How to Vent Through a Ceiling to the Second Floor

Joist Space

• The dimensions of the vent depend upon the spacing of the floor joists. Nearly all floor joists sit on 16-inch centers, meaning from the distance from the center of one joist to the center of the joist beside it should be 16 inches. Allowing for the width of the joist leaves you with a space between joists of 14 1/2 inches in which to install the vent. Vents up to 14 inches will fit between the joists.

Grille vs. Register

• On the ceiling of the floor through which the vent runs you may install a flat grille to cover the vent, but on the floor of the room above, you’ll need a register. The grille is lightweight and probably has no moveable louvers to damper airflow. The floor register is heavier because it must bear the weight of people walking or standing on it, and most registers have adjustable louvers that open and close.

Grille Installation and Blocking

• A stud finder helps locate the ceiling joists from below before cutting the drywall between the joists with a utility knife and a straight edge. To ensure that warm air flows through the vent into the room above and not into the joist space, blocking is necessary. Blocking consists of boards that are the same dimension as the joists, on either side of the vent hole. For a standard floor made with 2-by-10 joists, you would need two, 2-by-10 boards, cut 14 1/2 inches long. The boards are attached to the adjacent joists with nails or screws.

Register Installation

• After installing the blocking, the simplest way to determine where to cut out the floor above for a register with the same dimensions, is by drilling four holes, from below, through the four corners of the vent space. From upstairs, you’ll locate the holes and cut through the floor with a reciprocating saw. The floor register will slip into the hole and you’ll secure it with screws to the joists.