If you are going to install underfloor heating in a new building, or even in a refurbishment project, it will need to be screeded. However, before you can lay a screed there are several processes that need to be undertaken in order to install the underfloor heating correctly and to produce the most efficient form of heat transfer to a room.
The first thing that you need to do is to sweep the concrete subfloor to remove any debris and dust such as lumps of plaster and so on. The building itself needs to be weathertight with doors and windows fitted, but if not, the gaps should be covered with polythene.
If you are going to install insulation on the floor – usually something such as Jablite polystyrene board – it is available in varying thicknesses from 75mm to 150mm and some such as Stylite EPS 300 is as much as 200mm. The insulation boards need to be laid on the floor, and if there are any service pipes the boards need to be laid between them and then a second layer laid on top of that. It may also be necessary to cut grooves in the insulation boards to accommodate the pipes.
Once the insulation boards are in place, they need to be covered with a tanking membrane of 500-gauge polythene which needs to be tightly fitted with no creases or air pockets and overlaps of 100mm as a minimum. You should also leave 100mm to run up the wall at the sides so that the edging strip can be fitted to it: it should be tacked to the wall using a staple gun.
The next step is to install the underfloor heating, and the pipes should be stapled or screwed down every 400mm as a maximum, with more fittings where there are bends. The staples or screws will seal the membrane where they are fitted, so there will not be any leaks. The pipes then need to be filled with water and checked for leaks and pressure tested.
Now one can lay the screed. Traditionally you might have used a sand and cement screed in Devon, but at UK Screeds we use a liquid calcium sulphate screed, also known as anhydrite screed. This has very many advantages over sand and cement screed.
First and foremost, our liquid screed will totally envelop the heating pipes because it is liquid when poured and therefore doesn’t leave any voids or gaps. If you use a sand and cement screed and lay it by hand, it is almost impossible to ensure that all of the heating pipes are fully covered. Unfortunately, if you have any gaps or air pockets this will result in uneven transfer of heat into the room. Not only that, but our liquid screed has nearly twice the thermal conductivity of sand and cement, so the room will achieve the required temperature much faster.
In addition, one needs to take into consideration the depth of the screed. Sand and cement screed is usually laid to a depth of 75mm in order to fully cover heating pipes, but our liquid screed only needs to be laid to a depth of 30mm above the pipes making 45mm in all. This again helps with faster transmission of heat, and also saves on material costs. To be fair, liquid calcium sulphate screed costs more than sand and cement, but this is offset by the amount of material needed. Liquid screed is also preferable when carrying out a refurbishment programme because it takes up less space in the room so doesn’t increase the overall height of the floor as much as a sand and cement screed.
When we arrive on site we will check and set up levels using tripods working from a datum point. We also set up the pump and check that the amount of screed ordered is sufficient to cover the floor. When the mixer truck arrives, we then pump the screed into position and dapple it in two directions using a dappling bar in order to remove any air bubbles and ensure a perfect level.
The screed can then be left to dry for 24 – 48 hours, during which time it must not be walked on. After that, it is safe to walk on, so other contractors on site will not get held up.