Attic insulation under current building codes call for 6 1/2 inches of blanket insulation in the ceiling, giving you an R-value of 19. R-value stands for the ability of a material to impede the transfer of heat. The higher the number, the greater the ability to prevent heat or cold loss.
I feel twelve inches is the minimum for a house I plan to live in for more than four years. In terms of energy savings, it is the best bang for the buck. To add six inches to a 1,000 square foot house might cost about $300 if you do it yourself. If you use air-conditioning, you could save this investment in as few as two to three years. Not only will you save energy costs, your home will be much more comfortable.
Typically, the preferred insulation is fiberglass in a roll or batten style. If there is some existing insulation, use unfaced insulation. Only use faced insulation (with paper or foil) in new construction when the facing can be against the heated area. The facing acts as a vapor barrier, something you do not want between layers of insulation in the attic.
Blown in insulation, also known as cellulose (a chopped up, fireproofed paper), is the choice if there are many obstructions in the attic making using blanket type insulation too difficult to install.
One thing to prevent is putting so much insulation at the outside edges of the attic so it touches the roof boards. If the insulation touches the roof, it can melt the snow in the winter and create an ice dam.
An ice dam forms when snow on the roof melts and re-freezes at the gutter. This frozen dam does not let melting snow run off the roof, it traps it. Water is forced up under the shingles and rots the roof-boards, rafters and the fascia. Now you may know why the gutter keeps falling off.
Always keep the insulation at least an inch from the roof. The air gap will let air from the overhang vent into the attic. This fends off humidity and keeps the roof boards the same temperature as the outside air. This air gap allows any moisture that does get into the area evaporate away.
Less rot, less mold, less damage and a warmer or cooler home all for a one-time investment.