Insulation Performance: R-value

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).

Unless stated otherwise, we will use the non-SI values.

R-values depend on...

R-values vary with the insulating material: modern spray-on foams have far greater R-values per inch than traditional insulating materials like fiberglass, cellulose and mineral wool.

See, for details:

Spray-on foam insulation
Cellulose insulation
Fiberglass and Mineral Wool insulation

R-values may also depend also on...

- the number of layers of the insulating material to be installed
- density, in the case of loose-fill materials.
- the effectiveness of the installation.

A Proper Installation

An improperly installed insulation (e. g. a compressed batt insulation) doesn't provide the same R-value of a properly installed one. In the case of batts, compression and indentation may easily create air pockets which will reduce the insulation R-Value. That's just an example: things vary with the type of material and installation.

Extra-layers of insulation

Since the R-value per inch also depends on the thickness of the insulation material, installing an extra layer of insulation in your home will increase its R-value: the R-value of an installation with various layers of insulation is the sum of the R-values of the several layers of insulation.

Which R-value to apply

The R-value that should be applied in insulations depends on...

- particular climatic conditions
- particular parts of the house (ceilings, floor, walls...)

The demanded R-value may also depend (in a smaller scale) on the materials with which they interact: cement, tile, joists, studs... The insulation of a wall and the insulation of a ceiling may differ because the R-value also depends, in the last case, on factors as the heat that may flow through the joists and studs...

Un example

The table below shows some official recommended R-values for homes in the Pacific Northwest (a cold climate):

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 in colder climates

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

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 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. So, insulation materials should have a high Down r-value and a Low up r-value.

Examples of the R-Value of some insulating materials

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 on Insulation Materials and their uses:

Insulation materials
Spray-on foam insulation
Fiberglass, mineral wool & Home insulation