How to Avoid Air Pockets Around Underfloor Heating in Concrete Slabs

✔ Step-by-Step: How to Avoid Air Pockets Around UFH in Concrete Slabs

1. 1

   Use the Right Concrete Mix for UFH Pours

   Choose a concrete mix suitable for UFH installations — typically a self-compacting or semi-flowable mix that can flow around pipes without excess vibration. Mixes with too low a slump will trap air as they settle over pipes. For most slab pours, a slump of 100–160 mm works well. If using fibre-reinforced concrete, make sure the fibre dose doesn’t hinder flow. Always consult the ready-mix supplier and specify that the mix is being used over underfloor heating.

2. 2

   Place Concrete Slowly and Evenly Across Pipe Zones

   Pouring too quickly can trap air beneath pipes, especially on tight loops or returns. Place concrete gently using a pump hose or chute positioned close to the surface. Begin at one corner and work methodically across the slab. Avoid pouring large mounds directly onto pipes — instead, build up in even layers so the mix can flow and settle naturally around the pipework. Keep the nozzle or delivery head close to the surface to reduce impact splashing and turbulence.

3. 3

   Use a Vibrator — But Only Around the Pipes, Not Directly On Them

   Controlled vibration is essential for removing trapped air. Use a poker vibrator or surface vibrator around the perimeter of pipe runs, avoiding direct contact with the pipes. Vibrate in a grid pattern — 300–500 mm apart — and withdraw the poker slowly to allow air to escape. Over-vibrating or vibrating directly on top of UFH pipes can damage fixings or cause pipes to float. Always check that pipes remain fully seated and adjust if any lifting occurs during compaction.

4. 4

   Anchor Pipes Properly Before the Pour Begins

   Pipes that aren’t fixed securely are far more likely to float or shift, creating hollow pockets beneath or around them. Make sure every loop is tied to insulation or mesh at 300–500 mm intervals. Recheck bends and curves, where pipe tension is strongest. Pipes must remain in firm contact with the support surface throughout. Once you begin placing concrete, it’s often too late to reposition a floating pipe without compromising the pour.

5. 5

   Pour in Two Layers for Full Encapsulation

   Pour a first pass of 30–50 mm of concrete to bed in the pipes. Let this layer settle slightly, ensuring the mix surrounds the lower half of the pipe runs. Then follow with the second pass to bring the slab up to finished level. This two-layer method allows better consolidation and reduces the chance of large air voids forming around the curved underside of pipework. It’s especially useful on wide pours where vibration alone won’t reach all cavities.

6. 6

   Monitor Slab Depth and Rework If Air Pockets Are Found

   After pouring and before the concrete sets, probe the slab surface with a depth stick to check consistent coverage over the pipe zones. If you feel soft spots, voids, or see bubbling/slumping, it may indicate trapped air. Use a screed bar to gently rework these areas or top up with fresh mix as needed. Any visible surface depressions should be corrected immediately to avoid long-term performance loss or cosmetic failure after polishing.