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We've recently had an anhydrite liquid screed floor laid in our house. We are now waiting for it to dry before porcelain floor tiles are laid (over a decoupling layer).

There is some disagreement between the tradesmen involved about how to confirm the screed is dry, and what level of dryness is required.

  • The tiler (and screed supplier) says "maximum 70% relative humidity" (which seems to align with various online sources), but offers no suggestions on how to measure that, and all my searches around RH only seem to really reference calculating relative humidity in air.
  • The builder has this Brennenstuhl Moisture Detector (Manual), and says "this meter goes to up to a max of 33%, so that's the equivalent of 100%, so 70% of 33% means we need to target a reading of 23%" - As a software engineer I can't emphasise enough how completely wrong this calculation feels to me, but willing to accept I'm not an expert in this topic.

These are the ranges from the back of the meter (showing the max reading of 33% when in building materials mode. The builders suggested target of 23% also falls well into the 'high' category on the meter, another indication that seems to suggest he's very wrong):

Reading ranges for Brennenstuhl Moisture Detector Picture of Brennenstuhl Moisture Detector

So I suppose the core questions are:

  1. If 70% RH is the target, how do I convert that to the % reading this meter shows?
  2. What is this moisture content meter actually reading, as it's clearly not the RH?
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  • You're right that RH is completely irrelevant. It would be entirely dependent on temperature and air circulation and doesn't tell you anything about the state of your subfloor. What does the subfloor installer say?
    – isherwood
    Nov 14, 2023 at 22:02
  • And what's the decoupling layer? My concern here is mostly mold. The tile mortar doesn't really care, but you don't want to trap a lake under your new finished floor.
    – isherwood
    Nov 14, 2023 at 22:03
  • That meter probably detects moisture by volume. It doesn't go over 33% because more water than that just isn't likely in any solid material it would be used on. The correlation between 33% and 100% strikes me as absurd. The math tying RH to that is asinine.
    – isherwood
    Nov 14, 2023 at 22:04
  • @isherwood, Both the tiler and the screed supplier have given this figure of 70% RH (have updated to note that). It's just that the meter doesn't seem to measure RH, or have any clear indication of what it's actually measuring, and I can't work out what it means. decoupling layer is just to prevent cracking of the tiles as the screed settles. Probably not relevant, but didn't want to miss a detail (Basically this: tilingsuppliesdirect.co.uk/product-category/wetroom-systems/…). Nov 14, 2023 at 22:08
  • I know nothing about the actual question, but see bal-adhesives.com/blog/fail-safe-tiling-onto-anhydrite-screeds/….
    – isherwood
    Nov 14, 2023 at 22:09

1 Answer 1

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Your builders are wrong. That device measures moisture content. You're waiting for a moisture content of 0.5%, where 0.5% moisture content correlates with 75% relative humidity (unfortunately there's no citation or methodology given). The 0.5% moisture content is very dry, with the substrate weighing 100.5% of its dry weight, and the 75% relative humidity is measured at the surface of the substrate.

The device's manual says that it registers 50% or 33% if you bridge the probes with a conductor. This suggests that the meter is measuring an electrical property and empirically mapping that property to moisture content. The wood setting, then, empirically maps perfect conductance to 50%, and the construction material setting, then, empirically maps perfect conductance to 33%. Maybe vice versa, as the manual isn't clear.

The relative humidity measurement is for a hygrometer at the surface, where the relative humidity just above the surface correlates with the moisture content. There's a formalized version of the idea under ASTM F2170 for concrete slabs. A good ad hoc procedure would be to put the hygrometer inside a clear, impermeable box with its only open face pointed downward for some reasonable period of time. When the readings stop fluctuating, the box's relative humidity has reached equilibrium with the substrate, and you've got your relative humidity reading.

For very deep screeds, I imagine that there's a moisture gradient through the thickness of the substrate that can goof up your readings. The surface could be locally dry from fans, heating, etc., while high moisture from deep within the substrate is still taking its time to diffuse to the surface where it can evaporate.

Notes:

  • For targeting other moisture contents based on relative humidity, see https://www.contractflooringjournal.co.uk/most-read/a-creed-for-a-dry-screed/. Again, this source isn't absolutely trustworthy.
  • That 50% versus 33% could be as dumb as "construction materials" on average have a dry density that's 50/33 = 1.5 times the dry density of wood. The bottom end of the "construction materials" range hasn't scaled in proportion to the high end of the range, so maybe there's something complex going on there.
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  • Thanks. But I don't think your first paragraph is correct. This meter only goes down to 1.5%, so this can't be the same as the moisture content referred to in that article which says the target is 0.5%. And further, the contractflooringjournal graph of moisture content % by weight gives values from 0.5% to 5% mapping to an RH of 75% to 100%. But the meter goes up to 33%, more than 6 times the moisture content value that would supposedly correlate with 100% RH. Whatever the meter is reading, I do not think it is the same as the moisture content % by weight described in either of these articles. Nov 15, 2023 at 0:35
  • @Simon, you've merely discovered the useful bounds of relative humidity testing. It's only useful for moisture content bound by 0.5% and 5% (if that data is correct). You can always destructively test a sample by weighing it at its in situ state, burning off the water, weighing it again, and then doing some math. The fact that everybody quotes a relative humidity value for your application suggests that the targeted moisture content lies in the 0.5% to 5% moisture content range (if that data is reasonable). Your decoupling membrane supplier may spec a looser moisture content.
    – popham
    Nov 15, 2023 at 1:00
  • @Simon, that meter is effectively the same thing as a garden tester, just calibrated for a different dry weight material. I don't understand how its inadequacy for the job works as evidence.
    – popham
    Nov 15, 2023 at 1:04
  • @Simon, that meter is designed for measuring wood moisture content. See 5.5.1 at archive.org/details/gov.law.astm.d4442.1992/page/n3/mode/2up for how wood moisture content is computed. Do you believe that construction material mode uses a different methodology than wood mode?
    – popham
    Nov 15, 2023 at 1:19
  • @Simon, who said that 33% was supposed to correlate with 100% RH? Your builders are full of it.
    – popham
    Nov 15, 2023 at 1:23

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