Insulation, as practically everyone knows, has an R-value or Resistance-value. This is a measure of a material's resistance to heat flow. The higher the R-value of a material, the more it will slow the transfer of heat into or out of the house. Thus a material rated R-9 will only slow the heat flow half as much as a material rated R-18.
R-value is a function both of the kind of material and the thickness used. For example, Fiberglas insulation has an R-value of 3 for each inch of material (this isn't a hard number, other factors also enter in). Therefore, three inches of R-3 Fiberglas would provide an R-value of 9. Cellulose (recycled paper, chemically treated and in the form of small pellets) when densely packed has an R-value of 3.8 per inch, so 3 inches of that material would provide an R-value of 11.4.
R-values are established in a laboratory under specific conditions but these are not necessarily real world conditions and other factors will impact the effectiveness of insulation in this house or that one. Houle Insulation, Inc. on its website explains:
Scientists and engineers refer to building systems that separate the interior of a structure from the ambient environment as the "building thermal envelope." Many factors affect the energy efficiency of the thermal envelope. These include:
- Total R-Value of all system components
- Air infiltration due to leakage through gaps in the system
- Air infiltration due to permeability of system elements
- Convective flows within insulated systems
- Thermal bridging across the building envelope
- Thermal mass of building components
This would lead to technical discussions of thermal bridging and other engineering factors and we aren't going there. This kind of detail can be found at www.Houleinsulation.com and http://apps1.eere.energy.gov/consumer (insulation and air sealing.) The latter site also contains links to several insulation calculators.
Insulation comes in several forms. Batts are those pink rolls of Fiberglas which are sized to fit between wall studs and attic or floor joists. According to the Department of Energy, a 2x4 wall can accommodate R-13 or R-15 batts; 2x6 walls will hold R-19 to R-21 products. Batts can also be made of rock wool or other mineral fibers.
Loose-fill insulation is sold in the form of pellets or loose fibers and is blown into voids in walls and attics using special pneumatic equipment. Usually made of Fiberglas, rock wool, or cellulose, it is good for places where access is limited.
Rigid foam insulation is effective where space is limited but high R-values are needed. R-values range from R-4 to R-6.5 per inch, about twice that of other materials, but it is also more expensive and is difficult to use in a retrofit
Foam-in-place insulation comes in several forms including in pressurized cans for small D-I-Y jobs. It is blown into place where it expands to fill the void. Foam sprays are also being used in new construction to fill entire walls as it is less labor and time intensive than using batts and finds its way into tiny crevices like no other material.