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NZ Building Code · Fire & AcousticSTC acoustic ratings, explained for builders
STC is the single number that says how well a wall or floor blocks airborne sound — and NZBC G6 sets the legal minimum between dwellings.
Sound Transmission Class (STC) rates how well a wall or floor blocks airborne sound — voices, TV — where a higher number means quieter. It matters because NZBC clause G6 Airborne & Impact Sound sets a legal minimum between abutting household units (and certain other spaces), so if you’re building terraces or apartments you have to hit it.
What the code requires
STC is a single number that comes from a lab test. Between abutting household units, G6 sets the minimum at airborne STC ≥ 55, plus for floors an impact IIC ≥ 55.
Lab STC vs the field number
STC comes from an ideal lab test. The same wall built on site always performs a bit worse because of flanking — sound sneaking around via the floor, ceiling, junctions and penetrations.
- G6/VM1 can be demonstrated by field measurement (often quoted as DnT,w or R’w).
- Rule of thumb: spec a system rated several points above 55 so the as-built result still passes.
What the numbers feel like
- STC 30–35: loud speech audible — a normal internal stud wall.
- STC 40: loud speech a murmur; loud TV audible.
- STC 50: loud sounds faint; normal speech inaudible.
- STC 55: code minimum between dwellings — most speech gone.
- STC 60–67: most sound inaudible — GIB intertenancy systems land here.
What boosts STC
- Mass — extra plasterboard layers or heavier board reflect more sound.
- Decoupling — separate or staggered studs, resilient channels or isolation clips stop vibration crossing the wall. This is the biggest single gain.
- Absorption — sound-rated insulation (e.g. Pink Batts Silencer or denser wool) in the cavity.
- Sealing — acoustic sealant at floor, ceiling and junctions; any air gap is a sound gap.
Don’t put back-to-back power points or recessed fittings in a rated wall. GIB Intertenancy systems achieve STC 61–67 — comfortably above the G6 minimum, which is why they’re specced for terraces and apartments.
Plain-English guide, not advice. This page helps you understand and navigate the rules — it is general information, not design, engineering or consent advice, and it does not reproduce the copyrighted tables of NZS 3604 or any Standard. Always check the current Standard or Acceptable Solution and your BCA, and use a suitably qualified LBP, engineer or QS where it matters.
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Common questions
What is the minimum STC between dwellings in New Zealand?
NZBC clause G6 Airborne & Impact Sound sets the minimum between abutting household units at airborne STC ≥ 55. For floors there is also an impact requirement of IIC ≥ 55.
Why does a wall perform worse on site than its lab STC?
STC comes from an ideal lab test. The same wall built on site always performs a bit worse because of flanking — sound sneaking around via the floor, ceiling, junctions and penetrations. That’s why the rule of thumb is to spec a system rated several points above 55 so the as-built result still passes.
What has the biggest effect on a wall’s STC?
Decoupling is the biggest single gain — separate or staggered studs, resilient channels or isolation clips stop vibration crossing the wall. Mass (extra plasterboard or heavier board), absorption (sound-rated insulation in the cavity) and sealing all help too.
What STC do GIB Intertenancy systems reach?
GIB Intertenancy systems achieve STC 61–67, comfortably above the G6 minimum of 55, which is why they’re specced for terraces and apartments.
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