In an unheated loft, NF DTU 68.3 requires a ventilation duct to have a thermal resistance of at least 0.6 m².K/W (about 25 mm of wool), and 1.2 m².K/W (about 50 mm) on the supply and extract runs of a heat-recovery system. That is a floor: 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.
Ventilation duct insulation: which ducts to insulate in extract-only and MVHR systems
Short answer: in an unheated loft, every duct gets insulated, though not for the same reason. In an extract-only system, condensation of the extracted air is what matters; in an MVHR system, you also have to keep the heat the exchanger has just recovered.
| Duct | Air carried | In an unheated loft |
|---|---|---|
| Extract-only, extract run (grilles to unit) | Warm, humid indoor air | Essential: this is the one that condenses first |
| Extract-only, exhaust (unit to roof) | The same air, pushed outside | Insulate up to the outlet, or the water runs back to the unit |
| MVHR supply (exchanger to rooms) | Fresh air warmed by the exchanger | Heavy insulation: every degree lost erases part of the recovery |
| MVHR extract (grilles to exchanger) | Humid indoor air | As for extract-only, for condensation and efficiency |
| MVHR fresh-air intake and exhaust | Outdoor air, or air cooled by the exchanger | Insulated with a continuous external vapour barrier |
The MVHR trap: the intake and exhaust ducts are cold. In the loft they cause little trouble, but as soon as they cross the heated volume to a unit sitting in a utility room, it is their outer face that condenses. The vapour barrier must then be on the outside, continuous. And a poorly insulated supply run in the loft is enough to make a unit rated at 90 % recovery blow lukewarm air.
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. Outside the heated volume, NF DTU 68.3 asks for R ≥ 0.6 m².K/W on single-flow ducts and on the fresh-air and exhaust ducts of a heat-recovery system, which above all must not condense, and R ≥ 1.2 on the supply and extract ducts of a heat-recovery system, which must keep the recovered heat. Inside the heated volume, only the fresh-air and exhaust ducts of a heat-recovery system still need R ≥ 0.6. 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.
Pre-insulated duct or sleeve: 25 or 50 mm?
Pre-insulated flexible ducts are mostly sold with 25 and 50 mm of mineral wool; a sleeve goes over a bare duct. With R = e / λ and a common lambda of 0.040 W/(m·K), the orders of magnitude are as follows (the cylindrical shape and compression shift them a little: redo the calculation with the product's data sheet).
| Solution | Usual thickness | Approximate R (m².K/W) | Use in an unheated loft |
|---|---|---|---|
| Pre-insulated flexible duct | 25 mm | ≈ 0.6 | DTU minimum in single flow, and for fresh air and exhaust in heat recovery |
| Pre-insulated flexible duct | 50 mm | ≈ 1.2 | DTU minimum for heat-recovery supply and extract; the advised margin on single-flow wet-room runs |
| Wool sleeve with aluminium facing over a bare duct | 25 to 50 mm per layer | ≈ 0.6 to 1.2 per layer | Insulating an existing run without removing it, or doubling a 50 mm duct |
| Rigid duct with a shell or foil-faced wool | Per data sheet | To be calculated | MVHR, long straight runs |
The DTU minimum is therefore met by a standard pre-insulated duct, 25 mm in single flow and 50 mm on the supply and extract of a heat-recovery system. Beyond that, the dew point decides: on wet-room runs laid in the cold, go to 50 mm even in single flow, or run the duct under the loft insulation. A pre-insulated duct has a continuous factory vapour barrier; a sleeve is only as good as its joints.
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.
Ventilation duct buried in loft insulation: good idea or bad idea?
Thermally, it is a good idea: laid on the ceiling and covered by the insulation, the duct ends up on the warm side, better protected than with any sleeve. The bad configuration is the reverse, and very common: ductwork laid on top of the wool after it was blown, in the cold, and trodden on at every visit. Burying the duct calls for four precautions, because nobody will see it again.
- Joints sealed and photographed before covering: a leaking joint under the insulation blows humid air into the wool, invisible until the stain on the ceiling.
- No crushing: blown wool settles without load, while a roll laid flat squashes the duct. With rolls, run the duct between two strips, never underneath.
- A slope with no low point, set before blowing: once covered, it can no longer be checked.
- The sections leaving the insulation treated separately: the rise to the unit and the exhaust up to the roof stay in the cold and keep their 50 mm duct or their sleeve.
If the loft is being redone, the network is installed and tested before blowing in wool or cellulose, and the unit stays accessible.
Carrying out clean, durable duct insulation
Preparing the job: mapping the ventilation network and loft access
Before installing the insulation, secure access to the loft, light the area well, and plan a walkway so you don't compress the existing insulation. Clear measurements (diameters, lengths, crushed or damp zones) 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. A leak at a joint is also a leak of humid air into the loft insulation: the orders of magnitude and the tests are covered in our article on ventilation duct airtightness.
How to insulate an existing ventilation duct, step by step
- Switch the ventilation off and survey each duct, from the grilles to the exhaust: diameter, length, bends.
- Diagnose before insulating: a duct that is torn, crushed or full of water gets replaced, not wrapped; so does the wet insulation around it.
- Choose the sleeve: inner diameter equal to the duct's outer diameter, thickness according to the table above. On a duct that is too damaged, a new 50 mm pre-insulated duct is quicker.
- Disconnect the duct at the unit and slide the sleeve along its full length, without stretching it.
- Reconnect with a clamp, then bring the sleeve right up to the unit and to the grille, without a centimetre left bare.
- Close the vapour barrier: aluminium tape on every joint, bend and overlap.
- Hang the duct and set the slope, redoing any supports that pinched it (see below).
- Restart and measure the airflow at the grilles, then check the low points after a week of heating.
Reducing nuisances and optimizing the ventilation network in the loft
Limiting noise: duct fixing, hangers, and vibrations
The priority is decoupling: clamps on hangers, no contact with the roof frame, a flexible connector at the unit's outlet. The rest is covered in our articles on ventilation noise and on the two-speed unit, whose network is checked at both speeds.
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 continuous slope of around 1 %, with no low point, toward the unit or the drain point provided by the manufacturer. A sag between two hangers becomes a water pocket that holds on to the slightest condensation.
Avoiding crushing of the duct and its insulation
A crushed duct loses its airflow and its insulation: squeezed to half its thickness, an insulant loses roughly half its thermal resistance. Support it about every metre with a wide strap rather than wire, pull it taut to smooth the coils, and lay a boarded walkway so nobody steps on it.
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.
