ᐅ Cement screed without chemical additives does not properly dry.
Created on: 25 Nov 2025 13:00
B
BrnZck91
Hello everyone!
On September 3rd, we had 65mm (2.6 inches) of cement screed installed (without chemical additives).
The heating protocol was followed, and we ventilated the house three times daily for 10-20 minutes each time. Several calcium carbide (CM) measurements have already been taken, and even after following the heating schedule, we consistently maintained heating up to 30°C (86°F).
During this time, plasterers and painters were also present. However, that was almost three weeks ago now.
Tomorrow the screed will have been laid for 12 weeks, and the last CM measurement taken last Monday still showed 2.6%.
On the recommendation of the screed company, we have since increased the heating to 45-50°C (113-122°F) up to today and also set up a construction dryer and a smaller room dehumidifier. Someone from the screed company visited today and couldn’t explain the situation. Inside the house, there is no noticeable moisture. It does not smell damp. There is no condensation on the windows, and the humidity levels on three hygrometers placed throughout the house range between 35-44%.
Still, the electronic moisture meter shows values between 1.7 and 2.2, and when the meter is placed in the hole left by earlier CM measurements, it even rises up to 2.7. I am at a loss.
The screed installer contacted another floor installer who recommended lowering the heating to 25°C (77°F), so that the air temperature stabilizes around 20°C (68°F). The reasoning was that the devices do not work correctly at higher temperatures (currently the air temperature is over 35°C (95°F)). The electronic moisture meter even showed 3.0% on the concrete stairs, which seems unlikely. But the CM measurements were done by the tiler and flooring installer, often together using two different devices alternately.
We only have one more measuring point left, and the furniture is arriving on December 9th.
The flooring installer also suggested applying an epoxy resin coating to prevent moisture from rising. To allow space for any ascending moisture, he plans to leave out the baseboards for now. This is not possible in the bathrooms, of course, since there will be tiles on the floor and walls.
I really hope someone here can help. I am getting desperate.
On September 3rd, we had 65mm (2.6 inches) of cement screed installed (without chemical additives).
The heating protocol was followed, and we ventilated the house three times daily for 10-20 minutes each time. Several calcium carbide (CM) measurements have already been taken, and even after following the heating schedule, we consistently maintained heating up to 30°C (86°F).
During this time, plasterers and painters were also present. However, that was almost three weeks ago now.
Tomorrow the screed will have been laid for 12 weeks, and the last CM measurement taken last Monday still showed 2.6%.
On the recommendation of the screed company, we have since increased the heating to 45-50°C (113-122°F) up to today and also set up a construction dryer and a smaller room dehumidifier. Someone from the screed company visited today and couldn’t explain the situation. Inside the house, there is no noticeable moisture. It does not smell damp. There is no condensation on the windows, and the humidity levels on three hygrometers placed throughout the house range between 35-44%.
Still, the electronic moisture meter shows values between 1.7 and 2.2, and when the meter is placed in the hole left by earlier CM measurements, it even rises up to 2.7. I am at a loss.
The screed installer contacted another floor installer who recommended lowering the heating to 25°C (77°F), so that the air temperature stabilizes around 20°C (68°F). The reasoning was that the devices do not work correctly at higher temperatures (currently the air temperature is over 35°C (95°F)). The electronic moisture meter even showed 3.0% on the concrete stairs, which seems unlikely. But the CM measurements were done by the tiler and flooring installer, often together using two different devices alternately.
We only have one more measuring point left, and the furniture is arriving on December 9th.
The flooring installer also suggested applying an epoxy resin coating to prevent moisture from rising. To allow space for any ascending moisture, he plans to leave out the baseboards for now. This is not possible in the bathrooms, of course, since there will be tiles on the floor and walls.
I really hope someone here can help. I am getting desperate.
@BrnZck91:
It seems that quite a few things are out of order!
First of all: a 65mm (2.6 inches) cement screed is completely standard for a heated screed in residential construction! Whether with or without screed additives.
The official heating protocol according to "Interface Coordination for Surface Heating and Cooling Systems in New Buildings" does not include anything involving 30°C (86°F).
Now here’s the important part:
We differentiate between the initial heating phase, the pure functional heating (to check if all components of the heating system are working), the cooling down of the screed, and then the drying heating phase.
So there are two heating phases with a temperature reduction in between (usually for around 3 days).
In cement screeds, this can cause rewetting if the screed had already been dried by heating beforehand!
Inside the house itself there is no longer any noticeable moisture. It does not smell musty. There is nothing on the windows, and the humidity measured on three hygrometers distributed throughout the house is between 35-44%.
For me, after the initial adjustment period, the hygrometer reading is then decisive!
At these values, the screed no longer releases moisture into the indoor air, and furthermore, the walls appear to be dried out as well.
Electronic moisture meters can only be considered approximate testing tools despite their measurement scales and conversion tables.
Please disregard these values, even the 2.7%.
I would bet my hat (figuratively speaking) that these readings are wrong and too high!
I personally have always relied only on my own CM meter because I regularly checked it for tightness and calibrated it precisely with exactly 1.00ml of water.
My experience is that most professionals rarely maintain their CM meters and therefore doubt their results.
My advice here: don’t despair, but trust the readings from the hygrometers.
Even if they had an absolute error margin of 5%, the readings would still indicate that there is no significant moisture diffusing from the screed or wall surfaces!
So: be confident, I would install (or have installed) a floor covering — even one that is vapor diffusion-resistant — without hesitation!
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Best regards to everyone: KlaRa
It seems that quite a few things are out of order!
First of all: a 65mm (2.6 inches) cement screed is completely standard for a heated screed in residential construction! Whether with or without screed additives.
BrnZck91 schrieb:
The heating protocol was followed, we ventilated three times daily for 10-20 minutes each. Several CM (calcium carbide) moisture measurements have already been taken, and even after following the heating protocol, we continuously heated up to 30 degrees Celsius (86°F).
The official heating protocol according to "Interface Coordination for Surface Heating and Cooling Systems in New Buildings" does not include anything involving 30°C (86°F).
Now here’s the important part:
We differentiate between the initial heating phase, the pure functional heating (to check if all components of the heating system are working), the cooling down of the screed, and then the drying heating phase.
So there are two heating phases with a temperature reduction in between (usually for around 3 days).
BrnZck91 schrieb:
Meanwhile, plasterers and painters were also on site. However, that was almost 3 weeks ago now.
In cement screeds, this can cause rewetting if the screed had already been dried by heating beforehand!
BrnZck91 schrieb:
Tomorrow the screed will have been installed for 12 weeks, and the last CM measurement from Monday last week still showed 2.6%.
Inside the house itself there is no longer any noticeable moisture. It does not smell musty. There is nothing on the windows, and the humidity measured on three hygrometers distributed throughout the house is between 35-44%.
For me, after the initial adjustment period, the hygrometer reading is then decisive!
At these values, the screed no longer releases moisture into the indoor air, and furthermore, the walls appear to be dried out as well.
BrnZck91 schrieb:
"Nevertheless, the electronic moisture measurement is showing values between 1.7 and 2.2, and when holding the device in the hole created by earlier CM measurements, it even rises to 2.7."
Electronic moisture meters can only be considered approximate testing tools despite their measurement scales and conversion tables.
Please disregard these values, even the 2.7%.
I would bet my hat (figuratively speaking) that these readings are wrong and too high!
I personally have always relied only on my own CM meter because I regularly checked it for tightness and calibrated it precisely with exactly 1.00ml of water.
My experience is that most professionals rarely maintain their CM meters and therefore doubt their results.
My advice here: don’t despair, but trust the readings from the hygrometers.
Even if they had an absolute error margin of 5%, the readings would still indicate that there is no significant moisture diffusing from the screed or wall surfaces!
So: be confident, I would install (or have installed) a floor covering — even one that is vapor diffusion-resistant — without hesitation!
----------------------
Best regards to everyone: KlaRa
schubert79 schrieb:
A layer had formed on it. I don't remember the exact term anymore.You probably mean the efflorescence.N
nordanney26 Nov 2025 22:33BrnZck91 schrieb:
We are considering doing the test with a membrane as well. Does that make sense, or is it less reliable? At the beginning, yes, but not anymore. After such a long time, the membrane will only show "dry," but not how dry exactly.
BrnZck91 schrieb:
Thank you for the detailed answer!
We are considering whether to additionally perform the test using a plastic sheet. Does that make sense, or is it rather unreliable? The test with a plastic sheet placed on a heated screed is not a "standardized" procedure, but it certainly provides a reliable indication of whether significant moisture is still being released from the screed into the room air or not.
Place a PE (polyethylene) sheet, about 40cm x 40cm (16 inches x 16 inches), on the heated screed in a room, create a slight elevation underneath (ideally a hygrometer sealed under the sheet), secure the edges of the sheet with tape on the screed surface, heat the screed, and ensure a cool room climate by keeping windows open (which should not be a problem given the current winter temperatures).
Once the screed is heated, condensation should form under the PE sheet within approximately 8 to 12 hours. If condensation occurs, it means the screed is indeed still too moist for covering.
The hygrometer will give you a precise assessment of the situation:
Anything below 70% relative humidity is considered dry!
It does not matter how dry it is!
What counts is that the screed is sufficiently dry.
This method can be used on old (subsequently moistened) or new screeds, whether cement screed or calcium sulfate screed, but not on magnesite screeds!
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Good luck: KlaRa
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