Whether you need to insulate your roof rafters will depend on what you intend to do in the space directly beneath the rafters. For example, if the space beneath the rafters is going to be used for a living area, then you will need to insulate the roof rafters. Although there are many different circumstances that may call for insulating roof rafters, the means to determine the necessity for insulating rafters remains the same. By following a few simple steps for determining need and application, you can be sure that the right decision has been made.
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If your attic is going to be converted into livable space, then you will need to insulate the roof rafters. Install ventilation baffles into the space between the rafters before you install the insulation--usually by stapling into place. The baffles will allow circulation to continue even though insulation has been installed directly in the path of the rafter air movement; air circulates from the lower part of the roof overhang, through the rafters, and out the ridge vent of your roof.
Once the baffles have been installed, you will have two viable options: fiberglass roll insulation or sprayed cellulose insulation. The fiberglass insulation comes in rolls that are 16 to 24 inches wide. The rolls have a number of 8-foot pieces rolled onto one roll. The insulation should be installed with the paper facing out and attached using staples. The paper will fold over the rafter framing, thereby allowing for attachment with staples.
Blown cellulose insulation will require the installation of netting over the rafter pockets. This netting serves to hold the blown insulation--usually recycled newspaper--inside the rafter pockets until such time that ceiling materials can be installed over the insulation. You can rent a blowing machine from the same place that supplies the insulation.
If your attic is usable space but not lived in, then you won't need to insulate the roof rafters as long as the ceiling/floor that separates the roof from your living area below is insulated. Home ceilings are usually insulated in order to seal off the extreme temperatures in your attic. These temperatures may change dramatically by several degrees over the seasons because your rafters are not insulated, which is fine as long as you are not using the attic as livable space. Storing items in an attic that has uninsulated roof rafters is also OK.