This type of flooring uses the screed to conduct heat from the UFH pipe surface to the underside of the final floor finish. This is usually a sand and cement mix.
Structural Concrete Floors
In structural concrete floors the pipework is installed within the structural floor onto concrete reinforcing bar at a pre-determined depths.
Bonded Screed Floors
In Bonded screed Floors the screed is bonded to the substrate using a bonding agent or a primer. The underfloor heating pipework is then laid onto the top of the concrete subfloor and can be fixed usually with fixing rails secured to the floor.
A layer of screed is then laid on top of the pipework to form the heat transfer medium, and to level and strengthen the floor usually 25-40 mm.
Floating screed Floors
In floating screed floors, the underfloor heating pipework is then laid onto the top of the insulation and can be fixed by several methods including fixing rails and staples
A layer of screed is then laid on top of the pipework to form the heat transfer medium, and to level and strengthen the floor usually 65-75mm.
A 50 mm liquid screed or fibre screed can also be used. With liquid screed it is important to install a plastic membrane on top of the insulation and to use the edging strip around the perimeter of the walls.
Whichever construction is used the screed must always be allowed to cure naturally, usually 28 days.
The commissioning of the system should not be carried out for at least 28 days after the screed has been laid. The system should be o run for three days at 20-25°C and increased by 5°C every day until the design temperature has been reached.
BS EN 1264:4 requires that the underfloor heating pipe be installed with less than a +/- 10mm deviation from the installation plans and that less than a 5mm vertical movement possible during screeding. This is to ensure the pipes position and the systems performance is known.
A vapour barrier is should always be used on top of any structure that is porous to eliminate absorption of moisture from the screed. An example of this would be expanded polystyrene and polyurethane insulation.
The vapour barrier is not a damp proofing membrane, all structural slabs will require a damp proof membrane in accordance with the current regulations.
The vapour barrier must always be sealed to stop screed penetrating the insulation layer.
Heated screeds will expand and contract slightly during use. The perimeter insulation will allow for a certain amount of expansion in the slab but expansion joints may also be required.
As per BS EN 1264-4 expansion joints should be used for every 40m² at a maximum length of 8m and an aspect ratio of 2:1. One joint is always required between a heated screed slab and an unheated screed slab.
Wherever an expansion joint is crossed, the pipe must be enclosed within conduit, extending perpendicular to the joint for at least 300mm either side to enable the pipe to absorb the movement within the floor.
Edge/perimeter expansion strip
For all screed floors, heated or unheated, it is important to separate the floor from the walls to prevent damaged due to movement. Even without UFH a screed will expand and contract, both as it cures and then from season to season thereafter.
Fitting a simple, 10mm think compressible perimeter strip around the perimeter of and rooms to be screeded, and around any penetrations such as pipes rising through the screed, prevents structural damage from occurring.
This strip is typically made of a closed cell PE foam, providing a modest amount of thermal insulation, and reducing thermal bridging from the heated floor to the walls