The type of insulation you use will be determined by the nature of the spaces in the house that you plan to insulate. For example, since you cannot conveniently "pour" insulation into an overhead space, blankets, spray or board products, or reflective systems are used between the joists of an unfinished basement ceiling. The most economical way to fill closed cavities in finished walls is with blown-in insulation applied with pneumatic equipment or with foamed-in-place polyurethane foam. Table 1 provides a concise summary of the appropriate applications for the various types of thermal insulation.
It is important to know that the different forms of insulation can be used together. For example, you can add batt or roll insulation over loose-fill insulation, or vice-versa. Usually, material of higher density (weight per unit volume) should not be placed on top of lower density insulation that is easily compressed. Doing so will reduce the thickness of the material underneath and thereby lower its R-value.
In cold climates, some low-density loose-fill insulation allows air to circulate between the top of your ceiling and the attic. This air circulation can decrease the effective thermal resistance of the insulation and may be significant for regions with more than 5000 heating degree days, or north of a line running from New York to Pittsburgh to St. Louis to Topeka to Santa Fe to Reno and up to Portland, Oregon. You can eliminate this air circulation by covering the loose-fill insulation with a blanket insulation product or with a higher density loose-fill insulation.
Courtesy of the DOE