In an unfinished attic, the goal of insulation is to keep the rooms below cool in summer and warm in winter.

That means insulating the attic floor only—not the walls—and having vents in the roof. If the attic is finished, meaning that at least part of it is living space, then you also want to keep the attic warm in winter. So a finished attic should be insulated much like the rest of the house, with insulation in the walls and ceiling.

If your attic is already finished, it probably is at least partially insulated. However, most older attics are under-insulated so it’s a good idea to raise the R-value where possible. Unfortunately, you will not be able to get into all the openings. The ceiling will probably be closed off to you unless you remove the drywall or other finish material. You may have better luck getting behind sidewalls (also called knee walls).

There are two basic ways to insulate a finished attic. If only the living space will be insulated, wrap the insulation around the room’s walls and ceiling and then continue along the floor of the non-living space. In this case, the area behind the knee wall will be uncomfortably hot or cold. If you want to keep the area behind the sidewalls from freezing in winter, apply insulation from the roofline all the way down to the floor.

A roof should be able to “breathe,” meaning that air should blow from the eaves below to the roof above. Such ventilation keeps the roof cold in winter, which prevents ice dams that can damage your roof and even your house.

To provide ventilation in a roof that is insulated, install special rafter vents. These are stapled to the underside of the roof sheathing, between rafters. They allow air to move behind the insulation so the roof stays nice and cold while the room inside is insulated against the cold. Make sure the eave vents at the bottom of the rafter and the ridge vent or other vents at the top are free of obstructions so air can move freely.

Illustratons copyright Sunset Publishing Corporation