Biscuit Screed using lime?

[Atorph]

We're installing underfloor heating between floor joists in our Victorian stone terrace.
After the pipes go on top of the insulation we need to put a weak screed (biscuit screed) around them to spread the heat. Usually it's an 8:1 sand cement mix, but I was wondering if I can use an NHL 3.5 instead of cement and if I did what would be the ratios.
Thanks

 

[stuart45]

Is a firm doing the installation, or is it DIY?

 

[larry]

So I tried using AI today to generate a recipe for perlite based insulating lime plaster (just to put on as a base coat behind some wood fibre boards, so it doesn't need to be pretty). It gave me a mix which has seemed to go on very well. With that in mind, I asked Google Gemini your question and it's given me an umambiguous yes. I've copied the output below.

I have absolutely nothing to contribute to your question from any kind of professional knowledge or experience. Obviously, by copying this here I'm now providing another source for the AI model to run into its own echo chamber!!!

Make of this what you will!!!

NHL 3.5 Screed Ratio for Underfloor Heating​

You can absolutely use NHL 3.5. Given that you're aiming for a weak screed, similar to the 8:1 sand:cement mix, a recommended starting ratio for a moderately hydraulic lime screed is generally 1 part NHL 3.5 to 4 or 5 parts sand by volume.

Rationale​

• Flexibility and Breathability: Lime screeds, especially those using a moderate strength like NHL 3.5, have a lower modulus of elasticity than cement. This means they are more flexible, which is beneficial over a suspended timber floor structure and for accommodating the thermal movement from the underfloor heating pipes. The breathability allows moisture to escape, preventing damp issues often associated with cement in historic properties.
  
  
  
  
• Strength: An 8:1 sand:cement mix is very weak. A typical general-purpose lime mortar mix is often 1:3 (lime:sand). By going to a 1:4 or 1:5 mix with NHL 3.5, you create a screed that is weaker than standard structural mixes but still provides sufficient heat transfer and structural integrity for the thin biscuit screed application between the joists.

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Key Considerations for the Mix​

1. Sand Selection​

The type of sand is critical:

• Sharp, Coarse Sand: Use a clean, well-graded sharp sand (sometimes called coarse sand) for your aggregate. This type of sand provides better drainage and strength compared to soft builder's sand, which is less porous and can lead to a weaker mix. The coarser particles aid in heat transfer and prevent excessive shrinkage.
  
  
  

2. Consistency​

The mix should be a semi-dry/damp earth consistency—not wet or sloppy.

• You want just enough water for the mix to bind together when squeezed into a ball, but not so much that water is released. A semi-dry mix is less prone to shrinkage and cracking, which is especially important around heating pipes.
  
  
  

3. Mixing and Curing​

Lime cures more slowly and requires a different approach than cement:

• Mix Thoroughly: Mix the dry sand and NHL 3.5 first until the colour is uniform. Then, slowly add water and mix for a long time (around 10-20 minutes in a forced action mixer) to ensure all the lime is fully activated and dispersed.
  
  
  Curing: Lime screeds need to be kept damp and protected from freezing, drying winds, and direct sun for the first few days to a week. Proper curing is vital for the development of strength and to prevent cracking.
  
  
  
• Carbonation Time: The full strength of the lime screed develops over many months as it carbonates (reacts with CO2 in the air), but it will be firm enough to walk on and use the heating system much sooner (check with your supplier for specific guidance, but typically a few weeks).

4. Fibre Reinforcement​

Consider adding polypropylene or glass fibres to the mix.

• Fibre reinforcement helps to control shrinkage and increases the screed's flexibility and crack resistance, which is very useful when pouring around pipes and over insulation in a suspended floor structure. Follow the fibre manufacturer's instructions for the correct dosage.

https://tellinglime.com/unilit-floo...ithout a DPM will,laden water into the walls.

 

[Zebra]

Never heard the term "biscuit screed" before. Reminds me of the base of a cheesecake.

 

[stuart45]

It's easy to get the wrong mix if you Google biscuit mix. I think my wife used the 8/1 OPC/sharp sand on the last cheesecake she made after getting an online recipe.