How long does screed take to dry & other FAQs
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 for minimum screed depths and drying times.
What is the minimum screed depth for underfloor heating?
The minimum screed thickness you should use for warm water underfloor heating systems is:
- Minimum depth of 65mm for hand-mixed screed
- Minimum depth of 50mm for liquid screed
How does screed depth affect underfloor heating?
For a domestic property, it’s best not to exceed a screed thickness of 65mm. This would have an impact on the property’s SAP rating, as the additional screed depth means the underfloor heating system will have to work harder to heat up.
Screed underfloor heating systems’ heat up and cool down times are similar. How long that will be depends on the water temperature, the amount of insulation, and the resistance of the chosen floor covering e.g. tiles heat up quickest.
A typical underfloor heating system using screed and run by a boiler would take approximately one hour to heat up.
How long does screed take to dry before fitting the floor covering?
The minimum time you have to leave underfloor heating screed to dry depends on the type of screed you are using.
Hand mixed screed for underfloor heating
The recommended drying times for hand mixed screed are 1 day per mm for the first 50mm, then 2 days per mm for every mm over 50mm. So, for a 65mm underfloor heating screed there would be a recommended drying time of 80 days.
In practice, 4 weeks is a sensible minimum drying time to leave hand mixed underfloor heating screeds.
Liquid (self-levelling) screed
Drying times are significantly reduced by using a liquid screed, such as LoPro QuickSet self-levelling compound. Not only is the minimum screed thickness less, but liquid screed has better drying characteristics and can be dried using dehumidifiers.
Are there any specialist underfloor heating 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. For our retrofit LoPro systems, we recommend our LoPro QuickSet self-levelling compound.
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 – 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).
If you’re unsure about using underfloor heating screed, our friendly team can help you on 01404515867.
Is insulation needed beneath underfloor heating screed?
Insulation is a crucial consideration for underfloor heating. There should always be insulation below the underfloor heating pipe 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 insulation in the floor is important too. Traditionally, insulation is laid and a concrete slab is cast above it. With underfloor heating, the slab should be below the insulation where possible, so that the mass of material to be heated is less.
This positioning of insulation improves underfloor heating 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 when installing underfloor heating.
Can UFH be used on any type of floor substrate and what are the considerations?
Nu-Heat offers a wide range of underfloor heating systems suitable for all types of projects. 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
- Acoustic requirement
- 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. If you’re unsure which system to choose, Nu-Heat can visit your home and help you choose the best type for you.
Still have questions about underfloor heating screed, such as minimum depths and drying times?
Contact our friendly customer support team or give us a call on 01404515867.