On a construction project, ducting decisions made at design stage drive the cost of every later trade — first-fix carpentry, fire stopping, ceiling voids, plant room sizing, and commissioning. This guide is for site managers, M&E contractors, and self-builders who need to specify ducting on a UK construction project: types, sizing, material selection, fire compliance, and the mistakes we see go wrong on site. For a general overview of what ducting is and the types available, read our companion guide: What is Ducting? Types, Sizes & Materials Explained.
Why ducting specification matters in construction
Building Regulations Part F (ventilation) and Part L (energy efficiency) both require controlled airflow in occupied buildings. You cannot meet either without correctly-specified ducting — it is how air gets from fans, MVHR units, and air-handling systems to the rooms that need it, at the airflows the regs demand and at noise levels occupants will tolerate. Get specification wrong and you fail Part F airflow tests at handover, fail Part L SAP calculations, or build in a noise complaint that returns as a defect six months after practical completion.
In new-build residential projects, ducting connects bathroom and kitchen extract fans to external terminations and, increasingly, links MVHR units to every habitable room. In commercial construction — offices, schools, hospitals, restaurants — the ducting network is larger, fire compliance is more onerous (Part B), and integration with the structure has to be coordinated across trades from RIBA Stage 4 onwards.
The ducting system is one of the first second-fix items to go in on most construction projects. Get it wrong, and you're dealing with poor airflow, noise complaints, failed commissioning, and costly remediation.
Types of Ducting Used in Construction
Rigid PVC ducting
Rigid PVC ducting is the standard for domestic and light commercial ventilation. Available in round (100mm, 125mm, 150mm, 200mm) and rectangular flat channel (110×54mm, 204×60mm, 220×90mm) profiles. Round rigid ducting offers the best airflow efficiency. Flat channel is used where ceiling voids or floor spaces are too tight for round duct. Browse our full rigid ducting and fittings range.
Galvanised steel ducting
Galvanised steel ducting is the go-to for larger commercial and industrial installations. It handles higher pressures, longer runs, and elevated temperatures. Typically fabricated to order for projects above 200mm diameter or where fire-rated ductwork is specified.
Flexible aluminium ducting
Flexible aluminium ducting is common for final connections — the short runs between a rigid trunk and a ceiling valve or fan spigot. It's easy to route in tight spaces but creates significantly more air resistance than rigid duct. Keep flexible runs under 1.5 metres where possible and avoid tight bends. See our flexible ducting range.
Insulated ducting
Insulated ducting is used where condensation control or thermal performance matters — particularly in loft spaces, unheated areas, and MVHR installations where supply and extract ducts run in close proximity.
Ducting Specification: What to Get Right
Sizing
Undersized ducting restricts airflow, increases noise, and forces fans to work harder. Oversized ducting wastes space and budget. For domestic bathroom fans, 100mm is standard. Kitchen hoods typically need 150mm. MVHR systems use 125–200mm depending on the unit and number of rooms served. For commercial projects, ducting is sized by an M&E engineer based on calculated airflow rates, duct velocity limits, and pressure drop budgets.
Material
Match the material to the application. PVC for domestic. Galvanised steel for commercial. Insulated for exposed runs in cold spaces. Never use flexible ducting as a primary trunk run — it's a last resort for connections, not a system backbone.
Fire compliance
Any ducting that passes through a fire compartment boundary needs a fire damper at the penetration point. This is a Building Regulations Part B requirement that gets missed more often than it should. We stock circular fire dampers and intumescent blocks for common sizes.
Acoustic performance
Ducting transmits sound as well as air. For noise-sensitive spaces (bedrooms, offices, meeting rooms), specify inline silencers and acoustic-lined ductwork. Position fans away from occupied rooms where practical — a loft-mounted inline fan with rigid ducting to ceiling valves is the quietest domestic arrangement.
Common Ducting Mistakes on Construction Sites
Using flexible duct for everything. It's quick to install but wrecks airflow performance. A 3-metre run of flexible duct can halve a fan's effective extraction rate compared to rigid.
Running ducting through external walls without sealing. Air leakage around duct penetrations is a Part L failure waiting to happen. Use appropriate sleeves and sealant.
Forgetting condensation management. Warm, moist extract air running through cold loft ducting will condensate inside the duct. Insulate extract runs in unheated spaces, and ensure ducts fall slightly towards the external termination so any moisture drains outward.
Ignoring commissioning. Part F requires ventilation systems to be commissioned and airflow rates measured at each terminal. If your ducting is restrictive (too many bends, undersized, kinked flexible runs), you'll fail commissioning and need to re-do the installation.
Ducting Products for Construction Projects
At eFans Direct, we stock the full range of ducting and fittings for residential and commercial construction:
- Domus rigid PVC ducting in round and flat channel profiles, with all bends, connectors, and adaptors
- Flexible aluminium ducting in 100mm to 315mm diameters
- Insulated flexible ducting for loft installations and MVHR systems
- Systemair galvanised spiral duct for commercial specifications
- Fire dampers, silencers, and plenum boxes for compliant installations
All available with next-day delivery and trade pricing. Whether you're speccing a single bathroom extract or a full commercial ventilation system, get in touch for technical support on ducting layout, sizing, and product selection.