Insulation materials are not all equal at preventing heat loss and unwanted heat gains. Their thermal performance varies and is measured by the R-value (thermal resistance) or by the U-value (the reciprocal of the R-value) factors.
"R" stands for thermal performance. The thermal performance of specific materials per inch of thickness is measured by its R-value: standard fiberglass batts may have an Imperial R-value of 3.4, while the thermal performance of blown cellulose is R-3.2 to R-3.6.
The recommended insulation for a specific building assembly (a wall, a ceiling, a floor...) is also expressed in terms of R-value or U-value.
For instance: in cold climates, wall insulation should be R-30 to R-40 (U-value, Metric system: U-0.19 and U-0.14), which requires about 9.5 inches (24 cm) of fiberglass, 7.5 inches (19 cm) of expanded polystyrene, 8 inches (20 cm) of low-density polyurethane or 4.5 inches (12 cm) of polyso.
R-Value, U-Value, Imperial US System and Metric System
R-value is the reciprocal of U-value or U-factor (the Heat Transfer coefficient). A high U-value means a high overall heat transfer. Hence: the lower the U-value the better (similarly, the higher the R-value the better).
United States R-values are given in Imperial units (ft2 ºF hr/Btu), but R-values can also be expressed in Metric units. Most other countries use Metric units (SI units: m2 °C/W).
These two standards are a source of confusion. It’s easy confuse the US R-values with the SI R-values.
United States R-values are 5.682 times higher than SI R-values.
R-Value table Insulation materials
The table at right shows the approximate R-values of the most important insulation materials in both US Imperial units per inch and Metric SI Units per 50 mm of thickness.
They exact values vary with formulations and manufacturers.
R-value of fiberglass: between R-2.5 and R-4.2 (US Imperial system)
The R-value of fiberglass varies between 2.5 and 4.2 (US Imperial system), depending on their density and formulation.
Loose fill fiberglass has a lower R-value (about R-2.5) than fiberglass batts (R-3.4).
Dense pack fiberglass boards have a higher R: about 4, to a maximum of 4.2.
Compressing fiberglass (during its manufacturing phase) to a specific density increases its R-value. Fiberglass reaches a higher R-value at about 3.2 lb/ft3. Below or above this value, the R-value per inch decreases.
Note that compressing a fiberglass batt or blanket during installation doesn't increase its R. On the contrary. Compression, voids, gaps, splits or indentation during the installation, reduces its R-value.
Fiberglass is permeable to water vapor (though not to water), which also degrade its R-value. Wind and air convection, or air from inside or outside the building (carrying water vapor), can degrade the R-value of fiberglass up to 50%.
R-value of cellulose Insulation: between R-2.8 and R-3.6
Loose-fill cellulose has an R-value between R-3.2 and R-3.6, while the R-values of dense-pack cellulose and spray-on cellulose are between R-2.8 and R-3.4.
The R-value of cellulose insulation is greatly affected by water vapor and water (cellulose can absorb up to 130% water by weight, and is very permeable to water vapor).
Blown-cellulose (the typical type of cellulose) is not much affected by the issues mentioned above for fiberglass, but its R-value is highly affected by water or density variations.
R-value of mineral Wool: between R-2.6 and R-4.4
Rigid mineral wool boards have an R-value between R-4 and R-4.4 - more than the relatively uncommon loose-fill formulations (R-2.6 to R-3.4). See: Fiberglass vs. Mineral wool.
R-value of rigid Polystyrene : between R-3.6 and R-5
White rigid board expanded polystyrene has a R-value between R-3.6 and R-4.2, according to its density. It absorbs little water (4%) and comes in a variety of vapor permeances. Its R-value is affected by exposure to UV, or to high temperatures (above 165ºF).
Rigid board extruded polystyrene has an higher R-value than expanded polystyrene: about R-5. It's R-value is not significantly affected by water or water vapor, but as with expanded polystyrene, it should be protected from the sun's ultraviolet radiation or extreme temperatures.
R-value of polyurethane Sprays: between R-3.5 and R-7.0
Low-density polyurethane sprays have an R-value between R-3.5 and 3.8., which decreases with exposure to the sun's ultraviolet radiation and moisture.
High-density polyurethane sprays have a R-value of about R-6.0-7.0, which decreases slighty over time. These sprays share the same features of low-density polyurethanes. Polyurethane sprays are not affected by moisture or water vapor when properly coated with a sealant.
R-value of Polysocyanurate: between R-5.6 and R-7.2
Polysocyanurate rigid boards have the highest R-value per inch of any common insulation material: up to R-7.2. Their R-value decreases over time, though not significantly.
The R-value is not affected by air leakage (they are not permeable) or water absorption. They should be protected from fire.
R-value of Vermiculite and Perlite (Loose-fill): About R-2.1
Vermiculite and perlite have a low R-value per inch and are expensive and outdated materials.
Meeting A High R-Value with low R-Value insulation materials?
The mere fact that an insulation material has a higher R-value is not a reason to prefer it - especially if there is enough space to install the insulation.
That's the case of open attics: you don't have to use high-R-value materials to get high levels of attic insulation. As long as you have enough space, you may add as many inches of insulation as you want.
Whole R-values of walls, floors and Ceilings do not coincide with the R-value of the Insulation Materials
It’s important to distinguish the R-value of the insulation materials in a wall (or floor or attic) from the effective R-value of the wall as an assembly (the whole R-value). R-values are reduced by about 30% in two-by-four and two-by-six frame construction; the whole R-value of two-by-six wood frame walls, with 5 ½ fiberglass insulation, may amount to about R-12, while the R-value of the insulation materials in them is R-17.5 (5 ½ inches x 3). The reason: thermal bridging.
The effective R-value of an assembly can be higher or lower than the R-value of the insulation materials in them. Typically, it is lower, due to thermal bridging: heat transfer through the framing and other elements in the walls, floors or ceilings (image from Dow).
Note: thermal bridging can be minimized by installing some inches of foam or mineral wool on the outside or on the inside of external walls (see: Thermal Bridging).