Worked Example: Loft Conversion — BS 8233 Part E Compliance
80 m³ loft bedroom with sloped plasterboard ceiling, carpet, and Velux windows
A 80 m³ residential living designed to BS-8233 requires RT60 ≤ 0.5s. With Plasterboard on studs on the ceiling, the calculated RT60 is 0.52s — MARGINAL.
Room Setup
Total surface area: 131.6 m². Calculation method: Sabine equation (ISO 3382-2:2008 Annex A).
Surface Materials
| Surface | Material | Area (m²) | NRC |
|---|---|---|---|
| Ceiling — sloped plasterboard | Plasterboard on studs | 40.0 | 0.07 |
| Floor | Heavy carpet | 40.0 | 0.29 |
| Walls — plasterboard | Plasterboard on studs | 18.0 | 0.07 |
| Knee walls — plasterboard | Plasterboard on studs | 10.0 | 0.07 |
| Velux windows | Double glazing 6mm | 4.0 | 0.03 |
| Curtains | Curtains (medium weight) | 6.0 | 0.56 |
| Sofa bed + furnishings | Upholstered seating | 8.0 | 0.55 |
| Chimney breast — plaster | Painted plaster | 4.0 | 0.04 |
| Door | Solid timber door | 1.6 | 0.09 |
Absorption coefficients sourced from manufacturer datasheets and ISO 354 test reports. Browse the full acoustic materials database.
Step-by-Step Calculation
- 1
Room Volume
V = L × W × H = 8 × 5 × 2 = 80 m³
- 2
Total Surface Area
Stotal = 40.0 + 40.0 + 18.0 + 10.0 + 4.0 + 6.0 + 8.0 + 4.0 + 1.6 = 131.6 m²
- 3
Absorption per Surface at 500 Hz
Ceiling — sloped plasterboard: 40.0 m² × 0.05 = 2.00 Sabins
Floor: 40.0 m² × 0.14 = 5.60 Sabins
Walls — plasterboard: 18.0 m² × 0.05 = 0.90 Sabins
Knee walls — plasterboard: 10.0 m² × 0.05 = 0.50 Sabins
Velux windows: 4.0 m² × 0.04 = 0.16 Sabins
Curtains: 6.0 m² × 0.49 = 2.94 Sabins
Sofa bed + furnishings: 8.0 m² × 0.56 = 4.48 Sabins
Chimney breast — plaster: 4.0 m² × 0.04 = 0.16 Sabins
Door: 1.6 m² × 0.08 = 0.13 Sabins
- 4
Total Absorption at 500 Hz
Atotal = 2.00 + 5.60 + 0.90 + 0.50 + 0.16 + 2.94 + 4.48 + 0.16 + 0.13 = 16.87 Sabins
- 5
Sabine RT60 at 500 Hz
RT60 = 0.161 × V / A = 0.161 × 80 / 16.87 = 0.76s
- 6
Compare to Target
Mid-frequency RT60 (avg of 500, 1k, 2k Hz) = 0.52s. Target: 0.3--0.5s per BS-8233. Verdict: MARGINAL
Octave Band Results
| Frequency | 125Hz | 250Hz | 500Hz | 1kHz | 2kHz | 4kHz |
|---|---|---|---|---|---|---|
| Total Absorption (Sabins) | 23.3 | 14.5 | 16.9 | 27.8 | 38.3 | 40.9 |
| RT60 (s) | 0.55 | 0.89 | 0.76 | 0.46 | 0.34 | 0.32 |
| Target (s) | — | — | 0.50 | 0.50 | 0.50 | — |
| Verdict | — | — | FAIL | PASS | PASS | — |
Compliance Verdict
Reference:
Open the AcousPlan calculator pre-loaded with the exact room dimensions (8m × 5m × 2m) and RT60 target (0.5s).
Open CalculatorField Story
A family in Islington is converting the loft into a master bedroom to avoid moving house. The 8 m by 5 m room with average 2 m ceiling height (sloping from ridge to knee walls) must meet Approved Document E requirements. The sloped plasterboard ceiling on independent joists provides sound insulation and significant low-frequency panel absorption across its 40 m² area. Combined with wall and knee-wall plasterboard, the room has 72 m² of panel absorber — unusually high for residential. Heavy carpet on the new floor build-up damps mid and high frequencies. A sofa bed and curtains provide additional mid-frequency absorption. Velux roof windows are acoustically similar to double glazing. BS 8233 recommends RT60 below 0.5 s for bedrooms; the extensive plasterboard achieves this comfortably.
Frequently Asked Questions
What RT60 does a Loft Conversion — BS 8233 Part E Compliance achieve?
With the specified materials and dimensions (8m x 5m x 2m, volume 80m3), the calculated mid-frequency RT60 is 0.52s using the Sabine equation. The target under BS-8233 is 0.3--0.5s. The result is a MARGINAL.
What materials are used in this residential acoustic example?
This example uses Plasterboard on studs on the Ceiling — sloped plasterboard (NRC 0.07), Heavy carpet on the Floor (NRC 0.29), Plasterboard on studs on the Walls — plasterboard (NRC 0.07), Plasterboard on studs on the Knee walls — plasterboard (NRC 0.07), Double glazing 6mm on the Velux windows (NRC 0.03), Curtains (medium weight) on the Curtains (NRC 0.56), Upholstered seating on the Sofa bed + furnishings (NRC 0.55), Painted plaster on the Chimney breast — plaster (NRC 0.04), Solid timber door on the Door (NRC 0.09). The total absorption at 500 Hz is 16.9 Sabins.
How much does acoustic treatment cost for this room?
Cost data is not available for this example. Use the AcousPlan calculator to estimate treatment costs for your specific room configuration.
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All calculations are advisory and based on the Sabine equation (ISO 3382-2:2008 Annex A). Results require professional verification for compliance submissions. Absorption coefficients are sourced from manufacturer datasheets and ISO 354 test reports.