Which Floor Screed Is right for the job?
UNDERFLOOR HEATING SCREEDS
Traditional sand and cement screed is mixed in the ratio of 1:3-4.5 with water and polypropylene fibres which reduces surface micro-cracking within the screed and generally applied to a depth of 75mm. The screed is mixed semi dry to an earthy consistency and the snowball test is the simplest and most effective way of judging the correct quantity of water. The snowball test consists of creating a snowball like ball of screed and if it holds together without evidence of excess water this is deemed to be of the correct proportion. If the snowball does not hold together then more water should be added to the mix.
Traditional cement sand screeds are suitable for all manner of construction methods ranging from bonded, unbonded, floating and heated floors.
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. These types of screeds have additives mixed along with the cement sand mixture to improve their drying time.
Fast drying accelerated screeds are now very much in preference because of the quick drying time they offer. Many fast drying floor screeds allow foot traffic to proceed as early as 12 hours and facilitate the early installation of the final floor finish, allowing the heating system to be used much earlier. FlexiDry is a highly recommended fast drying screed that is found to give the best results when used over underfloor heating systems.
Unlike the usual screeds this screed is composed of a mixture of calcium sulphate, sand and water, and is liquid in consistency. These screeds have the advantage of being easy to install as the liquid can be poured quickly to cover a larger area. However the drawback is that they cannot be used in wet areas and have long drying times.
Free flowing anhydrite screeds 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.
The heating elements for the underfloor heating are usually encased in a layer of screed before the installation of the final floor covering. In the case of hydronic underfloor heating, the heating pipes are usually secured to insulation panels on the floor slab fitted with moisture barriers, and the screed layer is laid over this to provide a level surface for installing the final floor finish.
Though traditional sand, cement screeds are generally used for screeding over underfloor heating, this can be quite time consuming as the screeds require a long time period for drying sufficiently to take on the final floor covering. However, though an obvious option, the underfloor heating system cannot be utilised to accelerate the drying process as it can result in the cracking of screed due to quick drying.
Fast Drying Floor screeds help to overcome this issue, drying much faster than traditional screeds while allowing the screed mix to hydrate fully. They act as capillary pumps, removing excess water and locking in the rest of the water by crystallisation, thus preventing the cracking of screed. An example for Fast Drying screed is Isocrete K-Screed fast drying floor screeds. The different types of screed laid over underfloor heating are:
Underfloor heating(UFH) is a highly efficient form of space heating that generates heat through electric or hydronic heating elements embedded beneath the surface of the floor. The heating elements are typically secured to insulation panels on the floor slab and the screed layer is installed over the heating elements before laying the final floor finish.
Apart from serving as a level surface for taking on the final floor finish, a good screed plays the important role of conducting the heat evenly across the surface of the floor, without producing any hot or cold spots. A good quality screed, laid to the right depth helps to retain the heat for longer hours, reducing the energy required at the source and improving the energy efficiency of the UFH system. A wide range of screeding options are now available for underfloor heating systems: