A question that we get asked more and more frequently is how to screed when underfloor heating is being installed.
Underfloor heating is fantastic but poses a number of challenges for screed installers. The water pipes are inherently not a strong substrate and are susceptible to point loading damage, bonding to the slab is not normally possible and the heating and cooling effect of the heating system can damage the screed.
The best way to screed for underfloor heating is to follow these top tips:
• Use a high-performance engineered screed instead of a traditional sand cement screed. The screed should achieve at least a 30MPa compressive strength and have a high flexural strength. We recommend Mapei Topcem or Mapecem which we supply as part of our ScreedMax Pro range.
• Engineered screeds are particularly important if the screed is "floating" over foam insulation boards.
• Make sure there is a good overall thickness of screed and a good coverage over the top of the water pipes. Most engineered screeds require at least 25mm of screed over the top of the water pipes. This will help prevent point load cracking of the screed above the water pipes, which is the weakest point of the screed.
• Incorporate steel mesh or ScreedMax Fibres into the screed. This is particularly important if it is a floating screed. The mesh or Fibres will provide additional protection against the screed shrinking or cracking.
• Make sure that there is a strong compaction of the screed above, below and between any mesh. You don't want the mesh to inadvertently create a separating layer between the screeds above and below the mesh. Make sure the mesh is well incorporated into the screed as one cohesive screed.
• Do not be tempted to speed up the curing of the screed by turning on the heating system. Follow the screed guidelines for commissioning of the water system so that the screed cures at the appropriate rate to achieve maximum strength.
• Take extra care to ensure strong compaction of the screed. Air gaps or poorly compacted screed around the water pipes will lead to poor heat conductivity and increase the risk of the screed (and pipes) cracking. Compaction is key with all screed installations.
Here are some don'ts to pay attention to. Don't:
• Install a sand cement screed
• Install a thin screed
• Skip on mesh or fibres
• Take shortcuts on compaction
• Speed up curing by turning on the heating system too early
• Don't let the mesh create a separating layer in the screed
Underfloor heating systems are a significant investment and replacing them because of a poor screed design or installation would be an expensive mistake. Always err on the side of caution with a high quality engineered screed and you will enjoy your underfloor heating system for many years to come.