Tekno Training

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The radiant floor system...

2022-05-04 15:00:00

Standard screed or self-leveling screed? With what thicknesses?

A few days ago a couple of friends, during the renovation of their house, asked me the following question:

"Marco ... we are building the underfloor heating system ... what advice do you give us?"

My first request for clarification was: "Standard screed or self-leveling screed?"

Here ... what you see in the following image was exactly the expression of my friends:

I often visit construction sites and have seen many underfloor systems built. But unfortunately I must point out that even today the solution that is adopted in most cases is the one with a standard screed. Let's first explain the difference between the 2 solutions:

STANDARD SCREED

Cement-based mixture (cement, aggregates, water and anyadditives), prepared on site by the screed installer, based on his experience and professionalism, and laid over the pipes with a minimum thickness of 45 mm. Current standards require a minimum thickness of 30 mm for the standard screed (above the pipe), but practically all companies that market radiant systems recommend a minimum thickness of 45 mm (above the pipe), which is also the minimum thickness normally adopted in construction site.

As you can see from the image above, the consistency of a standard screed is very similar to "wet soil" (if the installer of the screed was good at dosing the ingredients) and is usually leveled and smoothed with a trowel.

SELF-LEVELING SCREED

The ingredients are more or less the same (the type of binder changes: it can be cement or anhydrite or gypsum, or even a mixture), but they are "premixed" and calibrated in the dosage with more than exact proportions (especially the aggregates). In most cases, the mixture is delivered to the construction site ready (or freshly prepared by modern concrete mixers equipped with "dosing computers") and "pumped" to cover the pipes.

The consistency is so fluid that you can count on self-leveling (hence the name of these screeds).

What is the substantial difference between the 2 screeds?

A self-leveling screed has decidedly superior qualities:

Faster installation speed, thanks to pumping and self-leveling.
Greater thermal performance. The fluidity guarantees the total absence of "voids" in the mixture and everyone knows that any air cavities (even millimetric) hinder the transfer of heat from the screed to the living spaces.
Shorter curing and drying times. A standard screed needs at least 28 days for curing, while a self-leveling product allows the final flooring to be laid even after only 6-10 days. This detail can be very interesting in those construction sites where delivery times are late!
Hygrometric shrinkage (phenomenon of reduction of the volume of the screed during setting and hardening) optimized to the maximum, with practically no risks of "curling" and "bleeding" (these are phenomena of deformation and cracking of the screed linked to an incorrect dosage of components.... side effects not so rare when using a standard screed!).
The last difference, which is decidedly the most important for the realization of a floor system, is the possibility of creating a screed of lesser thickness!

The great advantage of the reduced thickness

With a self-leveling product it is possible to create a very low thickness screed, up to only 1 centimeter above the pipe (ATTENTION: respecting certain parameters)!!

What does it mean to have only 1 centimeter of screed above the pipe?

It means having a very low thermal inertia system. It means eliminating the only drawback that the underfloor system has always had, that is the long times to reach the operating temperature and its slowness in cooling with the system off. These two factors have always been an obstacle to the adoption of the floor system. Even today (incredible but true!), When a radiant floor system is proposed, the objection we hear is the following:

“Yes…… nice but……… doesn't it take too long to warm up? And then... shouldn't it be kept on all day? "

The objection may make sense if it is a standard screed, a screed at least 45 mm above the pipe. But it loses all meaning when faced with a self-leveling screed just 10 mm above the pipe.

The following image shows the comparison between the behavior of a standard screed and that of a self-leveling product:

Let's analyze the curves better:

The "X" axis shows the time, expressed in minutes; the "Y" axis shows the supply water temperature in degrees centigrade.
The boiler is started and after 10 minutes the feed water reaches the value of 30° (red curve). This water temperature value is kept almost constant through continuous switching on and off phases of the boiler.
After 10 minutes, the self-leveling screed with a thickness of 1 cm above the pipe (ocher color curve) has reached a temperature of 23°.
The standard screed with a thickness of 45 mm above the pipe (black curve) can reach the same temperature value (23 °) only after 3 hours!

If we observe the behavior of the ocher curve, that is of the self-leveling screed, we can see that it undergoes temperature changes in correspondence with the ignition and shutdown phases of the boiler. The standard screed (black curve) has a completely different behavior, practically indifferent to changes in water temperature.

This analysis highlights very well the substantial difference between a standard screed (45 mm above the pipe) and a self-leveling screed (10 mm above the pipe):

The self-leveling screed of only 10 mm above the pipe guarantees very low thermal inertia and responds very quickly to starts and stops of the system!

If we think about it, until a few years ago the only radiant systems considered to have low thermal inertia were the ceiling system and the wall system. Why?

Because they were the only systems that could be built with a reduced inertial layer. The inertial layer is the layer entrusted with the task of transmitting thermal energy, from the water conveyed in the pipes to the air-conditioned rooms. This layer is reduced to a few millimeters in the case of the 2 systems in question: the thickness of the plaster in the case of the wall system and, for example, the thickness of the plasterboard in the case of a ceiling system.

With an inertial layer of 10 mm we can speak of a system with low thermal inertia! If the inertial layer reaches, for example, 20 mm, then we can speak of a system with medium thermal inertia.

That is why, with a self-leveling screed, the optimal solution would be to not exceed 10 mm above the pipe.

But be careful! It is not so simple and so obvious to make a radiant screed of only 10 mm above the pipe!

Those shown in the following image are just some of the self-leveling products available on the Italian market:

But we must be very careful, because not all of them are suitable for our purpose, which is to create a low thermal inertia screed of only 10 mm above the pipe! If we look for more details through the technical data sheets of these self-leveling products, we can dwell on some interesting details, which can significantly narrow the range of products to choose from:

As you can see in the table above (which is just an example... without wanting to exclude other newly marketed products), there are not many self-leveling products that allow us to reduce the inertial layer to just 10 mm above the pipe. A recommendation is a must: check the characteristics, in particular the minimum thickness above the pipe, using the technical data sheets!! As they say… “Verba volant, scripta manent”!!

For those wishing to download the table in PDF format, you can use the link at the end of the article.

Why is it difficult to find a self-leveler that guarantees 10 mm above the pipe?

The reason is that the insulation layer of a floor system behaves like an "elastic layer". Therefore, when making a very thin screed, the latter could be subject to cracks due to excessive bending of the screed itself. In fact, if we analyze the previous table, we can observe that all manufacturers prescribe a different minimum thickness based on the presence or absence of the insulating layer. Many manufacturers of radiant systems, in fact, have put special panels on the market: these are panels without an insulating layer, made of plastic material, with "perforated" knuckles to allow the screed mixture to fill the inside of the knuckles and form a perfectly adherent layer to the substrate.

With this type of panels it is possible to create a self-leveling screed with a thickness of only 5 mm above the pipe!

But we must be very careful!

This is a very particular solution which, despite being contemplated by current legislation, involves two major drawbacks:

There is no insulation layer under the pipes. It is evident that part of the energy is dispersed in the opposite direction with respect to the environment to be conditioned.
If the screed is made adherent to the substrate, it means that the inertial layer is no longer that of the self-leveling layer alone. The substrate also partly affects the thermal inertia of the underfloor system!

Therefore, with these so-called "perforated knuckles and without insulating layer" systems, the system is certainly a reduced thickness system but .... talking about a system with low thermal inertia is a real stretch!

Tekno Training tips

If we want to create a floor system with reduced thickness and low thermal inertia, the best solution is the following:

For the inertial layer we choose a self-leveling screed.
We build a comparison table between the various self-leveling products on the market and choose the one that represents for us the best compromise between the following characteristics: thermal conductivity (maximum possible), compressive strength (maximum possible) and minimum thickness above the pipe (the minimum possible , keeping the famous “10 mm above the tube” as a fixed point).
Regarding the insulating layer of our radiant system, regardless of the type of panel (flat or knuckle panel), we choose a panel that has good compressive strength (150-200-250 kPa). From this point of view, the adoption of an "industrial" type panel could represent a valid alternative.
Let's make sure that the screed installer has the right knowledge and good experience in the field of radiant floors. Personally I adopt this stratagem: I simply ask the operator if he has ever had to deal with cases of "curling" or "bleeding" (in a future article I will deal with these particular phenomena, related to the maturation of the screed!). The answer can give us an indication of the degree of professionalism of the person in front of us.

After explaining to my two friends all the details set out in this article…. well… there is no need to specify which screed the choice fell on!

An example of a radiant system with very low thickness and very low thermal inertia

The following is the radiant package that I have finally proposed to my friends:

Insulating layer: flat panels in expanded phenolic resin foam (conductivity 0.021 W/mK; resistance 0.95 mqK/W; compressive resistance 150 kPa)
Floor system pipes: multilayer polyethylene D.17 mm with aluminum oxygen barrier
Self-leveling screed: Leca Paris Slim (total thickness 27 mm; thickness above the pipe 10 mm; conductivity 1.66 W/mK; compressive strength 25 N/mmq)

The result is a super-performing floor system, with thermal inertia reduced to a minimum and manageable even in on-off (just like a ceiling or wall system)!!

Please note. Source of some images: web