Vapour barriers stop condensation from damaging the insulation when the interior's warm air meets the exterior's cold air. Damp insulation not only loses its effectiveness, it also allows mould and mildew to grow. Some manufacturers sell insulation with the vapour barrier attached. The attached barrier simply gets stapled to the ceiling joists. Install 6-mm polythene plastic sheeting as a separate vapour barrier if your insulation does not come with an attached barrier.
Check that your light fixtures have a vapour barrier box mounted behind them. If not, take the time to remove each light fixture and install boxes around them. If you do not, warm air will find its way out of these gaps.
Unroll the polythene sheeting across the floor, parallel to the ceiling joists. Use a scissors to cut the sheeting to the room's length.
Lift the sheeting's corner up to the room's corner and use construction staples to secure it to the wall's top plate -- the horizontal board running over the wall studs. Move down the length of the polythene sheeting and place a staple in the top plate, behind each wall stud. Remove all the wrinkles and folds as you go along.
Move 6 to 8 inches down and lift the polythene sheeting up to the ceiling joists. Remove all the wrinkles and staple the sheeting in place. Continue all the way down the sheeting's width, stapling it to each ceiling joist.
Move another 6 to 8 inches down the polythene sheeting's length. Lift the sheeting up to the ceiling joists, smooth it and staple it in place. Continue in this manner until you install the plastic sheeting from wall to wall.
Lift a second polythene sheeting up to the ceiling. Overlap this sheeting with the first sheeting by about 2 to 3 inches. Staple this sheeting to the ceiling joists. Continue until you install the vapour barrier over the entire ceiling.
Cut holes around the light fixtures with a utility knife. Reach inside and pull out the vapour barrier box and smooth it over the polythene sheeting. Run strips of seaming tape along all four sides to make it air tight. Run strips of seaming tape along the seam of the overlapping polythene sheeting.