Wet Underfloor Heating Installation Guide: Insulation, Pipe Fixing, Screed Thickness, Zoning, and Materials You Need

Wet underfloor heating is not just about buying a manifold and a coil of pipe. A proper system depends on the floor build-up underneath the pipe, the way the pipe is fixed, the screed or overlay above it, and the correct zoning and controls. If any of those parts are wrong, the system can be slow, inefficient, or awkward to control.

At APM Plumbing & Electrical, we supply practical wet UFH materials including manifolds, pipe, pipe staples, tile backer boards, levelling compound, and UFH accessories. This guide explains what goes under the pipe, what goes above it, how to fix the pipe down, what type of insulation to use, and the usual screed thickness ranges installers work to.

What goes under the underfloor heating pipe?

This is the part you were right to challenge. The layer under the pipe is one of the most important parts of the whole system.

A typical wet UFH floor build-up normally includes:

1. Structural base – concrete slab or suspended floor deck
2. DPM / vapour control layer if required by the floor construction
3. Rigid floor insulation – usually PIR, EPS, or XPS depending on the system and build-up
4. Perimeter edge insulation – around walls to allow expansion and reduce heat loss at the edge
5. Pipe fixing layer – staples, clip rails, castellated panels, or pre-grooved overlay boards
6. UFH pipe loops
7. Screed or low-profile overlay system
8. Final floor finish

Why insulation matters: if you do not insulate properly below the pipe, a lot of the heat goes down into the slab instead of up into the room. That means slower warm-up, lower efficiency, and higher running cost.

What type of insulation do you use under wet UFH?

The exact insulation type depends on the floor construction and the available height, but the common choices are:

• PIR floor insulation boards – high thermal performance, common where thickness needs to be controlled
• EPS floor insulation – often used below screeds in wider floor build-ups
• XPS insulation boards – useful where compressive strength and moisture resistance matter

On a full wet UFH screed floor, installers often use rigid floor insulation boards below the pipe, then staple or rail-fix the pipe directly onto the insulation layer or onto a fixing system over it.

On some low-profile retrofit builds, cement-faced or tile-backer style boards come into play above the structure as part of the overlay build-up rather than the main thermal floor insulation layer.

For example, APM stocks Embrass Peerless 1200mm x 600mm x 6mm Novatherm Tile Backer Board UK, which is useful in certain boarded / tiled build-ups, although it is not a replacement for a proper full-depth floor insulation design where the system requires that.

How do you fix the UFH pipe to the floor?

The pipe must be fixed securely before screeding or overlaying. The main fixing methods are:

• UFH pipe staples – pushed through the pipe fixing path into insulation
• Clip rails – the pipe clicks into rails at set spacings
• Castellated panels – the pipe sits between formed knobs in the panel
• Pre-grooved overlay boards – the pipe sits in factory-cut channels
• Staple systems on insulation boards – very common on screed floors

If you are asking about the “nails”, the usual wet UFH fixing item is actually the pipe staple, not a normal nail. These staples hold the pipe to the insulation before the screed goes down.

APM live link for pipe staples:

• 60mm Underfloor Heating Pipe Staples – Box of 300 UK

Typical staple fixing approach:

• lay the insulation boards tightly with no rocking
• mark the loop layout
• roll the pipe out carefully to avoid kinks
• staple at bends, returns, and regular straight intervals
• add extra staples anywhere the pipe tries to lift

The tighter the bend, the more important good fixing becomes. Poorly fixed pipe can float or move while screed is being laid.

What pipe do you use for wet underfloor heating?

Common wet UFH pipe choices include:

• 12mm multilayer pipe – useful where lower build-up and shorter loops are preferred
• 16mm multilayer pipe – very common for domestic systems
• Pert-Al-Pert / PEX-AL-PEX multilayer pipe – helps the pipe hold shape and suits UFH layouts well

Examples from APM include:

• Keyplumb Multilayer UFH 80m 12mm Pert-Al-Pert Coil - Boxed UK
• Multilayer Pert-Al-Pert UFH Pipe 200m 16mm x 2mm - Boxed UK

How do you calculate how much pipe you need?

A simple estimating method is:

Pipe length ≈ heated floor area ÷ pipe spacing

Then add extra for the flow and return tails back to the manifold.

Common spacing guide

• 100mm centres – higher heat demand areas
• 150mm centres – common domestic spacing
• 200mm centres – lighter demand areas with good insulation

Example

For a heated area of 20m² at 150mm centres:

• 20 ÷ 0.15 = 133m of pipe
• plus manifold tails / layout allowance = roughly 145m

That would usually be split into two loops, not left as one long loop.

Typical loop length guide

• 12mm pipe – often around 60m to 80m per loop
• 16mm pipe – often around 80m to 100m per loop

These are practical rules of thumb only. Final loop length should always be checked against the manufacturer’s design guidance, pressure drop, and heat-loss requirements.

How do you zone wet underfloor heating properly?

Each zone should reflect how the room is used, how much heat it loses, and how you want it controlled. Good zoning usually follows:

• room type – bathrooms often want more heat than bedrooms
• room size – large spaces often need multiple loops
• heat loss – external glazing, doors, and corners change output demand
• floor finish – tile transfers heat differently than timber or carpet
• timing – living spaces and bathrooms often have different schedules

A large open-plan room may have 2 or 3 loops but still be one zone on one thermostat. A bathroom may have only one loop but be kept on a separate control schedule.

Which manifold should you choose?

The manifold port count should match the number of loops, not just the number of rooms. APM stocks live manifold options including:

• KeyPlumb UFH 4 Port Manifold Underfloor Heating UK
• KeyPlumb UFH 5 Port Manifold Underfloor Heating UK
• Keyplumb UFH 7 Port Manifold Underfloor Heating UK
• KeyPlumb UFH 1" Isolation Ball Valve - Pair UK

What type of screed goes over wet UFH?

This is another part that needs real detail. The main screed choices are usually:

1. Traditional sand and cement screed

A common site choice. It is robust, familiar, and widely used on wet UFH projects.

2. Flowing / liquid screed

Often an anhydrite or cement-based flowing screed. It wraps the pipe well and can suit UFH very nicely when installed properly.

3. Low-profile retrofit overlay system

Used where floor height is limited. In these builds, the pipe often sits in a formed board system and the finish above may be a thin levelling / decoupling layer rather than a deep full screed.

Minimum and maximum screed thickness over wet UFH

There is no one universal thickness for every system because it depends on:

• pipe diameter
• traditional or flowing screed
• domestic or commercial loading
• manufacturer requirements
• whether the job is a full screed floor or a low-profile overlay system

That said, these are the typical practical ranges installers work to:

Traditional sand/cement screed

• around 65mm total thickness is a common domestic target on insulation
• around 75mm total thickness is often used where loading is higher or the spec calls for it
• a practical rule is to maintain roughly 25mm to 40mm cover above the pipe, depending on pipe size and screed spec

Flowing / liquid screed

• often 50mm to 55mm total on domestic builds, depending on the product and pipe size
• cover above pipe is often lower than traditional screed, but must follow the screed manufacturer’s data

Low-profile overlay systems

• can be far thinner, but the exact thickness depends entirely on the board and levelling system used
• these systems are not judged by the same “65mm screed” rule because they are built differently

Important: do not guess the screed thickness. Always check the pipe supplier, screed supplier, and UFH system data together before installation.

What do you use above the pipe before tiling?

That depends on the system type:

• on a full screed floor, the screed itself becomes the main layer above the pipe
• on a boarded or low-profile build, you may use tile backer / overlay products and then a levelling compound before the final finish

Useful live APM products for this side of the build-up include:

• Novatherm Tile Backer Board 1200 x 600 x 6mm
• Everbuild Self Level Compound 20Kg UK

Step-by-step wet UFH installation order

1. prepare the structural base
2. install DPM / vapour layer if the floor construction needs it
3. lay the rigid floor insulation tightly
4. fit perimeter edge insulation
5. mark the loop layout and manifold location
6. fix the pipe using staples, rails, or panels
7. connect all loops back to the manifold
8. pressure test the system before covering it
9. lay the screed or complete the approved overlay build-up
10. allow proper curing / drying time before commissioning and floor finishes

Best floor finishes over wet UFH

• tile and stone – excellent heat transfer
• engineered wood – often suitable if UFH-rated
• vinyl / LVT – often suitable within manufacturer limits
• laminate – can work if approved for UFH
• carpet – possible, but the total tog value must be checked carefully

APM wet underfloor heating buying checklist

1. measure the true heated area
2. choose the pipe spacing
3. estimate the total pipe length
4. split into sensible loop lengths
5. choose the correct manifold size
6. specify the correct insulation build-up
7. choose the pipe fixing method
8. confirm the screed or overlay type and thickness
9. choose the thermostat / zoning plan

Shop wet underfloor heating supplies from APM

• UFH manifolds
• UFH pipe coils
• UFH pipe staples
• manifold isolation valves
• tile backer board
• self levelling compound

Visit APM Plumbing & Electrical, 24 Western Avenue, Acton, London W3 7TZ, or shop online at apmi.uk.

Important: this is a practical installation and buying guide, not a substitute for a full heat-loss, screed, or structural design. Final insulation thickness, screed depth, and loop design must always be checked against the UFH system manufacturer, the screed supplier, and the project floor construction.