Soundproofing a ceiling: clips, channel, insulation and double layers

Two kinds of noise

A ceiling under a living space hears two things. Airborne sound, such as voices and television, is rated by STC, the same measure used for walls. Impact sound, such as footsteps, dropped objects and chairs scraping, travels through the structure itself and is rated by IIC, impact insulation class. The International Building Code requires both STC 50 and IIC 50 (45 if field tested) for floor-ceiling assemblies between dwelling units. Inside a single home there is no requirement; a basement ceiling under the kitchen or a bedroom under a playroom is a matter of choice.

Impact noise is the harder one. A footstep shakes the joists, and a ceiling screwed to the joists shakes with them. The answer is to hang the ceiling so it does not touch the joists directly.

Decoupling: clips and channel

The most effective method in homes is sound isolation clips with hat channel: rubber-isolated clips screwed to the joists, with 7/8 in steel hat (furring) channel snapped into them, and the drywall screwed only to the channel. The ceiling hangs on rubber, not on the joists.

The clip makers’ guidance, which the site’s calculator follows:

• Clips up to 48 in apart along the channel.
• Channel rows 24 in apart for one or two layers of board, 16 in for three.
• First and last rows within 6 in of the walls.
• Channel spliced with a 6 in overlap, between clips.
• A 1/4 in gap between the ceiling and the walls, filled with acoustic sealant.

Resilient channel is a cheaper alternative, screwed directly across the joists. It works when installed exactly to its instructions and fails easily when a screw finds a joist; the guide to soundproofing a wall explains why.

Insulation in the joist bays

Insulation between the joists absorbs sound in the cavity, and with a decoupled ceiling it adds much more than in a ceiling screwed straight to the joists. Fill the bays without compressing; full-depth batts help most.

Two layers, and damping

Two layers of 5/8 in board on the channel add mass; a damping compound such as Green Glue between them adds more. Two layers of 5/8 in board weigh about 4.4 lb per sq ft, which is why the load on each clip matters. The second layer’s joints are staggered from the first. The ceiling takes the same order of work as any other, ceiling first and walls after, covered in the guide to hanging ceilings first; with two layers of 5/8 in board overhead, a panel lift is close to essential.

Mass-loaded vinyl

Mass-loaded vinyl (MLV) is a dense, limp sheet, typically 1 lb per sq ft, hung between the joists or between layers. It adds mass where space is tight. On a ceiling it is heavy work: a 4 × 25 ft roll weighs about 100 lb.

Holes in the ceiling

A decoupled ceiling is only as good as its penetrations:

• Recessed lights are holes. Use fixtures rated for the purpose with covers, or surface-mounted fixtures.
• Electrical boxes get acoustic putty pads.
• Ducts and registers carry sound between floors; line the ducts or add bends.
• The perimeter gap is sealed with acoustic sealant, not taped or caulked hard.

Fixing impact noise from above

Most of the improvement in impact noise comes from the floor above: a thick carpet and pad does more for footsteps than anything under the joists. Where the floor above cannot change, the decoupled ceiling is the main tool. Check the ceiling’s own thickness and framing limits in the drywall thickness guide and with the clip maker’s instructions.

Common mistakes

• Screws through the channel into a joist. It shorts the decoupling.
• Ceiling board touching the walls. Leave and seal the gap.
• Clips overloaded. Check the weight per clip with three layers.
• Open recessed lights. They leak.
• Expecting a ceiling alone to stop footsteps. The floor above matters most.