How to Insulate Finished Sloped Ceilings

Turn off the electricity to the room, switching the breaker to "Off" or twisting the fuse to loosen and remove. Remove any ceiling light fixtures or ceiling fans. Run extension cords from another room, if needed, to supply light or power for tools.

Carefully pry paneling or wood planking away from the ceiling carefully. Encourage it to come up by starting in a corner, lifting at the edges with a crowbar or the claw of a hammer, and working it slowly loose. Rigid materials as will generally come off in one piece, allowing you to re-install it after insulating. For drywall ceilings, start a hole in a corner and use a combination of cutting, pulling, prying and breaking the material away. Discard the covering or set it aside.

Measure the width between the ceiling joists or rafters and the length of each row. Generally, sloped ceilings have a center peak and a row will be from one wall up to the peak. The distance from the peak down to the opposite wall is another section. Given a ceiling that slopes in one direction only, the length will be from one wall to the opposite instead.

Unroll a section of fiberglass insulation, of the proper width, to the length needed. Run a straight edge across the roll and mark before cutting with a utility knife. Fiberglass insulation comes in widths suited to common framing widths for ceilings and walls. On the length, however, you want to allow a couple inches extra to ensure it completely fills the cavity involved.

Hold the insulation in place, between the joists or rafters, in the ceiling. Start it snug against one wall and stretch it out the full length of the ceiling row. Staple the paper-like facing to the wood framing as you go, spaced about 6 to 8 inches apart, ensuring that you do not leave any gaps from wrinkled and sloppy stretches.

Repeat for each row of insulation. Cut small pieces and stuff in any gaps surrounding the ceiling, such as around the wall plates. Avoid any stuffing of insulation, which will cause the material to compress and lower its R-value.

Measure the entire room, both length- and width-wise. Add a couple of inches to each dimension to allow adequate coverage. Cut polyethylene -- plastic sheeting -- to stretch across the length of ceiling. Typically more than one row is needed; in this case, cut the sheets wide enough to overlap a couple feet at each seam.

Stretch the plastic across the ceiling, beginning in one corner and ensuring the plastic is snug and not wrinkled, and allow gaps from the ceiling surface. Staple to the wood rafters or joists every 6 to 8 inches. Work in rows, covering any seams with tape. While this vapor barrier is not strictly necessary when using faced insulation, it provides superior vapor and moisture protection.

Replace the ceiling finishing material or install new material. Re-install any light or ceiling fixtures before turning the electricity back on.