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.
Hello everyone and many thanks in advance for all your replies!
Here is an updated interim report:
This morning, I was on site with the floor installer and tile setter. We opened the last measuring point and conducted the CM (calcium carbide) measurement. Both devices showed a value slightly below 2.2%. This is already an improvement compared to last Monday’s measurement, which was around 2.7%.
Now regarding the agreed procedure:
We will now continue heating up to 50 degrees Celsius (122°F) until next Wednesday. Currently, the temperature is at 27.5 degrees Celsius (81.5°F). The construction dryer and dehumidifier are still running. Additionally, last night I placed a 50cm x 50cm (20in x 20in) sheet of foil in four locations inside the house, sealing it airtight. Under one foil sheet, there is a hygrometer. I already opened one location this morning after only 12 hours—the foil was dry. This evening, I will open a second spot, and the remaining two will be opened tomorrow. By then, the temperature should be higher again.
Today, we measured the depth of the openings for the CM measurement. We found depths between 7 and even 8 cm (2.8 – 3.1 inches), although according to the screed company’s report, the average thickness should be "65mm on average" (2.6 inches). Of course, I understand that strict millimeter precision on site is not realistic. I can only assume that due to differences in height of up to 1.5cm (0.6 inches) between the exterior walls and the precast concrete slabs, the screed is significantly deeper in some areas. This also explains the longer drying times—especially with 8 cm (3.1 inches) thickness. I just wonder if we should have started with a different heating plan from the beginning, allowing a longer heating duration.
There is no way around it: we have now agreed to continue heating until next Wednesday. The floor installer plans to pour the leveling compound next Sunday (I’m incredibly grateful as this is only possible with a one-person company). We will no longer be able to do a CM measurement. We hope that one week of high heating, together with the construction dryer, dehumidifier, and the three remaining measurement points under the foil, will provide some clarity.
What is your recommendation regarding ventilation?
Should I ventilate only twice a day for a maximum of 5 minutes so the house can heat up above 40 degrees Celsius (104°F) and ideally drive moisture out from below? Or would ventilating three times a day for 10-15 minutes be more effective, or perhaps leaving windows tilted open all day? I worry that tilting the windows all day will prevent the temperature from rising sufficiently.
I placed a 50cm x 50cm (20in x 20in) foil with a hygrometer yesterday evening around 9:30 PM. We are now starting to increase the heating. Is that okay, or should I put up the foil again this evening? Should the windows really be left tilted open in this cold weather? That way, I won’t be able to get the house heated at all.
Here is an updated interim report:
This morning, I was on site with the floor installer and tile setter. We opened the last measuring point and conducted the CM (calcium carbide) measurement. Both devices showed a value slightly below 2.2%. This is already an improvement compared to last Monday’s measurement, which was around 2.7%.
Now regarding the agreed procedure:
We will now continue heating up to 50 degrees Celsius (122°F) until next Wednesday. Currently, the temperature is at 27.5 degrees Celsius (81.5°F). The construction dryer and dehumidifier are still running. Additionally, last night I placed a 50cm x 50cm (20in x 20in) sheet of foil in four locations inside the house, sealing it airtight. Under one foil sheet, there is a hygrometer. I already opened one location this morning after only 12 hours—the foil was dry. This evening, I will open a second spot, and the remaining two will be opened tomorrow. By then, the temperature should be higher again.
Today, we measured the depth of the openings for the CM measurement. We found depths between 7 and even 8 cm (2.8 – 3.1 inches), although according to the screed company’s report, the average thickness should be "65mm on average" (2.6 inches). Of course, I understand that strict millimeter precision on site is not realistic. I can only assume that due to differences in height of up to 1.5cm (0.6 inches) between the exterior walls and the precast concrete slabs, the screed is significantly deeper in some areas. This also explains the longer drying times—especially with 8 cm (3.1 inches) thickness. I just wonder if we should have started with a different heating plan from the beginning, allowing a longer heating duration.
There is no way around it: we have now agreed to continue heating until next Wednesday. The floor installer plans to pour the leveling compound next Sunday (I’m incredibly grateful as this is only possible with a one-person company). We will no longer be able to do a CM measurement. We hope that one week of high heating, together with the construction dryer, dehumidifier, and the three remaining measurement points under the foil, will provide some clarity.
What is your recommendation regarding ventilation?
Should I ventilate only twice a day for a maximum of 5 minutes so the house can heat up above 40 degrees Celsius (104°F) and ideally drive moisture out from below? Or would ventilating three times a day for 10-15 minutes be more effective, or perhaps leaving windows tilted open all day? I worry that tilting the windows all day will prevent the temperature from rising sufficiently.
KlaRa schrieb:
Place a polyethylene (PE) foil, about 40cm x 40cm (16in x 16in), on the heated screed of a room, provide a small elevation underneath (ideally a hygrometer enclosed under the foil), fix the foil to the edges on the screed surface with adhesive tape, heat the screed, and create a cool indoor climate by opening windows (which should not be a problem given the current winter temperatures).
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Good luck : KlaRa
I placed a 50cm x 50cm (20in x 20in) foil with a hygrometer yesterday evening around 9:30 PM. We are now starting to increase the heating. Is that okay, or should I put up the foil again this evening? Should the windows really be left tilted open in this cold weather? That way, I won’t be able to get the house heated at all.
@ "BrnZck91":
You should not attempt to heat the house with this test.
To put it simply in physical terms: it is sufficient that the temperature above the vapor barrier, meaning in the room, is low enough so that the dew point beneath the barrier is reached.
To explain it more clearly and practically: you only need to significantly lower the temperature in one of the rooms while the screed is being heated (at maximum supply temperature).
Whether the other rooms are affected thermally by the "strict ventilation concept" is completely secondary.
The purpose is only to test whether the screed is still releasing moisture.
You have to accept opening the windows wide for a few hours to achieve this result.
If no condensation forms beneath the vapor barrier, you can skip further ventilation.
You should not attempt to heat the house with this test.
To put it simply in physical terms: it is sufficient that the temperature above the vapor barrier, meaning in the room, is low enough so that the dew point beneath the barrier is reached.
To explain it more clearly and practically: you only need to significantly lower the temperature in one of the rooms while the screed is being heated (at maximum supply temperature).
Whether the other rooms are affected thermally by the "strict ventilation concept" is completely secondary.
The purpose is only to test whether the screed is still releasing moisture.
You have to accept opening the windows wide for a few hours to achieve this result.
If no condensation forms beneath the vapor barrier, you can skip further ventilation.
Another thought occurred to me today: The limit for residual moisture in heated cement screed is 1.8%.
But isn’t there a difference between cement screed that is 4-5 cm (1.6-2.0 inches) thick and one that is 7-8 cm (2.8-3.1 inches) thick? In my (non-expert) opinion, a 7-8 cm (2.8-3.1 inches) thick cement screed can hold much more moisture without it rising to the surface compared to a 4-5 cm (1.6-2.0 inches) thick one. Therefore, I would suspect that such a limit cannot be applied universally to all thicknesses.
What do you think? Maybe this idea is complete nonsense.
But isn’t there a difference between cement screed that is 4-5 cm (1.6-2.0 inches) thick and one that is 7-8 cm (2.8-3.1 inches) thick? In my (non-expert) opinion, a 7-8 cm (2.8-3.1 inches) thick cement screed can hold much more moisture without it rising to the surface compared to a 4-5 cm (1.6-2.0 inches) thick one. Therefore, I would suspect that such a limit cannot be applied universally to all thicknesses.
What do you think? Maybe this idea is complete nonsense.
Your statement is correct, but your conclusion is nonsense because the moisture content is measured in percentage (%) and therefore represents a ratio.
However, you are right that thicker layers take longer to dry. Nevertheless, the percentage limit must be met regardless of the layer thickness.
However, you are right that thicker layers take longer to dry. Nevertheless, the percentage limit must be met regardless of the layer thickness.
No, no, the idea is completely correct.
However, I wanted to avoid “opening a can of worms” with an additional comment in this direction.
What does that mean?
Thicker screeds dry significantly slower. From 10 cm (4 inches) thickness upwards, it can be said that a non-heated screed never becomes ready for covering.
As an expert, I say: if 60 mm (2.4 inches) was agreed upon and the screed is 80 mm (3.1 inches) or thicker, I wonder how the larger screed volume was calculated by the screed installer.
If the screed installer notices during their work that, for example, greater screed thickness is necessary due to unevenness of the surface, they must inform the client.
If they do not, the undisclosed extra thickness is a defect in the trade work!
This means: all additional costs incurred, for example from drying equipment or added services from other trades, could be charged to them.
But since this is a sensitive issue that should only be dealt with under the supervision of an expert responsible for the trade, I chose not to introduce the aspect of “defect.”
What is true, however, is that thick screeds dry more slowly—sometimes not at all.
PS: Similar problems have occurred in the past with calcium sulfate screeds, which were allegedly ready for covering within 7 days.
Due to changes in the formulation that no one can recognize from the outside, only tiny pores formed in the screed matrix, completely interrupting capillary moisture transport, so drying could only occur by other, slower mechanisms.
But that is just a side note.
However, I wanted to avoid “opening a can of worms” with an additional comment in this direction.
What does that mean?
Thicker screeds dry significantly slower. From 10 cm (4 inches) thickness upwards, it can be said that a non-heated screed never becomes ready for covering.
As an expert, I say: if 60 mm (2.4 inches) was agreed upon and the screed is 80 mm (3.1 inches) or thicker, I wonder how the larger screed volume was calculated by the screed installer.
If the screed installer notices during their work that, for example, greater screed thickness is necessary due to unevenness of the surface, they must inform the client.
If they do not, the undisclosed extra thickness is a defect in the trade work!
This means: all additional costs incurred, for example from drying equipment or added services from other trades, could be charged to them.
But since this is a sensitive issue that should only be dealt with under the supervision of an expert responsible for the trade, I chose not to introduce the aspect of “defect.”
What is true, however, is that thick screeds dry more slowly—sometimes not at all.
PS: Similar problems have occurred in the past with calcium sulfate screeds, which were allegedly ready for covering within 7 days.
Due to changes in the formulation that no one can recognize from the outside, only tiny pores formed in the screed matrix, completely interrupting capillary moisture transport, so drying could only occur by other, slower mechanisms.
But that is just a side note.
Tolentino schrieb:
Your statement is correct, but your conclusion is nonsense because moisture content is measured in percentage, which indicates a ratio.
However, you are right that thicker layers take longer to dry. Nevertheless, the percentage limit must be met regardless of the layer thickness.Absolutely correct, nothing more to add!Similar topics