When does a moisture difference make a difference?

Just got a Flir B40 a couple of days ago. Looking in the attic of a condo building, we found evidence of moisture. The moisture reading of dry wood was 4.8. The reading of the affected areas ranged from 7 to 10. Could this be latent moisture from before the repairs were made, or is this evidence of a continuing problem? The repairs were made about two years ago.

What differences in moisture readings made a difference?


Sub flooring usually has to be 9%-10% before you can install hardwood flooring so that sounds pretty normal to me. I’ve taken readings of floor joists in unconditioned crawlspaces in the 12%-14% range.

Denny Denny Denny - Moisture and determining its causes and if it active or not takes alot of practice and experience. Now you have the Thermal Infrared Camera…Now is the time to go get trained on it. This piece of equipment is only as good as its operator. Did you find evidence of moisture or a thermal anamoly? The camera does not detect “moisture” the camera detects minute differences in temperature. I have had a thermal infrared camera for over 5 years now. There are still times you need to look and re look at the screen to determine what is going on, and then it sometimes even takes a little more than the camera, like a moisture meter.

In short - Go get professional training on how to use the camera, or instead of looking professional you will look like you have no idea what your doing. Good luck!

I should have said that the 7 to 10 readings were from a moisture meter. The condo is the one I live in. I’m just practicing and trying to get a feel for it. I plan on getting training, but it’s my experience that using the camera first will provide me with plenty of questions when I do get the training. Right now, I’m just playing with it. The only person to whom I will look like I don’t know what I’m doing is myself. And I know that.

We found what looks like evidence of a leak. The moisture meter confirmed a higher level of moisture. My question remains, what’s enough of a difference to make a difference to make a difference?


You will get your answers when you get educated with proper training and experience.

4.8% moisture in a standing home? Are you in Death Valley?

Answering this question depends on your moisture meter and the material tested. OSB for example has its properties *changed *after it becomes wet and then subsequently dries. If the leak existed for some time and was then repaired, chemicals leaching out of the shingles will change the composition of the OSB and give different dry readings than before the wood was damaged.

Meaning …OSB that has been exposed to moisture and roofing chemicals for a lenghty period of time and then dries, may exhibit slightly different moisture properties than undamaged OSB. Slightly different moisture meter readings aren’t that unusual. The only way to be certain is to take comparative readings after it rains. If the level goes up…you have a leak.

A DOE study in the late 1970’s found the summer moisture content (in Minneapolis) of wood in walls retrofitted with cellulose was down in the 3-4% range!!

Brian, would you post a link to that study? The average annual humidity for Minneapolisis about 70%. I’d like to learn how adding cellulose can reduce moisture levels that much. Basically it sounds like they were saying that adding cellulose insulation prevents wood framing members from reaching equilibrium moisture content with the homesite environment.

I hope no one is assuming that if the air is at 70% relative humidity that building materials will also be at 70%.

Standard air has the ability to hold a certain amount of water vapor at a specific temperature. This amount is 100% relative humidity.

Air and other building materials do not share the same properties (or the ability to hold the same amount of moisture per unit), therefore the amount of moisture in 100% relative humidity air at 70° is not the same amount that represents 100% relative humidity of a 2 x 4 (35%).

I have some documentation around here somewhere about the ratios of air and other materials but they’re not at hand, sorry.

[size=2]Percentages between air and wood are measured on different scales; relative humidity% for air and moisture content % in wood.
One of the ratios off my top of my head is that 20% relative humidity of air is equivalent to 5% moisture content of wood that has a fiber saturation point of 35%.

Seasonal fluctuation of 5%-15% is considered normal in bare wood.


When I suspect moisture intrusion I adjust my meter depending on material being tested, check surrounding areas to attain an average, then check suspected area/s in an attempt to determine if there is an active leak.

No matter what you find I think it’s always important to inform your client that the readings do not guarantee the presence, or lack of, an active leak. For example, if there has been no rain for a while those attic stains may be from an active leak that is inactive at the moment, if that makes sense. Or if that stain in the wall is from a plumbing leak it may have formed a temporary seal which of course will “unseal” sooner or later.

Someone tell me why we bother with moisture meters again?(8\)

To cover our butts.

InterNACHI has a free, online moisture course coming out in December.

The ASHRAE standards for normal moisture content is >15%mc, 16% - 19%mc = possible fungal growth, 20% + mc action level.