How to Add a Transfer Grille to a Closed Bedroom

Why this matters

A bedroom with a supply register and no return gets air pushed in and has nowhere to send it except the crack under the door. Close the door and the room pressurizes, the supply delivers less, and the room runs warm in summer and cold in winter. The pressure also pushes room air out through every leak in the exterior wall and ceiling, while the hallway and the rest of the house are pulled the other way. An offset transfer grille gives that air a path back to the hall through a stud bay without passing light or much sound.

Whether a grille is the right fix, rather than an undercut or a jump duct, is decided in Transfer Grilles, Jump Ducts and Door Undercuts. This card assumes that choice is made and covers the work: where the grilles go, what has to be true of the wall, how big, and how to prove it worked.

The path you are building

   BEDROOM SIDE          STUD BAY          HALL SIDE
                     +-------------+
  ceiling            |  top plate  |
  ===================+=============+==================
  [GRILLE A, high]-> |      |      |
   about 12 in below |      |      |
   the ceiling       |      v      |
                     |  air drops  |
                     |  down the   |
                     |  lined bay  |
                     |      |      |
                     |      v      | ->[GRILLE B, low]
                     |             |  about 8 in above
  ===================+=============+==================
  floor              | bottom plate|   the floor
                     +-------------+
  Bay 14-1/2 in wide (2x4 at 16 in o.c.), lined
  on all four inside faces, sealed at both plates

The grilles are offset vertically so the light from the hall and most sound have to turn two corners to reach the bedroom. The bay is the duct, so its cross section, about 14-1/2 by 3-1/2 inches, or roughly 50 square inches, is the most one bay can pass however large the grilles are.

Steps

1. Confirm the wall is not a fire separation, and is not shared with a garage. A wall between the house and an attached garage is a required fire separation under IRC R302.6 as adopted, and a wall between two dwelling units in a duplex or townhouse is a rated assembly. A grille through either breaks the protection the code requires, and no grille is installed there. In multifamily buildings, a wall between a unit and a corridor is often rated too. If you cannot tell, the building's plans or the local building department can, and the job waits for the answer. The bedroom side and hall side should also both be ordinary living space, not a bathroom, kitchen, laundry or a room with a combustion appliance, because the path moves air both ways when doors and fans change, and humid or flue-gas-bearing air does not belong in a bedroom.

2. Find what is already in the wall. Use a stud finder to locate studs and any wire it can detect, then check from above and below: in the attic, look for cables or pipes dropping through the top plate over that bay; in the basement, for anything coming up through the bottom plate. Look for outlets and switches on either face of the wall within that bay, because a box means a cable runs through it. A small inspection hole, an inch across, cut in the area the high grille will cover anyway, lets you look in with a light. Skip this and the saw finds the cable, at which point you have an energized conductor in a fresh cut. Before any cut in a bay that shows a cable, turn off and lock the breaker for that circuit, then prove it dead at the nearest device with a tester you have just checked on a known live source.

3. Size the opening to the room's supply. A common starting point is about 1 square inch of net free area for each cfm of supply air delivered to the room, which aims to hold the closed room within about 3 pascals of the hallway. Net free area is the open area of the grille after its louvers and frame, published by the manufacturer, and it is often well under the grille's face size. Use the room's measured or design supply cfm, not a guess from the register size. If one bay cannot carry the area, use two adjacent bays with a grille pair in each. The reference card's door-crack method refines this when a measurement is available.

4. Cut the two openings, offset. Mark grille A on the bedroom side high in the bay, about a foot below the ceiling, and grille B on the hall side low, about 8 inches above the floor, both centered between the same two studs. Cut to the grille manufacturer's rough-opening size, with a shallow blade depth set to the drywall thickness. Cutting drywall and joint compound releases fine dust, and compound in older houses can contain crystalline silica and, in some older products, asbestos; in a house built before 1978, painted surfaces may contain lead. Use a dust-collecting saw or a hand saw, a respirator matched to the dust under your shop's written program per 29 CFR 1910.134 (which 29 CFR 1926.103 applies to construction work), plastic below the cut, and HEPA cleanup. Where pre-1978 paint is being disturbed in a home, the EPA's Renovation, Repair and Painting rule, 40 CFR 745, may apply to your firm; read its scope and minor-repair limits rather than assuming you are under them. If the compound is suspected to contain asbestos, stop and have it tested before cutting.

5. Line the bay. Insulation in the bay comes out of the section between the grilles, and the bay is lined on its inside faces with sheet metal or drywall and sealed at both plates. An unlined bay pulls air from the rest of the wall cavity, which can connect to the attic, the floor system and every outlet in that wall, and a transfer path that draws from the attic is worse than no path. Lining also keeps the air off raw framing and insulation fibers. Your jurisdiction may have rules about using framing cavities for air; a lined, sealed bay used only for transfer between two rooms is the common approach, and the local inspector settles the question.

6. Fit and seal the grilles. Mount each grille into its opening and caulk or gasket its frame to the drywall so the grille is the only path. A grille that leaks around its frame into an unlined part of the wall reintroduces the problem step 5 solved. If you locked out a breaker in step 2, remove your lock and turn it back on only now, with the wall closed, then confirm the devices on that circuit are live with your tester and that the breaker holds without tripping.

7. Retest the room pressure, door closed, blower on high. With every interior door except this one open, measure the bedroom against the hallway with a manometer, tube under the door. Record it. Then compare with your starting reading. How to Check a Closed Room for Pressurization covers the measurement.

Worked example: a 90 cfm back bedroom

The house: a two-story with the air handler in the basement, and a back bedroom with one supply and no return. The owner says the room is the warmest in the house every summer night. The shared wall with the upstairs hall is a 2x4 interior partition at 16 inches on center, the attic is above, and the hall side of the wall has no outlets.

Starting reading. With the blower on high and the door shut, the bedroom reads 7 pascals above the hall. The existing undercut is about 3/4 inch on a 30 inch door, roughly 22 square inches, already doing what it can.

Sizing. The HVAC contractor's balancing report shows 90 cfm to this room. At 1 square inch per cfm, the target is 90 square inches of net free area. One bay at about 50 square inches carries only about 56 percent of that. Two adjacent bays carry about 101 square inches of cross section, which clears 90. Each grille pair must then publish at least 45 square inches of net free area per grille, or the grilles become the restriction. The chosen grille publishes 48 square inches per grille, so two pairs give 96, above the 90 target and below the 101 the bays allow.

Wall check. From the attic, no cables enter the top plate over the two chosen bays; a cable does drop over the third bay, which is avoided. The house was built in 1994, so pre-1978 lead paint is not in play. The wall is not shared with a garage or another unit, and both sides are bedroom and hall.

After the work. Door closed, blower on high, the bedroom now reads 2 pascals above the hall, inside the 3 pascal target. The owner's next report is the one that matters: after a week of nights, the room is within a degree or two of the hall thermostat.

What would change it. A bedroom with 160 cfm of supply would need about 160 square inches, more than even three bays carry (about 152), which pushes the job to four bays or to a jump duct through the attic. A bedroom whose only shared wall is with a bathroom has no acceptable wall path at all, and the answer moves to a jump duct or a ducted return.

How this goes wrong

Sizing to the grille face instead of its free area. A pair of grilles that look generous on the wall can publish half their face size as free area. Read the published number.

Lining nothing. The grille works on the first test and fails by winter, because the bay is drawing from the attic through a gap at the top plate. The bedroom gets dusty and the drywall above the grille shows a gray stain where filtered attic air has been passing.

A grille later taped over. The hall light shows through an offset grille less than through a straight one, but some still shows. Tell the owner why the grille is there and that closing it brings the problem back; write it on the invoice.

How to know it worked

The closed-door reading is at or below the 3 pascal target with the blower on high. Both grilles are sealed at their frames and no light shows through the bay from a phone flashlight held at grille B with the hall dark. The invoice records the wall checks (not a garage wall, not a rated wall, no cables in the chosen bays), the measured before-and-after pressures, and the grilles' published free area. If the reading is above target, add bay area rather than guessing: at the same airflow, the pressure varies roughly inversely with the square of the total free area, so doubling the area cuts the reading to about a quarter, as the reference card explains.

References

• See related: Transfer Grilles, Jump Ducts and Door Undercuts; Return Air Starvation and the Closed Bedroom Door
• See related: How to Check a Closed Room for Pressurization
• IRC R302.6, dwelling and garage separation, as adopted by your jurisdiction
• EPA Renovation, Repair and Painting rule, 40 CFR 745, for pre-1978 housing
• OSHA 29 CFR 1910.134, respiratory protection; 29 CFR 1926.103 applies it to construction work