4) Screed

Screed has become a science all of its own, with well over 25 different types of specialist, free flowing and traditional screeds. There are various additives available which substantially reduce drying times.

For the underfloor heating industry, we are generally dealing with two types of screed, free flowing and traditional.

Traditional Screed

Traditional screeds consist of four parts sand to one part of cement with a fibre additive. They are put down at a 75mm thickness, on basement, ground and upper concrete or block and beam floors that have been designed to take the weight.

Firstly insulation is laid on the slab and taped, edge insulation is put around the perimeter of the room which is also taped, and then the screed is laid on top of the insulation. Where there is an unheated area beneath the floor, 75mm of insulation is recommended, and for areas that do have heated areas below them, 25mm of insulation is recommended.

APPLICATION - on small projects, screed can be mixed either by hand or in a mixer and delivered in a wheel barrow. For larger projects, a screed pump is used and the screed pipe which delivers the mixed screed has a degree of movement which can dislodge the underfloor heating pipes. This can compromise an even heat output if the pipe is not clipped back into its original place. The insulation we prefer to use, EPS 100, is a commercial grade which has a density that gives secure bedding for the underfloor heating clips and which reduces the risk of pipes becoming dislodged during the screed process.

Examples of what can happen with incorrect installation

Traditional screed should be applied with care to ensure pipes are properly covered and that there are no holes or void spots which could become cold spots when the system is turned on.

PRESSURE GAUGE - all of our manifolds come complete with a pressure gauge. This is used firstly when the underfloor heating system is pressurised - usually to 3 bar. There is always some residual air in the system, and sometimes the pressure gauge will drop slightly from this reading. It is vital to check the pressure gauge before and during the screed process. Sometimes the edge of a shovel can puncture a pipe, and if this happens the pressure gauge drops to zero. Should this happen, screed must stop immediately and the pipe must be repaired.

DRYING TIME - traditional screeds can take foot traffic within 24-48 hours of being laid and the drying time is approximately 21 days before the underfloor heating system can be switched on. Once the 21 days have elapsed, the system can be turned on, but at a very low temperature. As each day passes, the temperature of the heating system can be gradually increased, but if the heat is turned up too high too quickly, the screed could crack.

However, due to developments in specialist additives, drying time can be reduced to as low as three hours and drying time down to 24 hours. These specialist additives obviously add to the cost of the screed, but these costs are offset by the ability to start other trades on site sooner than is possible with traditional screeds.

THICKNESS - as mentioned above, traditional screed should be laid at a depth of 75mm. There are circumstances where some projects have a lack of floor depth, and in these circumstances a thinner screed is required. A screed of less than 60mm thickness will require reinforcement; this can be achieved by laying chicken wire over the underfloor heating pipes prior to screeding.  This technique was used at a recent project at Lapstone (see Commercial section in Projects)

Free Flowing Screed

This type of screed has a number of advantages; due to the fact that it is much stronger than traditional screed, the recommended depth is 55mm compared to 75mm for traditional screed. Consequently as the slab is 20mm thinner, it heats up faster than thicker screeds.

Free flowing screeds are quicker to put down; 1,000 sm a day is possible, compared to 150 sm for traditional screeds.

SELF LEVELLING - water finds its lowest level and the same principle applies to free flowing screed, which has a smooth level finish. With traditional screed, a board is passed over the screed in order to get a level finish.

SITE CONDITIONS - liquid screed is delivered to site by a large lorry with a rotating mixer - a pipe then runs from the lorry to the screed pump. There must be adequate space for the screed lorry to access the site.

DESIGN CONSIDERATIONS - the heat output from screeded floors is 100 watts/m sq. The maximum floor temperature is normally 27°C which, with a room temperature of 18 to 21°C, gives radiant heat and a comfortable temperature. The temperature of the water in the floor coils is typically 55°C, i.e. much lower than for hot water radiators and fan coil heaters. The pipework in the floor is not accessible once it has been installed, pressure tested and screeded over, so it is important that the pipework has no joints in the screed (unless these are unavoidable repairs). The pipe has an oxygen barrier incorporated in its construction and is connected to a sealed heating system. If these precautions are taken, the risk of oxygen getting into the system is minimised as is the formation of sludge in the system which might eventually cause blockages in the underfloor heating pipes.

The water temperature to the floor coils should not exceed 55°C. To achieve this temperature a blending valve is fitted, which reduces the flow temperature of the water from the boiler by blending it with the cool return water from the floor pipes. This blending valve can either be a manually controlled blending valve or, on larger systems, a motorised three port valve which will fail safe in the blending position.

The blended water is circulated around the floor pipes by a separate circulating pump, located downstream of the blending valve.

TAPE - before an underfloor heating system is installed, it is imperative that the installers know what type of screed is going to be used. For a traditional screed, taping the joints in the floor and edge insulation is relatively straightforward and a commercial version of scotch tape can be used. But when a free flowing screed is going to be used a specialist tape is required and every single joint must be diligently taped. If this is not done, the liquid screed could find its way underneath the insulation and lift the whole lot up, condemning the installation in the process and resulting in thousands of pounds worth of additional cost.

TEMPERATURE - in order for screed to bind and last, it must be laid at a temperature that allows the molecular reaction to take place. If screed is laid at a temperature of 3 degrees C or less, the water in the mix freezes, and the binding of the molecules does not happen. Without the binding, the screed fails, and eventually cracks and crumbles. Screed laid in temperatures below 3 degrees C never sets properly.When screed is laid at temperatures above 35 degrees C, there are high levels of evaporation and water does not get properly absorbed into the mix. Once again the molecular reaction does not take place, the binding does not occur, and the screed will crack and crumble.

Screed Curing

A curing period of 21 days must elapse for traditional screeds and seven days for free flowing screeds must elapse before heat can be applied for preconditioning. After the period of 21/7 days has elapsed, the underfloor heating system can be turned on, initially delivering warm water at 25-30 deg C; after which the flow temperature can be raised by 5 deg C per day until the design temperature is reached.

Screed and Wood Floors

Before timber flooring is laid upon heated screeds, the screed must have been cured and conditioned previously to a moisture content of approx 0.5% (Carbide Method) by heating to the operating condition for five days. Timber flooring should have a moisture content of less than 8% - if it has a higher moisture content, there is a risk of the wood warping and shrinking. At all times the advice and instructions of the timber manufacturer must prevail.

The operational surface temperature of wood floors should not exceed 27 deg C - at temperatures above this there is a risk of warping. Furthermore, for some sports hall finishes, a 10 mm rubber matting is laid on the screed before the flooring is laid on the rubber matting. Temperatures above 27 deg C can melt the adhesives that secure the matting.

Damage to Screed

Recent development in screed technology mean that getting the best results from the screeding process and materials requires very specialised knowledge and practices. As a result we no longer offer integrated packages of underfloor heating and screed, preferring to work closely with a number of partner organisations who specialise in this area.  

We believe this ensures our clients get the best results and most up to date specialist knowledge and allows us to focus our expertise on delivering the best underfloor heating systems.

There are several steps to be followed in order to avoid damage to the screed:-

Correct insulation - If heavy plant is being driven across screeded floors, then a more dense floor insulation is required. The strength of the insulation determines the load capacity of the screed.

Heed specialist advice - A screeding specialist will advise on the various products which are available and how the through the various ranges of drying times the project programme can be optimised. He will also advise you when other trades can work on the screeded floor. Build this into you project plans and do not be tempted to reduce the schedule.

Allow the screed to cure - The biggest problem facing all screeding companies is with plant being driven over screeded floors before the screed has been allowed to cure properly. A damaged floor is the inevitable consequence.

Additives - The development of various additives to screed has vastly accelerated hardening and setting times. These fast drying screeds use super plasticisers to reduce the amount of water needed to make the mixture pliable, reduced water content means faster drying screed. However, remember that screed which can support foot traffic from other trades within a matter of days or even hours, is not ready to bear the weight of a cherry picker.

Protect the floor - when scissor lifts, cherry pickers or other heavy equipment is being driven over screeded floors, it is a wide precaution to protect the floor with plywood boards.

Cracks - All screeds have a risk of cracking as they dry and a liquid screed is more likely to crack than a standard screed. However there is a distinction between random small cracks and a damaged screed.

Floating Slab

A floating slab construction refers to the screed with the underfloor heating pipes in it being separated from any part of the building structure by the insulation beneath and around the perimeter. This allows the screed to expand and contract during the operation of the underfloor heating system without cracking or breaking up. This is why edge insulation is essential.

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UHMA - The trade association for surface heating and cooling.


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