Acoustic Design in Passivhaus Buildings — The MVHR Noise Problem Nobody Tells You About

Passivhaus buildings achieve external-to-internal noise reduction of 40-50 dB — roughly 15-20 dB better than conventional UK construction — by virtue of their triple-glazed windows, heavily insulated walls (often 300-400 mm of insulation), and airtight construction below 0.6 air changes per hour at 50 Pa. For occupants, this means external noise from roads, railways, and aircraft is dramatically reduced. A busy suburban road generating 65 dB LAeq at the facade produces only 15-25 dBA inside a Passivhaus bedroom — a level most people would describe as silent.

And then the MVHR system switches to boost mode and produces 35-45 dBA at the bedroom supply grille, drowning out the silence that the $50,000 building envelope premium was supposed to deliver.

This is the central acoustic irony of Passivhaus design: the building envelope is acoustically excellent, but the mechanical ventilation system required to make it habitable introduces noise sources that conventional naturally ventilated buildings do not have. Every Passivhaus project must address this challenge, and the evidence from post-occupancy evaluations suggests that many do not address it adequately.

Why Passivhaus Requires MVHR

The Passivhaus standard (PHI, Darmstadt) requires airtightness below 0.6 ACH at 50 Pa — approximately 10 times more airtight than a conventional UK dwelling (which typically achieves 5-8 ACH at 50 Pa). At this level of airtightness, natural infiltration provides insufficient fresh air for healthy indoor air quality. Opening windows for ventilation defeats the thermal performance of the envelope (each air change per hour of uncontrolled ventilation costs approximately 10-15 W/m² of heating load in a UK climate).

Mechanical Ventilation with Heat Recovery (MVHR) solves this by:

1. Extracting stale air from wet rooms (kitchens, bathrooms, utility rooms)
2. Passing extracted air through a heat exchanger to recover 85-95% of the thermal energy
3. Supplying pre-heated fresh air to habitable rooms (bedrooms, living rooms)

MVHR Noise Sources

The MVHR system generates noise through four mechanisms, each of which affects RT60 and background noise levels that determine STI and overall acoustic comfort:

1. Unit Noise (Fan and Motor)

The MVHR unit contains two fans (supply and extract) driven by EC motors. Noise generation depends on fan speed, fan type, and motor quality. Premium units (Zehnder ComfoAir Q, Paul Novus, Swegon CASA) achieve 25-30 dBA at minimum speed and 40-45 dBA at boost. Budget units can produce 35-40 dBA at minimum speed and 50+ dBA at boost.

The unit should be located in a utility room, plant cupboard, or loft space — never in a habitable room. Even with the door closed, a unit generating 45 dBA needs approximately STC 30 between the plant space and the nearest bedroom to achieve NR 25 (the target for sleeping areas).

2. Duct-Borne Noise

Fan noise propagates through the ductwork to the supply and extract terminals in each room. The noise level at the terminal depends on:

• Source noise level (at the unit's duct connection)
• Duct length and diameter (longer ducts = more attenuation; larger ducts = less flow noise)
• Number of bends (each 90° bend provides approximately 3-6 dB of broadband attenuation)
• Duct material (galvanized steel has higher breakout noise than insulated flexible duct)
• Terminal type (diffuser design affects high-frequency regenerated noise)

3. Flow-Generated Noise

Air moving through ducts, fittings, and terminals generates noise proportional to the velocity. Flow noise increases approximately 50-60 dB per decade of velocity (i.e., doubling velocity increases noise by 15-18 dB). The critical design parameter is air velocity in the ductwork:

Most MVHR installations use 100 mm or 125 mm diameter circular ducts. At the typical Passivhaus ventilation rate of 30 l/s per person (approximately 100-150 l/s for a 4-bedroom house), air velocities in 100 mm ducts can reach 4-5 m/s — too high for bedroom supply ducts. Using 125 mm or 150 mm ducts reduces velocity to 2.5-3.5 m/s, but at the cost of greater installation complexity and space requirements.

4. Crosstalk Between Rooms

Acoustic crosstalk is the transmission of sound from one room to another through the shared ductwork. This is the most insidious MVHR noise problem because it is not generated by the MVHR system itself — it is speech, music, or other room noise that enters the duct through a supply or extract terminal and emerges in another room.

Consider the acoustic path: speech in the living room (65 dBA at the extract grille) enters the extract duct, travels through the ductwork to the MVHR unit, and exits through the supply duct into a bedroom. Without attenuators, the attenuation through this path may be only 15-25 dB (depending on duct length and number of bends), meaning the speech arrives in the bedroom at 40-50 dBA — fully intelligible and highly intrusive at night.

The partition wall between the living room and bedroom might provide STC 45-55 of airborne sound insulation. If the duct crosstalk path provides only STC 20-25, it completely undermines the wall performance. The weakest acoustic path determines the effective insulation, regardless of how well the walls and floors are constructed.

MVHR Noise: Manufacturer Data Comparison

The price-noise correlation is striking: premium units (Zehnder, Paul, Swegon) at £3,000-4,000 are 6-8 dBA quieter at minimum speed than budget units at £1,200-1,800. In acoustic terms, 6 dB represents a halving of perceived loudness. The premium for a quieter unit (£1,500-2,500) is a fraction of the cost of acoustic remediation after installation.

Worked Example: 4-Bedroom Passivhaus

Building: 4-bedroom detached house, 150 m² gross internal area, Passivhaus certified MVHR: Zehnder ComfoAir Q350, installed in first-floor plant cupboard Ventilation rate: 140 l/s at boost (30 l/s per bedroom + 20 l/s kitchen + 20 l/s bathroom)

Duct Layout

The MVHR unit connects to:

• 4 × bedroom supply ducts (100 mm diameter, 3-6 m duct runs)
• 1 × living room supply duct (125 mm diameter, 5 m duct run)
• 1 × kitchen extract duct (125 mm diameter, 4 m duct run)
• 2 × bathroom extract ducts (100 mm diameter, 2-4 m duct runs)

Noise Calculation for Master Bedroom

Source noise at unit supply connection: 38 dBA at boost (manufacturer data for Zehnder Q350)

Duct attenuation:

• 5 m of 100 mm rigid steel duct: 5 × 1.0 dB/m = 5 dB
• 2 × 90° bends: 2 × 4 dB = 8 dB
• In-line rectangular attenuator (600 mm long, 100 mm): 18 dB at 250 Hz, 25 dB at 500 Hz, 22 dB at 1000 Hz (approximately 20 dB broadband)
• Terminal/diffuser end reflection: 5 dB at mid-frequencies

Noise at bedroom supply terminal: 38 - 38 = 0 dBA at boost (below the threshold of hearing)

At minimum speed (source 28 dBA): 28 - 38 = below threshold — effectively inaudible.

Without the attenuator: 38 - 18 = 20 dBA at boost, 28 - 18 = 10 dBA at minimum. Still quiet, but the attenuator provides essential margin for:

• Future fan degradation (bearings wear, increasing noise by 3-6 dB over 10-15 years)
• Filter loading (dirty filters increase pressure drop, increasing fan speed and noise)
• Crosstalk prevention (the attenuator also reduces speech transmission between rooms)

Crosstalk Calculation

Scenario: Conversation in living room (60 dBA at supply grille), path to master bedroom through ducts.

Path: Living room supply grille → 5 m supply duct → MVHR unit → 5 m supply duct → bedroom supply grille

Attenuation through path:

• Grille entry loss: 6 dB
• Living room supply duct (5 m + 2 bends): 5 + 8 = 13 dB
• MVHR unit (through heat exchanger): 12-18 dB (measured, varies by unit)
• Attenuator (living room branch): 20 dB
• Bedroom supply duct (5 m + 2 bends): 13 dB
• Attenuator (bedroom branch): 20 dB
• Grille exit loss: 3 dB

Speech level at bedroom: 60 - 90 = below threshold — inaudible. The two attenuators are doing most of the work (40 dB combined). Without attenuators, crosstalk attenuation drops to 50 dB, and speech arrives at 10 dBA — still quiet but detectable in the extremely quiet Passivhaus bedroom environment.

Key Design Rules for MVHR Acoustics

1. Select a quiet MVHR unit: The unit is the source of all mechanical noise. A 6 dB difference at the source means 6 dB less noise at every terminal. Spend the premium on a quiet unit rather than on attenuators to compensate for a noisy one.

1. Size ducts for low velocity: Target < 2.5 m/s in bedroom branches, < 3.5 m/s in living areas. Use 125 mm or 150 mm ducts where space permits, rather than forcing high velocities through 100 mm ducts.

1. Install in-line attenuators on every branch: One attenuator per supply and extract branch, located within 1 m of the MVHR unit. Budget £40-80 per attenuator — £400-800 total for a 4-bedroom house.

1. Locate the MVHR unit away from bedrooms: Plant cupboard, utility room, or loft space with STC 35+ between the unit and the nearest bedroom. Never install in a bedroom wardrobe or above a bed.

1. Commission acoustically: Measure noise levels at supply and extract terminals with the system running at normal and boost speeds. Target NR 25 in bedrooms, NR 30 in living rooms. If targets are not met, check: filter condition, duct connections (air leaks generate turbulence noise), fan speed settings, and attenuator installation quality.

1. Maintain the system: MVHR filters should be changed every 6-12 months. Clogged filters increase pressure drop, forcing higher fan speeds and more noise. A simple maintenance regime prevents the system from getting progressively louder over its 15-20 year lifespan.

Further Reading

• How Climate Change Is Making Acoustic Design Harder — the broader context of MVHR in climate-adapted buildings
• Acoustic Design in Net Zero Buildings — where thermal and acoustic performance conflict and align
• Building Acoustics vs Room Acoustics — understanding airborne and structure-borne transmission