Unfinished attics in almost all cases used Blown-in fiberglass.  Unlike batts or rolls Blown-In Insulation is a loose material that does the same and even a better jobs than batts and rolls.  It is a fast affordable, energy saving option.  Before this happens, however, it is critical to air- seal the entire attic comprehensively.  The Blown In material will not work to its best capacity if air is circulating through the material.  The best way to stop air circulation is to seal gaps between the attic and rooms below.   The blower takes bales of packed, dense fiberglass that is fed to a hopper which shreds and fluffs the fiberglass like a pillow. Then the machine lows it throughout he attic through a hose. The expert installer ensures even and complete coverage of the entire attic. In cold regions such as New England, the U.S Department of Energy recommend R-49 at a minimum.  A lot of houses fall under this desired level. Therefore it is a good idea to have the levels checked by a professional. The process of blown-in fiberglass can be done in one day or less. One complete savings will be evident almost immediately. Your costs can be reduced by up to 40%, which is well worth the cost in the long run, saving on both fuel and electricity