The lowdown on screed

The low down on screed

How long does screed take to dry & other popular questions

At Nu-Heat we’re regularly asked about underfloor heating (UFH) and screed. We thought it would be helpful to share some of the most common questions along with our recommendations.

What depth should screed be when used with UFH?

When used with warm water UFH, the minimum thickness of any screed you use should be 65mm for hand-mixed screed or 50mm for liquid screed.

How does the depth of the screed affect the UFH?

For a domestic property, it’s best not to exceed a thickness of 65mm of screed as this will have an impact on the property’s SAP rating, as the additional depth means the heating system will have to work harder to heat up.

Heat up and cool down times will be similar, but how long that will be depends on the water temperature, the amount of insulation, and the resistance of the chosen floor covering.

A typical UFH system in screed run by a boiler would take approximately one hour to heat up.

Are there any specialist screeds that Nu-Heat recommends?

A denser substance of liquid screed can be just 50mm in depth when used with UFH rather than the 65mm depth needed with a cement-based screed.

The density of liquid screed means it is also a good heat conductor, aiding the heat transfer around the tube and enhancing efficiency.  This is particularly good when combining UFH with a heat pump as it maximises the thermal output of the UFH system, allowing the heat pump to work at a lower temperature, improving its Coefficient of Performance (CoP).

Is insulation needed beneath the screed?

Insulation is a crucial consideration for UFH and there should always be insulation below the UFH tube to resist the heat going downwards.  As you would expect, you need as much heat as possible to flow upwards and into the room.

Floors must be insulated to Part L – 70mm of PIR (Celotex or Kingspan are examples) for ground floors.  Floors over heated areas should be insulated with 30mm PIR or equivalent.

The position of the insulation in the floor is important too.  Traditionally insulation is laid and a concrete slab is cast above it.  With UFH, the slab should be below the insulation where possible so that the mass of material to be heated is less.  This improves response times and reduces any downward heat losses.  It is much easier to have the build specified in this way than having to make allowances in the heating design process.

If possible, additional insulation should be used on any external walls.

Can UFH be used on any type of floor substrate and what are the considerations?

Nu-Heat offers a wide range of UFH systems suitable for all types of project.  The choice of system is usually influenced by the overall building construction and is dependent on several factors including floor substructure, available height build-up, any weight restrictions, required floor heat output, any acoustic requirement and joist spacing (if applicable).

The key to a solution that performs is to choose the right UFH system to suit the construction.  For example, a sand and cement or liquid screed over a solid concrete of beam and block floor, or a plated UFH system between a suspended timber joisted floor.

How long does screed take to dry before fitting the floor covering?

Hand mixed screed

The recommend drying times are 1 day per mm for the first 50mm and 2 days per mm for every mm over 50mm.  So, for a 65mm screed there is a recommended drying time of 80 days.

In practice, 4 weeks is a sensible minimum.

Liquid (self-levelling) screed

Drying times are significantly reduced by using a liquid screed.  Not only is this thickness less, but liquid screed has better drying characteristics and can be dried using dehumidifiers.

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