MVHR duct airtightness: reducing air losses

Understanding air losses on an MVHR system and their impact on the job

Where air is lost most often: connections, penetrations, take-offs and units

On an MVHR system, air losses mainly come from the joints. Loosely tightened sleeves, worn sealing tape, crushed flexible ducts. Untreated wall penetrations, poorly fitted take-offs, and units with a poorly sealed cover or cable gland let pressure escape.

Concrete consequences: airflow not held, noise, discomfort and overconsumption

When the network leaks, the airflow at the vent falls short. The fan compensates, so there's more noise and sometimes vibration. The result is moisture that lingers, odours that stagnate, and heating overconsumption because the fresh air isn't properly controlled.

Spotting the signs on site: leaks, condensation, dust and imbalances

On site, listen for whistling and draughts at the connections. Also look for condensation, dust trails, or drips in the loft. A flow-meter check at the vents quickly reveals a network imbalance between extraction and air inlets.

Choosing the right ducts and fittings for lasting airtightness

Flexible or rigid ducts: advantages, limits and uses by room

For an MVHR system, rigid duct (PVC or metal) is the simplest to make airtight along its length. It limits pressure drop and keeps its diameter. Flexible duct is mainly used for short connections, when you need to work around an obstacle. In lofts, make sure to suspend it and avoid crushing. In wet rooms, favour short, accessible runs for maintenance.

Fittings, sleeves, clips, seals: aiming for compatibility and long-term performance

Airtightness is often decided at the joints. Choose parts from the same range and the same diameter, with suitable seals.

• A sleeve with a seal or lip, rather than a dry push-fit.
• A correctly sized clip, without deforming the duct.
• A compatible sealant or sealing tape, for long-term performance.

Materials and airtightness classes: what to require in the quote

In the quote, ask for the material, thickness, fire reaction, and the airtightness class of ducts and fittings. Have the installation method specified (seals, sealant, tapes) and the sensitive points treated (take-offs, penetrations, connections to the unit). This avoids air leaks, noise, and "phantom" airflow. To go further on ductwork in lofts, see insulating ventilation ducts in unheated lofts.

Installation method: the steps that make MVHR ducts airtight

Preparing the substrates: clean cuts, deburring, dusting and alignment

Before connecting an MVHR duct, work as you would in plumbing. A clean square cut, deburring with a knife, then dusting off. Dry-fit the parts to check alignment. A sleeve fitted crooked is a guaranteed leak, even with the best tape.

Sealing correctly: tapes, sealants and seals (and mistakes to avoid)

On rigid networks, favour ducting tape (aluminium or butyl) or a suitable sealant. On fittings with a seal, check the seal's condition before assembly and push it fully home. Avoid "multi-purpose" cloth tape and wrapping tape onto a damp substrate. Aim for continuity with no gap at the joints.

Securing the fixings: clips, hangers, bend radius and crush points

Fix without straining. Regular clips and hangers to avoid sagging, especially in lofts. Respect a wide bend radius on flexible ducts, with no tight elbows or crushing where they pass through the framing. A pinched duct increases pressure drop and creates noise. Aim for the simplest line.

Checking and proving the result: verifications and key points in 2026

Self-check on site: airtightness, continuity and accessibility checklist

Before closing up ceilings and boxing, do a self-check of the MVHR system. Hunt for air leaks at sleeves and take-offs, crushed ducts, and unfilled penetrations. Nothing hidden should be left in doubt. Also keep simple access for maintenance.

• Connections taped and clips tightened, with no play.
• Network continuity, insulation in cold zones if planned.
• Fixings, slopes, and condensate drainage if needed.
• Access to the unit, filters, and vents clearly marked.

Measuring and balancing airflow: settings, vents and unit

Measure the airflow at each vent with a suitable flow meter or anemometer. Compare against the sizing, then adjust the vents and dampers without forcing the unit. Goal: stable airflow, good coverage of the wet rooms, and an acceptable noise level. Finish by checking the air inlets.

Documenting for the client and 2026 grants: photos, datasheets and commissioning report

For the client, and for MaPrimeRénov' or CEE in 2026, the proof needs to be simple. Dated photos of the unit, the network, and the sealed-up penetrations. Datasheets and instructions. A commissioning report with the model, serial number, settings, and measured airflow rates. A clean file means a smoother payment.

Common cases in renovation: gaining airtightness without redoing everything

Replacing duct sections: how to treat the joints with the existing network

When you replace a section of MVHR duct, the sensitive point is the joint. Keep the same diameter, cut cleanly, dust off the old duct, then connect with a suitable sleeve. Finish with sealing tape all the way round. A well-tightened joint is often worth more than a new duct badly connected.

Treating lofts and crawl spaces: condensation, insulation and penetrations

In cold lofts or crawl spaces, condensation shows up quickly if the extracted air cools down inside the duct. Favour an insulated duct, avoid low points, and limit lengths. Take care with penetrations (hatch, floor, party walls) using a flexible seal, without crushing the insulation, otherwise you create leaks and cold bridges.

Quick fixes: repairing leaks around vents, the unit and take-offs

For quick wins, target the most common leaks.

• Around the vents, refit a support foam and seal the connection to the finish.
• At the unit, check the clips, clamps, and connections. Even tightening is often enough.
• At the take-offs, redo the sleeves and tapes that lift over time.