are some insulation values:

R-Values Per Inch of Some Typical Insulating Materials
Mineral Wool Batts	3.5
Fiberglass Batts	3.1 - 3.5
Rock Wool (Loose Fill)	3.0 - 3.3
Cellulose (Loose Fill)	3.2 - 3.8
Fiberglass (Loose)	2.2 - 2.7
Perlite (Loose)	2.3 - 2.7
Polystyrene Boards (Rigid)	3.6 - 5.3
Polyurethane Boards (Rigid)	5.6 - 7.7

See, for more details:
Insulation materials
Cellulose insulation
Fiberglass and Mineral Wool insulation
Fiberglass, cellulose and rockwool loose-fill insulation
Sprayed-on fibrous insulation
Sprayed foam insulation
Blown-in cavity wall insulation
Rigid foam insulation
Reflective Insulation
Insulation Materials & Environment
Insulation best

Thickness and number layers

When calculating the R-value of several layers of insulation, the overall R-value is the sum of the insulation value of the several layers.

the Quality of the installation

But the insulation value of a given installation depends also on its quality. If the insulation batts are compressed they will not provide their full rated insulation value. Similarly, if the sprayed-on foam has voids or is fluffed, it will not provide the rated insulation.

R-value and thermal bridging

The overall R-value of a wall or ceiling - or any part of the house - is not equivalent to the insulation value of the materials involved, since some heat and coolness can easily flow around through the studs, joists or other structures. Only covering the home's envelope with a continuous layer of insulation we can achieve the maximum R-value.

R-value, climate and heating and Cooling Systems

Insulation is not useful in some hot-humid tropical climates. But that is the only big exception (See: Insulation and Cooling).

The broader rule is that colder climates require higher levels of insulation, and that it is advantageous - even in temperate climates - to apply lots of insulation, often more than that required by building codes.

R-value and Rooms and parts of the house

Different parts of the house require different insulation values due to issues like thermal bridging (heat escaping through studs, ducts and other structures) or to the fact that heat rises (the heat loss in our homes is greatest through the attic and ceiling)...

The table below shows official recommended R-values for different parts of the house, in a cold climate (Pacific Northwest):

Ceilings: R-38
Walls: R-11
Basements, garages and other crawl spaces under floor: R-19
Crawl Space Perimeter: R-10 (exterior) and R-19 (interior)
Ducts: R-11
Pipes: R-3 or more

For more details on R-values in different climate zones, see: Insulation and climate zones.

SI and non-SI Units

Some countries use the SI (International System of Units) system to express R-values, but most don’t. USA, UK and Canada don't use the SI units, contrary to countries like New-Zealand or Australia...

The SI/Non-SI units relation is approximately 1 (SI) to 5,67 (non SI).

Unless stated otherwise we will use the American standards, that is, non-SI units.

U-values

Sometimes the quality of an insulation material is expressed in U-values. The U-Value is the reciprocal of the R-value (U-value = 1/R-value).

For example: a fiberglass batt with a R-value of 5 has a U-value of 0,2 (1/5). The lower the U-value of an insulation product, the better.

Up R-values and Down R-values

In roofs, ceilings and floors, when using reflective insulation materials, it is usual to consider two types of R-values: Up r-values and Down r-values.

Up-R-values or Winter R-values define the resistance to upward heat flow (heat escaping to the outside through roofs or walls…).

Down-R-values or Summer R-values define the resistance to downward heat flow (heat entering into the house through roofs or walls…).

Note:

The difference between Summer and Winter R-values for common insulation materials (fiberglass, rock-wool, plastic foams) is generally marginal; they only are significant for reflective insulation. Summer and Winter R-Values are mainly applicable to reflective insulation (aluminum foils used in ceilings and roofs).

That means that common insulation isn't recommended in some hot-tropical climates. Only reflective insulation, with its limited role, largely restricted to roof cooling strategies.