What Floor Screed Do I Need and How Thick Should It Be?

Intro

Floor screed for underfloor heatingRecently, underfloor heating has become a more popular means of heating both domestic and commercial properties here in the UK.

Largely this has been due to the added efficiency that a well installed in-floor heating system can offer.

All of the various systems – wet or dry, pipe or cable – are designed with generating an up-flow of heat in mind, creating efficient radiant heat as opposed to the convection heating of old.

Of course, as heat rises, this means that the maximum benefit can be provided throughout the building while limiting wastage as far as possible.

Getting the thickness of screed right for use with an UFH system is one of the most important factors that needs to be taken into account before work can begin.

Here we’re going to take a look at the most common kinds of screed used in an UFH system, and which is the best thickness for both water fed and electric installations.

First things first – insulation for radiant heating systems

There’s little point in having a UFH system if it isn’t going to be installed to maximum efficiency.

Remember that unlike central heating systems that are dependent upon radiators, UFH is designed to require next to zero maintenance.

Indeed a properly insulated system should last for decades (saving energy bills and adding to property value too).

In order for it to work effectively, insulation needs to be laid beneath the UFH system so that the warmth generated doesn’t become absorbed downwards into the subfloor and the buildings foundations.

Insulation shouldn’t stop at the floor

Also, many people choose to review the overall insulation of their property prior to installing a new underfloor heating system to make sure that all of the walls, windows and even loft spaces are as well insulated as possible. Most contractors will typically be able to offer advice on maximising such efficiency.

Now we’ve covered the importance of insulation it’s time to take a look at the two most popular and efficient types of screed for UFH.

Semi-dry floor screed – what is it and what thickness is best?

This is the most common and, depending upon the site conditions, most popular type of screed used.

This traditional screed mix is made up from sand and cement at a 1:3-5 ratio and is then spread onto the floor and allowed to solidify around the UFH pipes or cables. Nowadays, most installers will use fibres to add strength and reinforcement to the flooring and will aim for a thickness around 75mm. This allows the best measure between heating efficiency (both retention and response time) and overall stability and strength for the floor itself.

The addition of fibres makes the floor far more stable, reducing the likelihood of cracking while also protecting against significant weight and impact. Some projects may require the need of thinner quantities of screed where the building lacks enough floor depth and screed laid thinner than 60mm will require adequate and sufficient reinforcement to remain sound.

Choose your contractor wisely

Screed really does need to be installed with care, and attention must be paid in order to avoid any gaps or cracks forming once the drying process begins.

Taking the time to research contractors in your area will pay dividends in the long run – this is not a stage to be rushed.

Drying time

This form of screed does require a degree of time to settle and that depends very much upon the environment and the premises themselves. However, as a general rule, a minimum of 21 days is recommended for the curing stage of most traditional screed mixes.

Bear in mind that while the floor may be usable for foot traffic, it’s recommended that the UFH isn’t turned on for at least a month. However, for solid subfloors this type of screed still offers the best general purpose efficiency.

To help with the movement of the screed whilst drying, almost always an edge insulation strip will be used as this is malleable to the screed and it will allow it to expand as it settles. Most architects and building control officers will recommend/expect this.

Those in a rush may like to consider fast drying screed which will – as expected – dry considerably quicker. In some instances foot traffic may be allowed after only 12 hours of drying.

Liquid screed for underfloor heating systems

Liquid (or free-flowing) screed is really designed for larger projects and is made from anhydrite – a liquefied form of screed that is poured instead of spread by hand, and designed to self-level. Most domestic installations of UFH will find that, unless their home is extremely large, traditional screed is considerably more cost-effective, however.

Huge coverage

Liquid screed is designed to cover up to 1000 square meters in one day compared to 150m² in a traditional screed floor. As this type of screed is pumped it is usually delivered in a mixing lorry and put down via a pumping pipe.

Because of this, it is important to bear in mind the access available to your site – a residential street may not be able to accommodate such disruption, for instance. So, unless you have a huge area to cover and the space to work with, a traditional mix is probably a better bet.

Advantages of liquid floor screed

The advantage of free flowing screed is that it is far more efficient and a lot stronger than its traditional cousin and it requires a layer of no more than 55mm to get the job done.

As a consequence, a radiant heating system that is laid inside this type of screed is correspondingly more efficient, taking less time to warm up. As a liquid, it also has the advantage of being self-levelling and doesn’t require the levelling boards used in a dry screed installation – saving both time and manpower.

Another advantage of liquid screed is that once it has been laid, the heating system can actually be turned on to assist in the drying process much earlier than with traditional screed which is prone to cracking. Usually only a week is needed before the heat can be passed through at a low level in order to assist the curing/drying.

This means that the floor will be usable in a much shorter space of time. The only real drawbacks for liquid screed are that it is more expensive and really demands a larger project to make it viable, such as a full floor screed installation across a large house or premises. This also works well with resin floors.

Conclusion

Hopefully this guide will provide some pointers as to which type of screed is best for your project. As mentioned throughout, the thickness and form of the screed will depend mainly upon the site, budget and size of the project.

Screed contractors are experts of this exact science so it’s well worth contacting your local firm to assist you in making the most informed decision about your particular project.

As ever, let us know how you get on.

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