Homestar Technical Clarification Rulings
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| Technical Question Title | Tool Version | Date Released | Credit Name | Tool Category | TC Link |
|---|---|---|---|---|---|
| TQ214 IT Wall STC and ASTC Difference | v4.1 | March 2024 | EHC9 – Sound Insulation | EHC | Link |
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Approved Ruling: NZGBC allows 5 dB difference provided the ASTC ( Performance measured in the field) is not more than 5 dB below the STC58 (Performance of a system in the laboratory). In your case, the specified IT wall system has an STC rating of 58, while the field test resulted in an ASTC rating of 53. A difference of 5 points between the specified rating and the field test result is not uncommon and may fall within acceptable tolerance levels in many cases. NZ Building Code specifies that walls and floor/ceiling assemblies must achieve a Sound Transmission Class rating of at least STC 55 in laboratory. A 5 point leeway is permitted on site, so that the minimum requirement on site is ASTC 50 (Field Sound Transmission Class). |
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