What to use will depend on factors such as budget and what you have currently. Insulation is measured in R-values. The higher the R-value, the better, so the first step is to determine the type and thickness or R-value of the insulation you have now. The Dept. of Energy site www.ornl.gov/~roofs/zip/ziphome.html can help you determine what R-value you have and then what is recommended for your particular area and situation.

Insulation can be made from a variety of materials but it usually comes in four basic types:

Loose-fill insulation is loose fibers or fiber pellets usually made of rock wool, fiberglass, or cellulose and should be blown into spaces using pneumatic equip-ment.  It conforms well to building cavities and attics and has R-values ranging from R-2.2 - 3.5 per inch.

Rolls, batts or blankets are made from fibers, such as fiberglass and rock wool.  They are available in widths suited to standard spac

ings of wall studs and attic or floor joists and will provide an R-value of around 3.25 per inch.

Rigid foam insulation - foam insulation typically is more expensive than fiber insulation but is very effective where space limitations and higher R-values are needed.  Foam insulation R-values range from R-4 to R-6.5 per inch.

Foam-in-place insulation - this type can be blown into walls and attics and can reduce air leakage, if blown into cracks, such as around window and door frames.  It has R-values ranging from 4 - 6 depending on the type used.  While DIY kits are available for the foams, often professional installation is best.