Why Do Homes Have Twice the Amount of Insulation in the Roof Than the Walls?

Attic Insulation

• Attics, the space between the ceiling and the roof, have the most open air space and the most places where hot air can escape. Since heat rises, they are the areas which need the most insulation protection. Insulation normally is installed in an attic in the areas between the roof joists, which form the ceiling underneath. The most common type of attic insulation is loose fill cellulose, which fills spaces between joists and around the edges of the attic.

R Values

• Insulation is rated by R value, its resistance to heat; the higher the number, the greater the heat resistance. Exterior walls generally are insulated with material, usually fiberglass batts, with an R value of 13 to 21. Attic insulation usually should have R values of 30 to 38, to effectively block heat from below from escaping through the roof. Original attic or roof insulation also needs a moisture barrier on the house side to prevent moisture from accumulating.

Finished Attics

• A finished attic, one with a finished room inside, needs insulation under the roof, between the rafters or truss top chords, and on top of the finished ceiling in that room. If the room abuts an unfinished attic area, the wall between those two areas also needs insulation. These areas must be insulated to prevent warm air from escaping, through the wall to the unfinished attic or through the roof above the room's ceiling.

Adding Insulation

• Older houses frequently lack proper attic insulation or have loose fill insulation that over the years has compacted or blown away to reduce its effectiveness. A common solution to this problem is to lay fiberglass batts across the roof joists. In that case, batts must be unfaced, so they do not add a second moisture barrier which could wind up trapping moisture inside the insulation. Adding attic insulation is the easiest way to improve heating and energy efficiency.