Moisture content in floor systems

For those that work in areas with crawlspaces, how many of you know when and why 18 percent moisture content in a floor system is a major concern?

Pest inspectors OK it as long as it’s not 20% or more. Similarly, I’ve never cited an issue as long as it’s < 20%. Willing to learn from others however.

FYI. From NACHI’s mold inspection course:

“Mold requires moisture to survive, so protecting lumber and wood structures from moisture will prevent mold growth. Mold growth can be limited if the MC of wood can be kept below 20%. Below a 17% MC of wood, virtually no microbial growth will occur on even the most susceptible materials. Southern Pine dimensional lumber is typically kiln-dried to maximum 19% MC or less. The moisture content will be identified on the grade stamp. Keep in mind, moisture content is related directly to particular substrates or materials.”

Not sure how it can be a “major” concern at 18% if it comes out of the kiln at 19%.

One has to be careful about measuring moisture content without various indicators to do such, I speak in particular of the Carolina’s where we have high humidity…add to the problem irrigation systems that are not properly set as well as the rain we have had lately, you are going to get a distorted view as to the moisture content of the joists.

Like most inspectors I have the cool gadgets however many times the best tool is simply a screw driver and a good set of eyes with the understanding of what one should be looking for.

!2% mc on trim is acceptable prior to installation…12% on hardwood flooring is not. 19 - 20% on framing members is fine…over that you end up having what looks like bad cuts…due to shrinkage… when inspectors see what appear to be bad cuts, many times its not…framers are simply working with what they have and under conditions they normally shouldn’t be installing lumber. How many of you have ever seen a framing crew cover up their lumber…rarely…even more so today with many construction crews consisting of illegal alliens that have no clue about building science…but I digress.

A sustained moisture laden crawlspace creates havoc with numerous components and system…be it creating conditions right for wood destroying insects, joist rotting away, insulation fallen down, hardwood cupping (the picket complaint from homeowners) as well as health issues as a result of fungi growth…all is moisture related. Moisture is the number one reason that homes have problems…be it attic, interior walls (plumbing), to the crawlspace.

Knowing how to manage the moisture will aid in the life of the home.



I have seen one flooring contractor drying subflooring to bring it to the 9%-10% range for installing hardwood tongue and groove according to manufacturers specs.

You will not find any kiln dried lumber anywhere near 18 percent, it comes out of the kiln at an average of 12 percent and is constantly air drying before it can reach the dealer. In some weather conditions it will end up being around 15 percent. When installed in a proper crawlspace it will vary slightly and end up still around 12 to 15 percent, maybe 16 during a very humid spell. (go to home cheapo and stick a probe into some wood there or just measure every house for practice) This does not include treated lumber, unless something has changed lately that stuff is around 40 percent when they get it to the dealer.

The house I did the other day had 18 percent all over which raised a red flag because:

The house had a vapor barrier already covering about 95 percent.
The block foundation was wet, very little efflorescence present but just wet.
I knew the wood was increasing in moisture, it had I-joists which are even drier that sawn lumber to start with.
You could also feel some excessive dampness under the vapor barrier.
With no actions taken, this house will be above 20 percent within 2 years.

Bruce i had the same thing here last week I beams where 25%
Sub floor same thing , Area was damp , down spouts where not away from the home
Steep hill with in 50 feet of the home
Fan running all vents open
Vapor barrier had moisture laying on top
Ducts where condensing water
I just simple put down the facts and statement there could a mold issue if not corrected this was a new Home
Insulation was already wet to touch
I think there a combination of issues Slope , down spouts, and being a new home where the soil has been disturbed,
I have no idea how they dry this But it will be a problem down the road.
Yeah it even had O-pipe around the foundation which dumped into a ditch No water was coming out though. Been raining hard here for the last 2 weeks . Builder didn’t think it should be a problem . and gutters with splash block was all they ever installed.

In western Washington, 18% is prime Anobiid infestation territory (13 to 18 %). Here I would definitely call it out.

According to IICRC 15-16% max is allowed, but they even say that at 15% mold growth can still occur.:wink:
I just make sure the drywall and carpeting above the floor is no more than 12%, the crawlspace as plenty of vents, and it has moisture barrier.