By the AHU Guide UK – Air Handling Units for British Homes Team · Updated June 2026 · Independent, reader-supported

Air Handling Unit Ducting Guide for UK Homes: Materials, Layouts & Costs

If you're installing an air handling unit (AHU) in your UK home, the ducting system is just as critical as the unit itself. Poor duct design kills efficiency, creates noise, and wastes energy. This guide covers what you actually need to know about AHU ducting: the materials that work in UK homes, how to size and layout ducts, insulation standards, and what you'll realistically pay.

What AHU Ducting Does

An AHU moves fresh air through your home and extracts stale air—that's a heat recovery ventilation (HRV) or demand-controlled ventilation (DCV) system. The ducts are the highways that air travels on. They need to be airtight, properly insulated, acoustically treated, and sized right. Undersized ducts cause excessive noise and pressure drop; oversized ones waste space and cost more.

Rigid vs. Flexible Duct: Pros and Cons

Rigid duct is the gold standard for AHU systems. Usually made from galvanised steel, aluminium, or plastic (PVC/phenolic), it's:

• More durable and longer-lasting than flexible
• Better airtightness when properly sealed
• Easier to clean and maintain
• Doesn't sag or develop dead spots where dust collects
• Takes up slightly less space (same nominal diameter, thinner walls)

The trade-off: rigid duct costs more, requires bends and fittings, and takes longer to install.

Flexible duct (insulated flex hose with spiral wire core) is cheaper and easier to route around obstacles. However:

• Interior corrugations trap dust and reduce airflow
• Prone to kinks and crushing if not properly supported
• Acoustic performance is worse than rigid with proper lining
• Less durable in attics where rodents or UV light are concerns
• Airtightness depends entirely on whether joints are properly taped

Most UK installers use rigid duct for main runs and flex only for final connections to terminal boxes or diffusers. This is a sensible compromise.

Insulation and Building Regulations

In the UK, AHU ductwork must comply with Building Regulations Part L (Conservation of Fuel and Power). The current standard requires:

• Ducts carrying conditioned air should be insulated to minimise heat loss
• Minimum 25 mm insulation is typical; 40 mm is better for long runs or lofts
• Insulation must have a λ-value (thermal conductivity) of around 0.032–0.040 W/mK
• All joints and penetrations must be sealed to prevent air leakage

In practice, most rigid ducts come pre-insulated. If you're using steel or aluminium without factory insulation, you'll need to wrap it with mineral wool or spray-on foam. Don't skimp on this—uninsulated ducts in an attic can lose 15–20% of heating energy.

Acoustic Performance

AHUs and ducting can be noisy. Supply air velocity creates rush noise; extract air creates rattle. The solutions:

• Flex duct: Has some inherent damping, but corrugations generate turbulence noise
• Rigid duct with acoustic lining: Rigid ducts lined with 25–50 mm rockwool or melamine foam reduce noise by 10–15 dB
• Silencers: Box-type or duct-mounted attenuators with baffled insulation absorb fan noise before it travels the ductwork
• Pipe lagging: Wrap extract ducts with external insulation to reduce breakout noise

For residential systems running 24/7, acoustic lining or a small silencer on the supply outlet is worthwhile. Extract ducts are less critical unless bedrooms are near them.

Duct Sizing and Layout

Oversizing ducts by 20% is common practice in the UK to keep velocity low (below 2 m/s in supply, 1.5 m/s in extract). This reduces noise and pressure drop. Rough sizing:

• 150–200 mm diameter for supply/extract in a three-bed semi
• 75–100 mm for room-to-room distribution
• Larger ductwork = quieter operation, slightly more space

Layouts fall into two categories:

Centrally routed: One main trunk from the AHU, branches to each room. Neater, easier to balance, works well in new builds with accessible lofts or voids.

Distributed: Smaller ducts routed individually to each room, meeting at the AHU. More complex, but avoids long trunk runs and suits retrofit installations.

Extract ducts should slope downward toward the AHU (or have a sump) to prevent condensation pooling. Supply ducts benefit from insulation to avoid moisture on the outside in summer.

Sealing and Airtightness

Airtightness is often overlooked and causes real problems:

• Leaky ducts reduce the volume of air reaching rooms
• Pressure imbalances develop, forcing air through unintended gaps
• Energy efficiency drops

Use foil tape rated for HVAC (not general-purpose duct tape, which fails within years). Seal all joints, bends, and penetrations. For flex ducts, wrap the outer diameter with tape; for rigid, seal the inside at joints before connecting sections.

Testing with a pressure gauge (held at a known pressure drop) can verify airtightness. Most installers aim for <10% leakage at design pressure.

Costs and Installation

Materials cost per metre (supply duct):

• Rigid galvanised steel, pre-insulated: £8–15/m
• Rigid aluminium, pre-insulated: £6–12/m
• Flexible insulated duct: £2–4/m

A typical three-bed home needs 40–60 metres of main trunk and branches: expect £400–900 in materials. Fittings, silencers, and acoustic lining add another £200–400.

Installation labour is variable. A skilled HVAC technician charges £40–60/hour. A complete ductwork package (design, materials, installation, testing, commissioning) for a new build typically costs £1500–3500. Retrofit installations cost more because of access constraints.

Many installers now include airtightness testing as standard, which is worthwhile.

Common Mistakes

• Running ducts through cold attics without insulation
• Using extract flex duct that's not properly supported (sagging reduces flow)
• Mixing duct sizes without proper reducers (causes noise and poor distribution)
• Over-relying on acoustic foam without actually reducing noise at source
• Leaving ductwork unsealed—air leaks undermine the whole system

Final Thoughts

Good AHU ducting is invisible when it works: quiet, efficient, and forgotten. Take time to specify the right materials, insulate properly, size ducts for your home, and seal everything carefully. It's worth spending a bit more upfront to avoid years of poor ventilation or unexpected noise.