Underfloor Air Distribution (AFD) with Flat Duct: Complete Design and Installation Guide

Underfloor air distribution (AFD) has become the signature of premium fresh air installations in apartments and villas across Europe, the Middle East and Asia. Instead of overhead ducts, supply air travels through slim insulated flat ducts inside the screed and enters the room through floor grilles. Done right, it is invisible, silent and draft-free. Done wrong, it generates condensation damage that no one can reach. This guide covers the complete system: components, design rules and installation practice.

What Is Underfloor Air Distribution?

In an AFD system, the ventilation unit (usually an MVHR/heat recovery unit) supplies conditioned air through flat ducts laid within the floor screed. Air rises gently from floor-level outlets and displaces room air from below — an arrangement that delivers fresh air directly into the occupied zone while keeping the ceiling free of ducts and diffusers.

The Five Components of a Complete System

1. Insulated Flat Duct

The main artery. A slim profile — typically 130×30 mm for branch runs and 256×48 mm for mains — carries dual air channels inside a rigid core, wrapped in closed-cell insulation and aluminum foil. The insulation is not optional: cool supply air against warm screed is a condensation risk that must be engineered out. See our insulated PVC flat duct 130×30 mm and triple-channel 256×48 mm duct.

2. Elbows and Connectors

Direction changes use dedicated insulated elbows such as the 132×40 mm insulated flat duct elbow, keeping the thermal envelope continuous through every turn.

3. Distribution Boxes

Air from the riser is split by an underfloor distribution box — available in ABS 1-in-4/5-way, galvanized steel pressure-resistant and custom insulated metal versions. The box must be pressure-tight: it sits buried in screed for the life of the building.

4. Floor Outlets and Grilles

Terminals include linear aluminum grilles with mounting sub-frames, open-close adjustable vents and long-strip multi-connection outlets. Choose adjustable models for rooms with varying load; fixed linear grilles for a clean architectural look.

5. Pipe-Crossing-Beam Solutions

Where a duct must cross a structural beam, dedicated over-beam devices preserve the duct's cross-section instead of crushing it flat.

Design Rules That Prevent Callbacks

• Insulate every buried section — including boxes and elbows, not just straight runs
• Plan routes before pouring — mark duct runs, outlet positions and riser penetrations on the slab
• Keep runs straight where possible — every elbow adds pressure loss; use gentle offsets instead of sharp turns
• Seal joints pressure-tight — buried leaks are unrepairable
• Cover duct ends during construction — screed slurry inside a duct is a classic and costly mistake

Installation Sequence

1. Complete slab leveling and moisture barrier
2. Lay main flat ducts from the distribution box along marked routes
3. Install elbows and branch connections, verify every joint
4. Set outlet boxes at final floor level using sub-frames
5. Pressure-test the network before screed is poured
6. Pour screed carefully around — never over open duct ends
7. After curing, mount grilles and balance airflow at each terminal

Common Mistakes and How to Avoid Them

FAQ

Q: Can underfloor supply be used for both heating and cooling?
A: Yes, tempered supply air works in both seasons. The duct's insulation layer protects against both cold-weather heat loss and summer condensation.

Q: What screed depth is required?
A: The 130×30 mm duct profile plus cover typically fits a standard screed build-up; always coordinate with the structural specification of the project.

Q: How do rooms get balanced after installation?
A: Adjustable floor vents allow airflow balancing at each terminal; distributor boxes provide zone-level control.