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MVHR duct design basics
Duct design is the single biggest factor in whether an MVHR system is quiet, efficient and effective. The unit gets the attention, but the ducting decides how well it performs. This guide covers the basics: the types of ducting, why runs should be short and smooth, and why cheap flexible ducting is generally discouraged.
By VentRight Editorial · Last updated 2026-07-08 · Impartial · Sourced
A whole-house MVHR system supplies fresh air to living rooms and bedrooms and extracts stale air from wet rooms, all through ducting to a central unit.
Whole-house MVHR duct layout — labels
- unitMVHR unit — Usually in a loft or plant space. Fresh air enters and stale air is exhausted here.
- supplySupply ducts — Fresh, filtered air is ducted to the habitable rooms such as bedrooms and living rooms.
- extractExtract ducts — Stale, damp air is drawn from the wet rooms such as the kitchen and bathroom back to the unit.
Ducts running through cold spaces such as a loft should be insulated, or the warm moist air inside condenses into water.
Why ducts in cold spaces need insulating — labels
- uninsulatedUninsulated duct — In a cold loft, the warm moist air inside the duct cools at the duct wall and condenses into water, which can drip and cause damp.
- insulatedInsulated duct — Insulation keeps the air in the duct warm, so it stays above the point where moisture condenses. No condensation forms.
Why does MVHR duct design matter so much?
The ducting carries air to and from every room, so its layout decides how much air actually reaches each terminal, how hard the fans work, and how much noise the system makes. A great unit in poor ducting performs worse than an average unit in good ducting. Duct design affects airflow, running cost and noise all at once.
It is the most under-appreciated part of an MVHR system. Buyers compare units on efficiency and price, then let the ducting be an afterthought, which is exactly backwards: the ducting is where most real-world performance is won or lost. Getting it right is worth more than a slightly better unit.
What is the difference between rigid, semi-rigid and flexible ducting?
Rigid ducting, in metal or rigid plastic, has smooth walls and low resistance, and is the best performer. Semi-rigid ducting uses smooth-bore flexible pipes that are easier to route while still performing well, and is popular for domestic MVHR. Flexible ducting, the concertina type, is cheapest but has a ridged inside that adds a lot of resistance and noise.
Semi-rigid is the sweet spot for many homes: it routes more easily than rigid metal but keeps a smooth bore, so airflow stays clean. Rigid is the best on performance and the choice for main runs where space allows. Flexible has its place only for very short final connections, if at all.
Why is flexible ducting discouraged for MVHR?
Flexible concertina ducting is cheap and easy to fit, but its ridged inner surface creates high resistance and turbulence, which cuts airflow, makes the fans work harder and generates noise. It also sags and can trap condensation. Rigid or semi-rigid ducting performs far better, and good installers avoid long runs of flexible ducting for the main system.
A system built on long flexible runs is the classic cause of a weak, noisy MVHR that never quite delivers. The fans have to fight the resistance, which raises running cost and noise, and the sags can collect moisture. It is a false economy that undermines an otherwise good unit.
What makes a good MVHR duct layout?
Short, direct runs with gentle bends, correctly sized ducts, and the unit sited centrally to keep runs even. Sharp bends, long runs and undersized ducts all add resistance. Ducts passing through cold spaces such as a loft should be insulated to stop condensation forming inside them. Good layout is planned early, ideally before the structure is closed up.
The best time to design the ducting is at the design stage of a build or renovation, when the routes can be chosen freely. Retrofitting good ducting into a finished house is harder, which is one reason MVHR is easier to do well in a new build. Plan the routes, size the ducts, and keep them smooth.
Should MVHR ducts be insulated?
Ducts running through cold, unheated spaces such as a loft should be insulated. Warm, moist air in an uninsulated duct in a cold loft can cool enough for condensation to form inside, which can drip and cause damp. Insulating those runs keeps the air warm and prevents condensation. The intake and exhaust ducts to outside especially need it.
This is a common and avoidable fault. The supply and extract ducts carry humid air, and any stretch running through a cold void is a place condensation can form. Insulating those runs, and especially the ducts connecting to the outside, keeps the air above the point where moisture condenses.
Questions
- Can I use flexible ducting for MVHR?
- It is discouraged for the main runs. Flexible concertina ducting adds resistance and noise and can trap condensation. Rigid or semi-rigid ducting performs far better; flexible is best kept to very short final connections, if used at all.
- What is the best ducting for MVHR?
- Rigid ducting performs best, with semi-rigid smooth-bore ducting a popular and easier-to-route choice for domestic systems. Both keep a smooth internal surface so airflow stays clean.
- Do MVHR ducts need insulating?
- Yes, where they run through cold spaces such as a loft, and on the intake and exhaust ducts to outside. Insulation stops condensation forming inside the ducts, which can cause damp.
- Why is my MVHR weak or noisy?
- Very often the duct design: long or flexible runs, tight bends or undersized ducts add resistance, which cuts airflow and adds noise. Improving the ducting usually fixes it.