Electric or Hydronic (Water) Underfloor Heating
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:
- 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.
When the traditional sand and cement screed is the economic choice for small areas, the anhydrite free flowing screeds facilitate easy installation over larger areas. However, the waiting period is generally longer for these screeds as it is important to ensure the optimal drying of the screed before the installation of the final floor covering.
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.
The concept of underfloor heating is said to have been inspired by the ‘hypocaust’ used in early Roman construction. The hypocaust which derives its meaning from the Greek words ‘hypo’ (under) and ‘caust’ (burnt), is documented to be the earliest form of underfloor heating that used hot air from furnaces to heat the floors from below. The ancient Roman baths and buildings were typically built with empty spaces beneath the floor to facilitate the movement of hot air from burning furnaces to heat the floors above.
A much closer version of the modern day underfloor heating is the electric underfloor heating system that came into vogue in the 1960’s. Electric resistance cables inlaid in concrete were used to heat the floors during the off-peak hours at night, when electricity was cheaper. But this system did not gain much popularity as it presented several technical glitches and was expensive to run. But the present day scenario has drastically changed from the rudimentary version of the 60s. With technological advancements, availability of sophisticated controls and better home insulation, underfloor heating has evolved into one of the most efficient forms of space heating available. And, with the growing potential of integrating solar technology to the underfloor heating systems and with newer avenues opening up for selling back to the grid through feed in tariffs, the underfloor heating graph could well be on the rise in terms of energy efficiency as well as economy.
The modern day underfloor heating is a highly evolved and extremely efficient version of this form of space heating, and is one of the most efficient forms of heating available today. However, a major advantage attributed to the underfloor heating system is, water is not required to be heated to a very high temperature as with the traditional radiator heating system. Another advantage is this system does not produce vector currents, but allows the heat to rise up evenly, generating an ambient temperature and comfortable heating.
Screeding for underfloor heating
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.
Enhanced 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 enhanced screed is FlexiDry fast drying floor screeds.
The different types of screed laid over underfloor heating are:
Traditional Screed: Cement and sand is mixed in a ratio of 1:3-5 with water and mixed with PP fibres or other micro fibres to reduce cracking.
Fast drying Screeds: This type of screeds have additives mixed along with the cement sand mixture to improve their drying time. The additives might come as ready mix or can be site mixed as per requirement.
Anhydrite Screeds: 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.