Monitoring moisture condition

What if a humidity sensor in a sleeve in a brick substrate were to register 80%? Would this wall still be significantly damp? This can be a difficult question for either:

• those who need to know when to switch off drying equipment; or
• a building surveyor who needs to state in a report that a wall is significantly damp.

The author has devised and carried out tests on brick, timber and cement screeds that could help us understand likely threshold relative humidity for damp or dry material.

Fletton brick

The new brick was oven dried to a lowest weight of 1825.3g. The brick was then left soaking in water until a maximum weight of 2223.8g was obtained. The absorption was calculated to be 21.8%.

The brick was then placed in an office environment (RH 50%) and allowed to naturally dry out from saturation.

Every time the sample was checked by the various instruments, it was accurately weighed and its moisture content calculated using the simple formula:

Key findings High ERH readings in the 90% range persisted as the brick dried down from saturation to 1.3% mc. Implication: you could deduce from static ERH readings recorded that masonry material is not drying when it actually is. A carbide test would confirm the drying that has taken place. Pin probe, search mode and deep probe readings all drop as the brick dries, but the rate of the decline in readings is not easy to interpret. Implication: Do not solely rely on EMM readings. ERH readings decline very definitely, from 90.4% to 33.3%, as the brick dries from 1.3% to 0.68% actual moisture content (AMC). At 75% ERH the brick has a moisture content of around 1%. Implication: Although the ERH of a drilled hole may lock for a long time at a high level, it will obviously drop as the brick moves into the final phase of drying, underlining the usefulness of this method. As the brick becomes acceptably dry pin probe readings and deep probe readings converge. Implication: This could help you know material is drying.

Figure 1: Fletton brick under test

What is acceptable dryness in walls?

There is no definitive level of dryness applicable to all buildings and situations.

It is probably the case that many older buildings, built without damp courses, cavity walls, etc. are a little damper than their more modern cousins. So the ERH in a 1930s house wall just above a skirting board may be lower than in an 1875 Victorian terraced house.

Older houses tend to operate with moisture contents in walls and floors often dangerously close to being ‘significantly damp’. Such a building would be 'on the edge', and would survive as the walls and floors breathe — taking in and giving out moisture to maintain a safe(ish) equilibrium. So anything from ERH 75% to 85% may be the band of safety in walls or floors.

Acceptable dryness depends on the type of building and the sensitivity of wall finishes and decorations. Remember that the brick substrate is just underneath plaster in the habitable space, probably less than an inch from wallpapers and linings. Brick in this position needs to be fairly dry to help keep decorations unblemished.

Purely as a guide, at 75% RH this type of brickwork would hold around 1% of its dry weight as moisture. If masonry is much damper than this there could well be problems for decorations, for embedded metals (e.g. socket boxes, fixings) and for any timber either built-in or in contact with the masonry.

You might find it useful to compare moisture content in a dry part of a building to that of the subject damp zone. This will give you quite a useful indication of the kind of moisture content to aim for in drying the wet zone.

There are no British Standards available or much published independent research, so acceptable dryness (i.e. what the client requires) can only be determined by what you find from your own experience.

Floor screed panel

Flooring contractors have traditionally checked floor screeds for dryness using mechanical hygrometers. However, electronic sensors that offer a reading of both relative humidity and temperature are now available. The tests on a screed panel described below offer some insight into how the various moisture measuring instruments track moisture change.

It is easy to see why flooring contractors have acquired the skills to measure floor dryness: if they lay a moisture sensitive floor on a damp substrate it will probably fail.

The test floor screed panel sized 600 × 600 × 65mm is shown in Figure 2. A 4:1 sharp sand:cement mix was used.

Figure 2: A flooring test panel during testing – humidity box to the left, synthetic hair mechanical hygrometer to the right, and to the front left an inserted RH/temperature sensor. The moisture meter is recording a reading of 77.4% RH for the humidity box. Moisture meter pin probe readings and search mode readings are taken at ‘C’, with deep probe readings taken further to the right where the 2 holes are seen plugged with sealant

Actual moisture content, as ascertained from staged carbide testing of drilled samples, reduced from 4.8% to 1.2% during the monitoring.

The actual time taken for the screed to dry from mixing and placing is not an important feature of this test, but out of interest the drying commenced on 27 March 2003, with the 31st check made on 25 April 2003. The flooring industry considers that a typical floor screed dries at a rate of 1mm of its thickness per day.

Key findings: The results for mechanical and electronic hygrometers, both housed in similar insulating boxes, are almost identical. With a little adjustment of the mechanical hygrometer we could in fact produce 2 identical graphs. Implication: If both hygrometers produce virtually identical ERH graphs, I would strongly advocate using electronic sensors. They are much more user-friendly. The sensor in the drilled hole seemed to produce the most reliable result — it was probably less affected by surface temperatures. Implication: From this test it would seem to be well worth drilling a hole and inserting a sensor, as long as there is no drilling risk. The ERH graphs drop quite reliably from the beginning to the end of the curing. Implication: This test shows a very reliable dropping of ERH readings as the screed dries/cures — so contractors would not face such difficulties in interpreting the readings as for sensors in brickwork. Results would be similar for a drying as opposed to a curing screed. Search mode (i.e. capacitance) readings follow quite an erratic downward trend, so are not that easy to interpret. The readings do, however, decline inevitably to give a guide to the state of changing wetness or dryness. Implication: You really need to back up search mode readings with other checks and tests due to the relative difficulty in their interpretation.

When did the screed reach an acceptable dryness?

• According to the flooring BS, a screed is dry enough to lay moisture sensitive floor coverings on when RH has dropped to 75% using a floor-mounted test box.
• Acceptable dryness would seem to be reached around check 20, when drilled hole ERH is 73.6%, deep probe reading 18R/R, carbide test reading 1.65%, humidity box reading 74.7% (i.e. fitted with electronic sensor) and mechanical hygrometer 83%, but on the 20th check the search mode reading was still quite high at 521 — which supports the advice to assess dampness using as many methods as you can. It is likely the mechanical hygrometer needed some further adjustment — there is just too much of a difference between the electronic and mechanical hygrometer readings.

Looking at the graphs, your eye might be drawn to the deep probe reading graph. In the test, deep probe readings tracked the drying out very positively.

Implication: deep probe readings might on occasion give you a good indication of state of dryness, but this is an indirect method of measuring moisture. Use this method sometimes as a back-up perhaps, as it is quick and efficient.

Figure 3: Floor screed monitoring (curing) – checks over time

Monitoring advice gleaned from tests:

• Consider using more than one monitoring technique.
• Do not expect ERH of masonry substrates to decline predictably as a material dries.
• When deep probe readings and surface pin probe readings converge, it is an indication that a masonry substrate is dry at surface and at probe depth.
• Knowing whether a material is ‘dry’ or ‘damp’ is a grey area.
• A deep probe reading of less than 17 offers an indication of acceptable dryness at probe depth.
• As you can see in the graphs, all the measuring instruments eventually tell us whether a material is drying or wetting.
• Search mode readings can be a little erratic and difficult to interpret.
• Pin probe readings at the surface reflect surface moisture condition.
• Capacitance (i.e. search mode) meter readings are influenced by the state of charge of the batteries.

The '3 S rule’ for monitoring

No 2 surveyors will obtain the same moisture meter reading at any given position, particularly when using pin probes or a capacitance meter with a curved contact. So when monitoring, remember the ‘3 S rule’:

The same surveyor must use the same instrument, and must take the readings in exactly the same places.

Consistency in readings is also improved if readings are taken at the same time of day.

Although drilling holes for testing AMC of drillings is destructive and too time-consuming for some, the author strongly recommends testing by carbide meter at the beginning of the monitoring phase, perhaps during the monitoring, and certainly when you are convinced drying down has concluded, to confirm the readings taken from other instruments.

Additional moisture meter tests on plasters

Lime plaster test

As explained in Surveying equipment and tests, EMM readings in material other than timber are referred to as ‘relative readings’. It has long been assumed that a moisture meter reading, even if not an actual percentage reading in a material other than timber, at least gives us a relative indication of dampness. But the author’s tests in materials confirms this may not be the case. This changes the way you might consider moisture meter readings either across a surface or from time to time, as you monitor dampness.

For example, traditional lime plaster can easily soak in 25% of its dry weight as moisture. But as the plaster dries, do not expect your moisture meter to always tell you when it's drying.

Key findings: As the lime plaster sample dried from saturation (24.5%) to around 12% actual moisture content (as confirmed from oven drying) the pin probe readings recorded a maximum reading all the time. As the lime plaster sample dried from saturation to around 5% ,the meter in capacitance (search mode) recorded a constant reading. Implications: Check a wall in position A, note a maximum reading, then check the same wall at position B, recording again a maximum reading (but it is possible one area of the wall has wetter plaster than the other). Take moisture meter readings in plaster on date X, and again on date Y. You might deduce from the same readings that the plaster got no damper or drier — but it may well have. If you take moisture meter readings in the same position on date X, and again in the same position on date Y, and the readings change higher or lower, it will be likely the wall has dried down or wetted up. On all graphed results shown when moisture meter readings continue to rise or fall, moisture content of the material under investigation is also increasing or decreasing, but it can only be at the depth in the material at which the moisture meter actually operates.

Figure 3: EMM readings as a lime plaster sample wets or dries. For this plaster the moisture meter calibration seemed to work quite well. When the meter read 20R/R, the actual moisture content of this lime plaster was found to be 2%. The author considers the plaster to be just beginning to be significantly damp at this moisture condition – the vertical dotted line shows this point in the moisture cycle

Salted and unsalted plaster

The test that follows produces a rather infuriating result. It would be extremely useful if capacitance readings were uninfluenced by salts. We would then be able to know more about changing moisture content across plasters. Surveyors have known for a long time that pin probe readings are influenced by salts, but the results produced from the most simple of tests alerts us to problems with capacitance meters too.

A sample of plaster was bench tested using a capacitance meter and resistance meter, to check the relationship between meter readings and actual moisture content. The sample was tested first by saturating in water and then drying it out. The test was then repeated on the same sample, but after soaking it in salty water. The influence of the salt in the plaster can be seen clearly.

Key findings: In capacitance mode, higher readings were obtained along the full range of moisture content of the sample — e.g. at 4% actual moisture content, a capacitance reading of the salted sample is 170, as against 145 without the salt. A flatter curve of readings results, with capacitance readings of the salted sample only slightly rising from 3% actual moisture content. In pin probe mode higher readings were also obtained for the salted sample, e.g. at 4% actual moisture content a pin probe reading of around 30 in the unsalted sample, and 65 in the salted. Pin probe readings in the salted sample hit a maximum reading of 100 R/R when the sample had a moisture content of 5% — which seemed to make sense, as so often when a plaster looks salt-damaged we find maximum 100 R/R readings using pin probes. Implications: When a plaster is salt contaminated, moisture meter readings using pin probes or capacitance function exaggerate the amount of moisture present. Other measuring methods would then need to be used to assess any change to moisture content of the subject plaster. Using pin probes, we are more often alert to a salt damp problem, as recording maximum readings tells us there is either very damp or very salt-contaminated plaster. We would then be able to flag up a likely damp problem and recommend further investigation.

It is often obvious when plaster has suffered salt damage: it will usually be badly stained and blistered. When you do obtain a 100 pin probe or a 1,000 search mode reading, it might be worth actually checking for salts using a salt detector, or identifying the type of salt using a salts analysis kit.

If highly hygroscopic salt is present, this would usually mean there is a ground-sourced damp problem, and it will be difficult to dry the plaster or decorate it, as it will just continue to attract moisture from the air.

Figure 4: Salted and unsalted 1:1:6 plaster samples – comparison of moisture meter readings