Cavity Depth Guide: How Far Back Should the Panel Sit?

Acoustic wants depth

Sound reduction improves as the air gap grows, because a deeper gap decouples the two panes more completely and lowers the mass-air-mass resonance out of the range that matters. The gain is steep up to about 100 mm, still worthwhile to 150–200 mm, and largely academic beyond that. For any acoustic job we take the deepest cavity the reveal will physically allow.

• 50 mm: modest acoustic gain, good thermal
• 100 mm: the acoustic standard, 40–47 dB Rw typical
• 150–200 mm: best for low-frequency traffic and rail

Thermal wants restraint

For heat loss the opposite applies. Beyond roughly 20 mm the trapped air starts to convect within the cavity, carrying heat from the warm pane to the cold one, so thermal performance plateaus and then very slightly declines. A 50 mm cavity is close to optimal thermally; a 150 mm one is marginally worse thermally but vastly better acoustically.

How we resolve the conflict

We ask what the room is for. A front bedroom on a bus route gets depth. A north-facing study with no noise problem gets a shallow cavity and Low-E glass. Most houses end up with a mixed specification room by room, which costs no more than a uniform one.

What limits you physically

The reveal depth, the position of the architrave, any window seat or radiator below the sill, and on sash windows the projection of the staff bead. We measure all of these at survey before promising a cavity, because a depth that will not fit is not a specification.