Liquid Screed and Underfloor Heating

J

joshea

Registered User
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Can some intelectual please tell me whats the difference between anahydrate and hemidrate when it comes to screed and underfloor heating

Is one better than the other
What are the pros and cons of these

Ta
 
Alpha hemi-hydrite and anhydrite floor screeds are both based on Calcium sulphate mixed with sand and water to give a fast curing, free flowing self levelling floor screed.

They both seem to be the same product marketed under different product names. They both do the same job.

Both are suited to underfloor heating with a minimum cover of 20-25mm and a guided nominal cover of 40-45mm form maximum efficiency of the system.

Both require treatment with a primer to allow tile adhesive to bond and this seems to be the only downfall, getting your finished floor to stick.

If you were comparing these flowable screeds with sand cement screeds there is a world of a difference in terms of bearing strengths vs thickness, efficiency of heat transfer, curing time, installation time etc.

All in all I would favour the flowable screed as drying time alone is 1 week per 10mm, thus 4.5 weeks for optimum flowable. Sand cement is the same 1 week per 10mm up to 50mm and then 2 weeks per 10mm after that. Thus equivalent sand cement would be 75mm taking 9 weeks to dry the stage of flooring.

Am I intellectual enough?
 
Hi Sconhome, during that drying time can you to continue to work on your house or is that just the time before you can start to switch on the heating system. Excuse the ignorance just interested in the lead time it would have in building/renovating.

Joshea - hope you don't mind me intruding.
 
Majj,

You can continue to work in the house. Generally it is best to install the screed on Friday morning, close up early and take a weekend off. It's advised that you leave for 2 days drying before walking around on the surface.

You would normally try and get the screed in as early as possible in the build so that it has fully dried out to the core by the time you are ready for floor finishes.

The heat can slowly be introduced after a few days and over the course of a couple of weeks stepped up to normal operating temperature. The underfloor heating should not be used to force dry the screed as you will cause structural faults in the core.

We would program our planned finish date back the calculated dry time plus 2 weeks to schedule when we need our screed poured in order to achieve our deadlines.
 

Thanks for that sconhome, much appreciated
 
Just one more question,
What is the normal/best width to allow for expansion joints along perimeter walls for this screed.
Also I have heard that its best to use aeroboard rather than foam insulation strips, the reason being that aeroboard is not as rigid and allows for more expansion. Is this true??
 
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