Non Invasive Moisture Scanning - How it works, Advantages, Limitations.
Excess moisture and humidity left within a building environment or its structural elements is the cause of damage to structural materials such as rot, mold growth, reduced insulation values, failure of flooring, roofing and wall support, it costly to repair and in some instances detrimental to your health.
The signs of moisture ingress are not always visible, and most homeowners are not aware that moisture ingress is occuring until it has escalated to a large problem.
Scanning walls with non-invasive moisture meter is important because it could identify areas where moisture intrusion is occuring at potentially early stages but still not visible by a 'naked' eye.
However, along with these advantages , they also have significant limitations.
Having a "clean" report with "no elevated readings were noted" during the inspection done at that particular day does not always mean that moisture related problems are not exist!
That's why during our inspection we also put a lot of time and effort to identify any visual signs of water ingress as water stains, damage, rotten or rusted elements - all what we call "Warning Signs".
Using our three level approach - checking on particular attributes and risk areas, visuall assessment of warning signs and providing non-invasive moisture scan we can give our clients a better understanding of current state of a house which decrease the risk of buying the house with inherited issues.
Some of our instruments.
HOW IT WORKS:
How does a Moisture Meter work?
Most pinless (non-invasive) meters use the capacitance method, which uses the relationship between the moisture content and the dielectric properties of the wood. Using the principle of electrical resistance, pin-type meters use the board as an element in a circuit by driving two pins or electrodes into it. This method works because moisture conducts electricity well and dry wood is an effective insulator.
Both the resistance and the dielectric properties of wood change in direct proportion to its moisture content, within a specific range.
ADANTAGES:
Scanning walls with non-invasive moisture meter could identify areas where moisture intrusion is occuring but still not visible by a 'naked' eye.
While an invasive testing is best to test the element, is not always possible or permissible. Non-invasive meters are very effective for initial troubleshooting and inspection, giving a quick indication of the location of elevated moisture and its source as they leave no trace of their use and cause no damage to the materials being tested.
Some examples: Moisture related issues found with help of Non-Invasive Moisture Scan
Bedroom. "Issue was fixed" said homeowner. No visual signs of ingress but High moisture reading were spotted. Invasive inspection - damage, rot.
Shower. Hidden issue - waterproofing failure. Spotted by high readings by meter and stains in an adjacent wardrobe. Extensive damage.
LIMITATIONS:
The capacitance signals used in these instruments to identify moisture lose strength rapidly as penetrative depth increases. Therefore the readings tend to provide more information about the cladding, or lining, than the underlying framing. They are also affected by density changes in the material, for example, knots in timber, and to other factors as weather season, particular day conditions, house sun and wind orientation, not to mention that moisture presence can be tempered by the owner disgusing issues by drying or replacing some elements, etc.
Non-Invasive moisture meter will not detect or measure moisture through any electrically conductive materials including metal sheeting or cladding, black EPDM roofing, butyl roofing, aluminum siding or wet surfaces.
Extract from incodo.co.nz by Paul Probett (Characteristics and Defects - a study of Weathertightness Determinations Sue Clark September 2006)
“30% of buildings showed elevated moisture levels when using non invasive moisture meters, but more than 65% of buildings had elevated levels when invasive moisture tests were performed” In other words non invasive moisture testing is failing over 50% of the time. The inference is that some buildings maybe determined as dry in whole or in part when they are not.
Example: Moisture scan with non invasive moisture meter - a DRY, high density GIB board is "blocking" an accurate reading, providing FALSE negative.
Competely wet peace of timber.
Direct Scan - HIGH readings.
READINGS USEFUL FOR COMPARISON ONLY:
Non Invasive Moisture can be used to detect and identify areas of elevated moisture within or behind most types of wall and floor coverings. These meters detect moisture in underlying substrates like framing and normally have a search depth of 16–100 mm., but results must be treated with caution.
Using moisture meters on materials other than exposed timber is not a reliable indication of the amount of moisture below the surface. As calibration is not practical because of the variation in composition of these types of construction, tests should be carried out on only a comparative basis.
The information gained can only be useful in comparative terms, not in absolute terms – when readings in known dry areas are low but are higher in a suspect location of similar construction, there may be elevated moisture levels.
IMPORTANT:
Please also refer to other sources of information and your legal aid to learn more about this subject as risk associated with moisture damage to the dwelling is very high and cost of remediation could be very substantial. Remember - your general pre-purchase/ pre-sale inspection is NOT a Weathertightness Assessment.
With regards to monolithic clad (plaster) houses, especially build with no cavity and questionable timber treatment (usually 1998-2004) the full Weathertighness report done by certified Assessor is highly recommended.
Source and more information could be found here:
www.buildmagazine.org.nz-Using-Moisture-Meters-Correctly
Example: Moisture Inspection Process (Scanning)
Room to Scan: Areas to Scan are identified Scan under the Window corner: No Elevations Scan at the bottom corner: Elevated readings.
DIFFERENT TYPES OF MOISTURE METERS
There are two types of moisture meters commonly used in lumber and woodworking applications.
A pin-type uses two pins that penetrate into wood at the users desired depth. Pin-type meters are the only instruments that indicate the moisture conditions inside a board or a piece of wood. Using insulated pins, only the uncoated tips are exposed to the wood fiber, providing more accurate readings of moisture content at various levels of penetration. Using a pin type meter with insulated pins is still the most effective method to determine moisture gradient, which is the difference between shell and core moisture content.
Pinless moisture meters read moisture closest to the source of the magnetic field, in this case, at the surface. For quickly scanning finished product, a pin-less meter is a convenient way to identify a problem area. However, a pinless meter cannot differentiate between shell and core moisture content and will not detect a moisture gradient. Also, readings provided by a pinless meters are affected by surface moisture.
Please refer to the diagram below for a basic look at the difference between readings obtained from a pin-type meter compared to a surface meter.
This diagram illustrates that pin-type meters and surface meters provide different information about the moisture content in a board. Each technology may be appropriate for different applications.
Scan "through" attached GIB - DRY readings.