The term Underfloor Heating Screed is mentioned a lot in conjunction with floor heating systems, but what exactly is it?
The screed is the word used to describe the thin layer of sand cement mixture poured on top of the heating pipes which are attached to the structural concrete slab or insulation, this makes up the underfloor heating screed.
We recommend 4 layers of which involves the screed when laying Underfloor Heating screed:-
Layer 1 – The Subfloor. This is the bottom layer and is usually simple concrete or a slab substrate.
Layer 2 – The Insulation. In order the minimise downward heat and also make the system a lot more efficient by forcing heat upwards we recommend using a good quality foil backed insulation such as brands like Kingspan or Celotex.
Layer 3 – Floor Heating Pipes or In Screed Electric Heating Cable. Normally secured on top of the insulation using clips or fixing rails
Layer 4 – The Screed. This is the layer that is poured over the heating pipes or cables, giving you a smooth, level surface to lay your flooring materials on top.
Once laid, the exact curing time will depend on the temperature and humidity in the building, but traditional mixes will need at least 75 days before switching on this allows 1mm per day of drying time if laying a 75mm screed on top. We also recommend introducing the temperature gradually to the screed to warm up slowly.
There are different types of screeds: –
- Traditional sand and cement screed: Sand and cement mixed in the ratio of 1:3-5 is generally applied to a depth of 75mm as a semi dry screed.
- Fast drying screeds: These are enhanced screeds with accelerated drying times, and are generally recommended as the ideal screed systems for underfloor heating. These screeds yield the best results when applied using forced action mixers such as screed pumps or pan mixers. E.g.) FlexiDry
- Free flowing anhydrite screeds: These are gypsum based liquid screeds that can be applied at reduced depths compared to the traditional screeds. Ideal for screeding over large areas. But the major drawbacks are, these screeds are not suitable for wet areas and are generally not compatible with cementitious adhesives. The surface usually requires sanding and priming, before a bonded finish can be applied. Some of the commonly recommended anhydrite screeds are Tarmac Truflo, Cemex Supaflo, Larfage Gyvlon
Choosing the right screed and properly installing is key to any successful Underfloor Heating installation and could prevent expensive problems further down the line if not laid poured properly and to the correct specification.