About moisture meters
Moisture meters are used to show the amount of moisture in a building. There are two types: conductive and capacitance meters. Some moisture meters use a flashing light or audible tone to indicate changes in moisture content, while others just have a meter dial giving a digital read-out of the results.
In a conductivity meter, an electrical circuit is completed through the meter by placing two probes on the surface or embedding them in the wall. The moisture meter then measures the electrical resistance, which is influenced by the moisture content in the wall.
With a capacitance meter, either the meter itself (carrying conductive plates) or a separate head (carrying conducting concentric rings) are placed on the surface of the wall that you are measuring for moisture. The meter readings measure the fringe capacitance in the sensor, which is influenced by the level of moisture in the wall.
Moisture meters can give misleading results
However, moisture meters can also be used to give misleading results, either deliberately or because of inexperience. While moisture meters do measure the amount of damp in a property, they can give an artificially high reading if there are salts in the wall. This can be an issue as damp and salt frequently go together due to the hygroscopic (water-attracting) properties of many salts.
In addition, moisture meters need to be used on appropriate materials. Most moisture meters tend to be calibrated for use on an average softwood. As such, when they are used on brick, plaster, stone or mortar, they can only give a qualitative reading: you will be able to tell whether a wall has consistent levels of damp throughout, or whether it is higher at the base of the wall and lower at the top (which can indicate rising damp, though this is not always the case with such a reading). However, the percentage moisture that appears is not the actual level of dampness in the wall.
Reading soft woods
Moisture meters can be used to assess how damp wood is. It’s worth remembering that wood is never fully dry as it absorbs humidity from the air, so there’s rarely cause to concern if you have a reading of 10-12 percent. However, if moisture levels are over 20 percent, the wood can become vulnerable to rot. Saturation is unlikely to occur from humidity alone and generally indicates another source of moisture, such as a leak, rather than simply condensation.
The reason that moisture meters tend to be calibrated for use on softwood is that masonry is much more variable. The size of the pores in the material (eg, concrete) can have an effect on the reading as a moisture meter simply measures how conductive a material is. This is also the reason that salts can affect a reading, as can foil-backed wallpaper or insulation material.
Reading soft woods
As an alternative to moisture meters, you can use the carbide method to identify levels of damp, though this is more destructive, as it requires taking samples. These samples are then mixed with calcium carbide powder in a pressure vessel fitted with a gauge. The calcium carbide reacts with any water in the sample to produce acetylene. The volume produced indicates the moisture content present in the wall.
The carbide method can be carried out on site taking around 3-5 minutes per sample. However, it cannot distinguish between ground moisture and hygroscopic moisture, though comparisons between samples taken from the surface and deeper in the wall can give a good indication.
Restoration UK in-house laboratory
To get a true moisture content for plaster, brick, stone or mortar, it is best to have samples of the masonry tested in the laboratory, using the gravimetric method (drill out one or more samples of the material, depending on what you want the results to tell you). While this is also more time consuming and destructive than using a moisture meter, as it requires drilling holes in the wall and sending them to a laboratory for testing, the results are more accurate than both the carbide method and using a moisture meter. However, if samples are to be removed from site for testing, they should be placed in an airtight container to avoid any change in the level of moisture in transit.
Restoration UK has our own in-house lab, and can calculate the moisture content (MC) and hygroscopic moisture content (HMC) of a material, along with identifying the presence of common salts. Using multiple samples will provide a profile of the type of damp you’re dealing with – and what remedies are most likely to be required.