Wood is absorbent and contains moisture depending on its surrounding environment. The factual way is to remove a 100mm long piece of framing 100mm*50mm; weigh it, then dry it, then express the lost weight over the final dry weight as a percentage.
The reliable way developed to measure moisture contents of walls is using conductance meters. These have 2 needles that are inserted into the wood and pass a small current through the wood which is converted into moisture contents. More moisture in the wood means less resistance and more current the higher the reading. The needles need to be inserted at least half way into the framing otherwise temporary condensation or surface wetting would distort the reading. The needles they must be insulated otherwise the sides may be in contact with something other than wood which may influence the current and give a false reading. This is called invasive readings.
The Mdu Probe has insulated sides and the length of the needles is designed to reach into the timber framing where moisture of concern may be found. The Mdu Probes are permanent so second and third readings can be undertaken with accuracy at the same location as the cladding or GIB has not been compromised.
How often should the probes be read?
An accurate survey of your home will include taking moisture content readings at least in a summer to see how dry walls become and in winter to see how and where framing gets damp. If the timber is well treated and the readings are fine ongoing readings could be done every 2-3 years as little if any damage can be caused even if a new leak develops because the treatment protects the framing. The less treated framing will require more regular readings as exposure to dampness can cause significant damage in as little as 6 months. Another good time to read the probes is after repairs to ensure they were effective and met your expectations.
How many probes should be installed?
The first probe is the important one. This will prove to you all other inspections and testing was a waste of time and money. The more probes the more informed you will be. It is important to test the higher risk detailing like beside doors, under windows, below gutter intersections with walls, below rain heads and under decks. Basically anywhere where doubt exists.
What is a ‘normal’ moisture reading?
This is a bit like genetics. Different cladding systems perform differently. Ground floors are always cooler than upper levels so moisture is higher around the ground. South walls get less sun so they have higher readings. Cavity construction allows ventilation so this alters the humidity which affects the readings. Acceptable moisture contents range between 9% very dry to 14%. This does not mean 15% is high. What we find is most of the readings will be in this range so if any deviate from this normal range something will explain the reason. The wetter homes are Tripple S, Stucco and Harditex as they are absorbent claddings and the insulation is inside the walls. Bricks are a little drier. The driest homes are generally EIFS or poly claddings.
How much do moisture readings vary between summer and winter?
Normal variations are 1%-3% depending on the cladding type and how the cavity works. Where moisture variations exceed this it is likely the wall is becoming wet through a leak, wicking, and capillary or vapour transfer. These are the spikes indicating work is required.
What moisture level does decay start?
Decay begins by germination which requires liquid wetting normally for at least 10 days. This can be by rainwater through a leak, or by condensation. NB: Treated timber becomes toxic in water so prevents successful germination. Once germinated decay fungi need moisture levels of around fibre saturation of about 30%-36%MC to become what is termed incipient decay. NB: Treated timber prevents the development of incipient decay unless the germination occurred in nearby untreated material eg flooring. This level of decay is not threatening to the structure. After a couple of months continual wetting decay move to advanced decay which is where it becomes visual and by then affected at least 30% of the integrity of the framing. Once the decay is mature it can continue growing in as little as 16%MC as recorded house 217 and as low as 12%MC house 985. It is believed once decay establishes in untreated timber they evolve into reproductive phase and digest cellulose mixed with air to produce their own moisture. Historically treated timber homes (pre 1987) had levels of treatment to manage the severity of decay.
Does treatment chemical affect readings?
Yes. Copper, Tin and Boron are all conductors so reduce resistance which increases the moisture readings. MC readings of 22% can be lowered approximately 2%. Surface applied treatments like Framesaver and injected RotStop will allow even more current and require greater adjustment.
Does temperature affect readings?
Yes but only marginal so no adjustments are considered necessary.
Does surface condensation affect MC accuracy?
Yes substantially. Condensation is liquid water so readings can be generated up to 100% yet after a few moments evaporation will read lower and maybe less than 14%MC. You can try this by testing MC with needles and then wiping a damp cloth over the surface and it will instantly read 30%MC or more.
Invasive tests can be used to benchmark other forms of non-invasive readings but not the other way around. Relying on non-invasive moisture readings can be expensive. Think twice.
What is the cost to have moisture testing done?
Get the Mdu Probes installed first. Follow up readings are currently $225 Auckland area. By the 3rd or 4th reading your investment has paid off. Getting accurate results often saves owners a reclad. At the worst the results may confirm a reclad is necessary, but may be able to be delayed for a few years to allow finances
Do we do non-invasive readings?
Yes. There is a time and place for these generally after we have confirmation of leaks – we use these devices to narrow down the likely sources instead of removing cladding. We use the standard qualifications re limited reliability and we will not be held responsible for any inaccuracies.
What is proof of condition?
This relates to presence of timber treatment, decay and moisture. In other words if the framing is well treated and not already decayed and moisture content readings are all in the acceptable range that is evidence of good condition – but only if adequate probing is done, the results are recent and include a winter reading. Relying on moisture contents only is misleading and can lead to surprises.
How many probes have you installed and what do they tell us?
Figure below shows the first 27156 Probe results comparing incidents of decay (visual colour rating C and D) and moisture content readings. Of concern is 6044 probes show VCR C yet dry and 719 probes were decayed and dry. Scanners at best would pick up 1407 VCR C and 843 VCR D or less than 12% of problems. To date we have around 250,000 probes installed.Does decayed framing hold moisture?
No. Well not reliably as moisture is absorbed by the fibres of the framing. When the fibres are damaged they don’t hold water hence what is called false positives have evolved. Immediately after rain a probe may read 100% as the water is shorting it, but after a few hours may read zero again. This is why it is important to consider treatment, wood condition and moisture contents. None in isolation.