wrong technique screed drying?

[Post and beam]

I bought one of the moisture testers from Pro tiler tools. Fix it to the floor and leave it for 24 hours. Easy.  I got a reading and then fired up a large fan for the day. Reading %is lower at the end of this, excellent we are moving in the right direction. But the following morning the % RHI is back above 90! Bugger.       I have been repeating this for a week now.    Am i doing this wrong?

[Kelvin]

What type of screed? How thick?
 

It takes a long time to fully dry. I ran the ASHP in floor drying mode which is a two week process. I also had three dehumidifiers running although we were also plastering at the same time so there was a lot of moisture in the house. Plus all the moisture from the timber etc. 

Edited June 28 by Kelvin

[saveasteading]

Check the instructions precisely.

I had a flooring company fit one over a floor  (150 concrete) .   We put in more and more fans and heaters and dehumidifiers for 2 weeks as the dial was not dropping.

Then the flooring company explained that the box being sealed means the numbers stay high. It was OK.

That's a bit vague I know....but do check the procedure.

 

 

 

[Post and beam]

50 minutes ago, Kelvin said:

t takes a long time to fully dry.

Thats not the issue, well it is an issue but not relevant to this question but thank you.

[Post and beam]

40 minutes ago, saveasteading said:

 

Then the flooring company explained that the box being sealed means the numbers stay high. It was OK.

That's a bit vague I know....but do check the procedure.

Thanks dude. Throuble is, i followed the instructions. Now i dont know how to get a meaningful figure. It might be that leaving the tester fixed to the floor for more than 24 hours is the issue. Does not say that anywhere though.  My fans move 100 cubic feet a minute. Thats twice the kitchen volumne every minute! Ought to be enough.  Very confused

[Conor]

1 hour ago, Post and beam said:

I bought one of the moisture testers from Pro tiler tools. Fix it to the floor and leave it for 24 hours. Easy.  I got a reading and then fired up a large fan for the day. Reading %is lower at the end of this, excellent we are moving in the right direction. But the following morning the % RHI is back above 90! Bugger.       I have been repeating this for a week now.    Am i doing this wrong?

The screed will continually release moisture for weeks so the humidity will remain high. A fan won't change things. The humidity might have dropped temporarily due to a curing reaction or something else entirely.

[SteamyTea]

Concrete, well cement, uses water as part of the exothermic curing reaction.

Excess water is used as a lubricant to help the mix flow.

 

If the air has high relative humidity, no amount of fanning it will reduce the amount in the cement mix.

This is almost the definition of Relative Humidity.  At 100% RH, the partial pressure is equal to the vapour pressure. At that point, no meaningful transfer takes place.

You may be better off fitting a dehumidifier if you are in a hurry.

Portland cement hardens quite fast, but can take weeks to cure.

[Kelvin]

1 hour ago, Post and beam said:

Thats not the issue, well it is an issue but not relevant to this question but thank you.

 
Sure but our screed released moisture for a long time which created high humidity levels in the house generally. Are you sure the box is properly sealed to the floor etc 

[Post and beam]

1 hour ago, Conor said:

A fan won't change things.

And yet all the advice is, 'ventilation  is key' 

[Post and beam]

2 minutes ago, Kelvin said:

Are you sure the box is properly sealed to the floor etc 

Yes.

[Kelvin]

2 hours ago, saveasteading said:

Check the instructions precisely.

I had a flooring company fit one over a floor  (150 concrete) .   We put in more and more fans and heaters and dehumidifiers for 2 weeks as the dial was not dropping.

Then the flooring company explained that the box being sealed means the numbers stay high. It was OK.

That's a bit vague I know....but do check the procedure.

 

 

 

This is the point I don’t follow with these boxes as that’s what you’d expect to happen. 

[SteamyTea]

8 minutes ago, Kelvin said:

This is the point I don’t follow with these boxes as that’s what you’d expect to happen. 

If the box covered area continues to show high humidity, then I think that it shows that there is chemical curing happening i.e. the exothermic heating is driving out excess moisture.

It may be the case that there is a sudden decrease in RH as the process stops.

 

I am rather guessing here, but can't think of any other reason to check the RH in an airtight box above a wet floor.

[Kelvin]

52 minutes ago, Post and beam said:

And yet all the advice is, 'ventilation  is key' 

The ventilation is getting the moisture out of the house as the atmospheric conditions are a big factor. There’s an analysis of this by a screed company. I’ll try and find it. Our Cemfloor screed was down for a few months before we tiled. I’d spent three weeks or so after it was laid running dehumidifiers and ventilating the house. This was during the winter so was having to close windows over night then running the dehumidifiers during the night. Two weeks prior to tiling we put the ASHP in floor drying mode which is one of the advantages of liquid screeds and there was still quite a lot of moisture in the floor. 
 

The process I followed was 48 hours with the house sealed up, then ventilated for 72 hours then started dehumidifying which I kept going off and on until there was  a significant reduction in the amount of moisture condensing on all the cold surfaces. Our house was unheated at this point.

Edited June 28 by Kelvin

[Nickfromwales]

48 minutes ago, Post and beam said:

Yes.

I suggest moving the box to sample a different area each day vs one spot which will ‘stagnate’ the reading…..if that’s the correct word.

[Nickfromwales]

If the fans move moist air away from an area, they will be of huge benefit for the relative humidity, but would only ‘dry’ the screed if it was pointed at the floor. Then it would dry that one spot. 
 

Then you’d move the fan to another spot, and see the previous one become ‘wet’ again. The only way to dry a screed quickly is to have used a fast drying product (like Ardex A35 etc) in the mix. That can have ceramic tiles laid 45 mins after it being laid! Used it on one job where we had a 7 day window to complete a summer room floor, and I was seriously impressed.

 

If you can have windows / doors open to cross ventilate, and fans on in the room, providing the outside relative humidity doesn’t then exceed that of the room, then you’ll aid drying / curing ‘a bit’.

 

Move the box, re-test, be patient   

[Kelvin]

I can’t find that report. From memory it talked about the general atmospheric conditions in the house and above the screed. If you don’t ventilate the air becomes saturated and it slows the floor drying down. Therefore ventilation aids the drying process. 

[Nickfromwales]

1 hour ago, Kelvin said:

I can’t find that report. From memory it talked about the general atmospheric conditions in the house and above the screed. If you don’t ventilate the air becomes saturated and it slows the floor drying down. Therefore ventilation aids the drying process. 

Indeed. But you’ve got to move the air out of the house, not just the room. 

[SteamyTea]

1 hour ago, Nickfromwales said:

got to move the air out of the house, not just the room. 

Really just got to replace the air, for drier air, at the boundary layer.

[Nickfromwales]

1 hour ago, Nickfromwales said:

providing the outside relative humidity doesn’t then exceed that of the room

👍

[Temp]

I was told to allow 1 day per mm of thickness so 60mm screed takes 60 days to dry.

[SteamyTea]

56 minutes ago, Nickfromwales said:

providing the outside relative humidity doesn’t then exceed that of the room

Isn’t that what this means

 

58 minutes ago, SteamyTea said:

replace the air, for drier air

 

As the difference is a pressure i.e. Pa =  kg⋅m⁻¹⋅s⁻²

Then a relatively small difference can, over time, have a large affect.

[saveasteading]

1 hour ago, Temp said:

was told to allow 1 day per mm of thickness so 60mm screed takes 60 days to dry.

Often said but never by experts. 

This would only apply with thick concrete with far too much water in it (so that it flows for the builder's convenience but  also greatly reducing quality.) ie stated as if it was science whereas it is as an excuse.

An industrial slab of 200mm does not take 7 months, but about 2. A 60mm screed does not take 2 months, but 2 weeks.

[Kelvin]

3 hours ago, Nickfromwales said:

Indeed. But you’ve got to move the air out of the house, not just the room. 

 What else does ventilation mean  😉 Therefore, contrary to what  post and beam said in reply to my first post the general level of moisture in the house has an impact on the drying speed of the screed. 

[Nickfromwales]

4 hours ago, Kelvin said:

 What else does ventilation mean  😉 Therefore, contrary to what  post and beam said in reply to my first post the general level of moisture in the house has an impact on the drying speed of the screed. 

Agreed! Let's stay up and fight, it's Saturday night after all lol.

[Kelvin]

😂