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Myths surrounding Moisture Content

This page is aimed at dispelling some of the myths associated with the moisture content of framing - and the moisture meters used to obtain the readings.

The most common meters fall into one of two categories:

(a) scanning type - no mark left by their use

(b) invasive type - a pair of holes is drilled, either from inside or outside

(I don't consider a thermal imaging camera to be a "moisture meter" by any stretch of the imagination, this tool is discussed here )

Almost all prepurchase inspectors have one of the scanning type meters nowadays and I have seen many reports where readings from these meters are listed, giving the impression they represent actual moisture contents. When I see that, my heart sinks and any respect for the author evaporates instantly.

Why? Because the readings are only useful relative to each other, they don't state the actual moisture content of anything, they are at best indicative and at worst positively misleading.

The "oven-dry" test

So what is this moisture content to which so much importance is attached? One way to understand it is to visualise the following: imagine taking a bit of framing timber and sticking it in an oven for a while - until you are sure every trace of moisture has evaporated from it. Now take it out and weigh it instantly. That is your "oven - dry" weight. Now just leave it alone for a few weeks, say in your bedroom under the bed. Then weigh it again. In Auckland, it will probably be about 9 - 14% heavier in summer, and maybe anywhere from 10% to 20% heavier in winter - just by virtue of having absorbed that much water vapour from the air and achieving "EMC" (Equilibrium Moisture Content). Then leave it outside in a wet ditch for a couple more weeks, that will probably add over 100% to the weight. Amazing how absorbant Pinus Radiata is!

The probe meters (where you stick a couple of needles in) do some internal analysis of the resistance between the ends of the probes where buried in the wood and produce a figure that (hopefully) approximates what you would get if you did the "oven-dry" test. These are the meters used by your building inspector who doesn't want you to line the inside of your house until the framing has dried out enough. Works reasonably well on nice new framing timber - but the more decayed the timber is, the less accurate these meters become. That's not the only problem - other things also mess with their internal calibrations - e.g., various salts and chemicals used to treat the wood to stop it rotting.

The short version is, most meters are easily fooled and may be giving very different readings to what would be found if the formal "oven dry" test was done.

So what is the problem with the timber being a bit wet anyway?

Decay fungi and moisture levels in Pine framing

Moisture content percentages are very commonly quoted in connection with fungal decay. We have moved now beyond your common prepurchase report and into the realms of a WHRS or other weathertightness report. Such reports invariably have dire things to say about the inevitable onset of fungal decay in wet framing.

How realistic is this?

Well, the bottom line is, the spores of decay fungi are omnipresent as are almost all the conditions necessary for their rapid proliferation: oxygen, the right temperature range and a food source (e.g. un/lightly treated timber framing). Basically, the only ingredient missing is moisture.

So yes, the risk is real - and some species are so determined that even quite nasty treatments do not hold them at bay for very long once water is added. It just becomes a question of how much water, for how long. Very few fungal species like completely saturated wood - there is not enough oxygen - but around 40% MC is suitable to a large group and it doesn't take a very big leak to allow Pine to soak up that amount of water.

So if your report contains a few (accurate / resistance meter) readings of around 40% and the author thinks the wood has probably been that wet for a while, the chances of decay are pretty high.

It is also true that readings of 35 - 40% are generally not particularly controversial. Probably everyone agrees there is a problem. The more interesting discussion is to be had when the readings are much lower, say 15%. I see so many reports that give the impression a reading of less than 18% means there is no problem.

How decay messes with so called "accurate" (resistance meter) readings

The resistance meter measures conductivity between the two probes where they are shoved into the wood. It assumes the wood is clean, with no decay. Great for building inspectors looking at new houses. But what if the timber is decayed? For starters, the simple volume of fibres between the prongs is now much less (the fungi have had lunch). Most of the little I know about density comes from this Radiata Pine Breeding Company bulletin publised in 2003. The essential point is that density is closely related to structural strength, and after decay fungi have had a feed, the density (and strength) is likely to be dramatically reduced. For example, when decayed, the timber may weigh only half of what it did yet still look remarkably similar. All the resistance meters are calibrated on the basis of a presumed density - if the density of the wood being tested is only half what the meter is assuming it get the picture.

For example:

rotted timber

This timber is very obviously severely decayed - it quite literally came away in my hand, although there was not the slightest visual indication either externally or internally in this area - this advanced decay was completely hidden.

However, the real point of these photos is to show how timber very close to the obvious decay appeared clean, hard and dry:


The accurate meter is showing 14%, it is a heavy set of prongs which is driven in and the timber is (just) strong enough to support their weight, despite its decayed state. 14% is generally considered a nice, safe, secure reading.

This illustrates why it can be unwise to rely just on accurate readings. And it is one of the reasons why I like The Probes - shavings are collected and the timber strength tested at the same time the accurate readings are taken so you get three very useful bits of information from each one. A probe inserted in the bottom plate in the corner of this room from inside would have at least found an area which required further investigation.

More about the non - invasive meters

Which are even less reliable than the "accurate" readings from a resistance meter described above. How unreliable?

Well, here are a few simple experiments you can do in your own backyard. The Humitest MC50 is a commonly used meter - and an extremely useful tool too, many times it has found areas for me to investigate further which, without it, I probably would have missed altogether. But it generally doesn't go any further than the first material. For instance, if you are using it inside and the gib (plaster board lining) is wet - it is just great at finding that, even when there is not the slightest hint visible to the naked eye. However, say the gib is dry .... then even if the stud behind is soaking - this tool is no use whatsoever.

Don't believe me?


The set up:

bone dry gib, bone dry H3 4x2 to the left, soaking wet 4x2 to the right

The Humitest ranged around 8 - 12 in the gib where it was not above any "studs"

No significant change over the dry stud

Negligible change over the soaking stud

Now lets try a similar test, but using a couple of thick nesses of fibre cement, starting with really thin stuff (4.5 mm)

Here it is where there is just empty space under the sheet - 23.9

Lets try it over the wet "stud":

30.5 - very small rise. Better try it again in a few places where there is nothing underneath at all, just to make sure:

28.8 - these are all within the range you can expect even when there is no problem!

Now lets have a look using texture coated 7.5 mm fibre cement - a common enough cladding in the real world -

13.8 in the middle (nothing underneath)

12.8 over the soaking wet "stud" - less!!!

So, clearly some limitations with this tool!

The microwave is another commonly used one - a bit more expensive and quite a lot harder to use in the sense that it really does read quite a long way - definitely beyond the surface material - and there can be so many "false positives"

What's a "false positive?

A classic example I saw in one report turned out to be the meter "reading" a metal corner behind the plaster. Other examples: pipes, wires, steel pates,'s a long list.

I have even had the experience of using this meter on a very rainy day (when the outside face of the cladding was obviously wet) and getting high readings from inside which I eventually figured out were due to the meter reading right through the wall to that water on the outside face!

About 5.1 over the area where there is nothing under the gib

Up a significant amount over the tanalised stud, to 18.3 and

Up a whole bunch more over the soaking stud. This is more like it!

Similar results over the two different thick nesses of fibrecement. This meter really can read through the first material. Now you just have to learn to deal with all those "false positives"!

So I hope this makes a few things clear:

1) Not all non - invasive meters are the same

2) None of them give you figures which represent the actual moisture content of anything

3) All moisture content readings, even those obtained by resistance meters, must be considered in context - the reading alone means virtually nothing in terms of decay or damage.

These fruiting bodies are growing on a bit of framing that was at 15% ("accurate" - Ha!).

At the other end of the scale, I have sent off samples of kiln dried untreated framing at 40% (accurate) and had the lab tell me: "no decay fungi, nothing at all wrong with this sample".

But that is rare, it is far more common for me to send off a sample at 18% or less, as above, and have them tell me: "Boy, is that sample had it, a large percentage of the structural integrity is gone, bits like that must be replaced now!"