In an unheated loft, a ventilation duct is insulated to reach a thermal resistance of at least 1.5 m².K/W, but what really sets the thickness is the dew point of the air being carried. Bathroom air at 22 °C and 85 % relative humidity condenses as soon as it meets 19.4 °C: in a loft at 5 °C, a bare duct condenses continuously through the whole heating season. The wet-room runs therefore need more than the WC run, and a single fitting left bare is enough to create the failure point. The same reasoning applies in reverse to the ducts of a ducted air-to-air heat pump installed in the loft, where it is the heated or cooled supply air that has to hold its temperature.
Why insulate every MVHR duct in an unheated loft
Limiting heat loss and stabilizing the ventilation airflow
In an unheated loft, the extracted air passes through a cold zone. Without insulation, the duct behaves like a small reverse radiator. It leaks heat and cools the air, which can shift the pressure balance and make the grilles less consistent. By insulating each duct, you keep a more stable temperature throughout the network, and the ventilation stays more predictable.
Avoiding condensation in the duct and damage in the loft
When humid air from the bathroom or kitchen cools too quickly, it can reach its dew point. The result: water appears inside the duct, then trickles down. This leads to stains on the ceiling, wet insulation, mould, and corrosion on the fittings. An insulated duct limits this cooling and greatly reduces the risk.
Improving comfort and air quality without extra energy use
The real calculation is an after-sales calculation. A poorly insulated duct does not announce itself at handover, it announces itself six months later as a stain on the ceiling, and the callback costs you loft access, strip-out, replacement of the soaked insulation and redecoration, for a line item worth a few tens of euros on the original quotation. It is the worst ratio in the whole ventilation package. Price duct insulation as a named line on the quotation, with the thickness and the R value stated, rather than as a fitting buried in a lump sum: a quotation that carries over the materials, labour and fittings from the works plan line by line shows it for what it is, and that is what lets you defend it against a cheaper competitor who has simply left it out.
Choosing the right insulation for the duct: performance, durability, and safety
Insulating sleeve, wool, shell: which solution for which duct
Three families cover most of the runs you meet in a loft.
| Solution | Duct type | What it brings |
|---|---|---|
| Elastomeric foam or PE sleeve | MVHR duct in a loft | Quick to install, limits condensation |
| Mineral wool with an aluminium facing | Rigid metal duct | Holds up well if you secure it with strapping and clamps |
| Rigid shell | Straight run | Stable installation and clean joints |
Thickness, thermal resistance, and vapour barrier: what to target
Don't choose at random. In an unheated volume, target a thermal resistance around R ≥ 1.5 m².K/W to reduce losses and avoid cold air inside the duct. Favour a continuous vapour barrier on the heated side, either built into the sleeve or added with an aluminium facing and suitable tape. Zero holes, or moisture will creep in.
The dew point sets the thickness, not the catalogue
The right thickness isn't a matter of feel: it follows from the dew point of the air you are moving. As long as the inner face of the duct stays above that temperature, nothing happens. The moment it drops below, water appears. Here are the figures for the extract air you actually meet in dwellings.
| Extract air | Temperature | Relative humidity | Dew point |
|---|---|---|---|
| Living room, bedroom, dry ambient air | 20 °C | 50 % | 9.3 °C |
| Occupied dwelling, winter | 20 °C | 60 % | 12.0 °C |
| Kitchen in use | 20 °C | 70 % | 14.4 °C |
| Bathroom, ordinary use | 22 °C | 70 % | 16.3 °C |
| Bathroom just after a shower | 22 °C | 85 % | 19.4 °C |
| Shower cubicle, direct extract | 24 °C | 90 % | 22.3 °C |
Read the last row, then think about your loft temperature in February. Bathroom air at 22 °C and 85 % condenses as soon as it meets 19.4 °C: in a loft at 5 °C, a bare duct condenses continuously through the whole heating season, not just on frosty days. That is why the wet-room runs deserve more attention than the WC run, and why one uninsulated fitting on three metres of perfectly insulated duct is enough to create the failure point.
France's Agence Qualité Construction lists ventilation among its twelve lessons on moisture in construction, with one finding that keeps coming back: the ductwork gets treated as dry plumbing when it is in fact carrying water. Check the airflow rates you actually achieve too, since an under-performing network cools its air further and condenses sooner: the instruments and the method are covered in our article on measuring ventilation airflow.
Fire reaction and compatibility: points to watch near the loft
Near the loft and at penetrations, check the Euroclass rating and compatibility with the duct, the tapes, and the hangers.
Carrying out clean, durable duct insulation
Preparing the job: mapping the ventilation network and loft access
Before installing the insulation, map out the entire ventilation network. Follow each duct from the unit to the grilles, note diameters and lengths, and identify any compressed or damp zones. In the loft, secure access, light the area well, and plan a walkway so you don't compress the existing insulation. Clear measurements and photos save you time.
Treating the junctions: elbows, branch fittings, connections, and collars
Losses and condensation often start at the junctions. Install the insulation continuously, with sufficient overlap, without compressing it. At elbows and branch fittings, cut cleanly and complete the coverage with add-on pieces to avoid thermal bridges. Tighten with suitable clamps and keep an even surface. Thermal continuity is the rule.
Ensuring airtightness and protecting the insulation in difficult zones
Close every seam with a compatible, durable tape, then protect the sections in cold zones (hatch, gable ends, roof frame) with mechanical protection to prevent tears and rodent damage. Around sensitive points, favour a vapour-barrier finish on the warm side and check for the absence of air leaks. Careful sealing means fewer losses.
Reducing nuisances and optimizing the ventilation network in the loft
Limiting noise: duct fixing, hangers, and vibrations
To quiet down an MVHR system in the loft, the priority is decoupling. Fix each duct with clamps on hangers, with no direct contact with the roof frame. Avoid rigid contact points, leave a slight amount of play, and plan a short flexible connection at the unit's outlet to cut vibrations.
Optimizing lengths and slopes: avoiding water pockets and pressure losses
Take the shortest route. Multiply the elbows and you multiply the pressure losses. Keep wide bend radii, maintain the correct diameter, and give a slight continuous slope toward the condensate drain to avoid water pockets. An insulated duct also limits condensation.
Getting the roof outlet and exhaust right: insulation-ventilation continuity
At the penetration, treat both air and water sealing. Install an insulated duct all the way to the outlet, avoid any break in insulation at the roof opening, and secure the cap to limit water entry and whistling. Check that the exhaust air can't be re-drawn back in. To go further, see also installing a dual-flow MVHR system.
Checks, maintenance, and compliance points in 2026
Checking proper operation: airflow, grilles, and balancing after insulation
After insulation work, the MVHR system can end up choked. Check the measured airflow at each grille, the condition of the dampers, and the balancing. A disconnected duct or a crushed branch fitting is enough to lose half the airflow. Adjust the grilles, check the tightness of the connections, and confirm the presence of air inlets.
Monitoring over time: condensation, duct crushing, rodents in the loft
In cold lofts, watch for condensation on the ducts. A poorly insulated duct, installed with no slope or with a sag, retains water. Also look for crushing, chafing against the roof frame, and signs of rodents. An annual visual check and cleaning the grilles keep the ventilation from failing.
Job-site traceability: photos, product data sheets, and recommendations for the client
In 2026, a subsidy can be audited after the work.
What stays in the file
- Dated photos before, during, after.
- Technical data sheets, references, diameters, duct insulation, and instructions.
- A simple sheet for the client: where the grilles are, how to clean them, when to have the unit checked, and who to call if odors come back.
