Crawlspace moisture question


I am an inspector in training with question regarding crawlspace moisture. Are we measuring the moisture content of the framing members, block, etc, or the humidity of the crawlspace air, or both?

Also any recommendations about testing devices for any or all these considerations would be greatly appreciated.


You need tools to measure all of the above. buy good equipment, and take pictures of the readings you find.
Crawlspaces are the most challenging of all to inspect. No one can teach you. YOur best learning tool is experience and evaluation based on how the space is performing. Aged homes can teach you a ton about what not to do.

Your eyes and flashlite and a long thin screwdriver are your most valuable tools for crawlspace. No need for expensive high tech gear. Remember- its a visual non invasive inspection. Look at the foundation for signs of water staining and cracks. Joists for cracks ,plumbers cuts , support and water stains- this is where the screwdriver comes in handy when you see if that tool can go through the wood. Check pipes , insulation etc. if you get any gadget- get a moisture meter for checking those wet or stained joists to see if still wet or prior damage. Plus you need to crawl around in about a dozen plus crawlspaces and then get your own feel for what you think You need tool wise . Its a never ending learning career in my opinion Good luck and welcome

Agreed. I wouldn’t want to take anything expensive into most crawls I encounter. Besides, I usually only have room for a flashlight and poker :slight_smile:

This is a great crawlspace tool box :slight_smile:

Don’t get the folgers one with the crappy lid. It will just keep coming off.

I would say none of the above. For a crawlspace vented to the exterior, measuring moisture content is not going to matter as it will be changing with the weather. For one vented to the interior(or exterior for that matter), the visible installation will either be adequate to resist collecting moisture or it won’t.

For instance, basically:

Vented to the exterior, needed - proper amount and placement of vents to allow airflow throughout the space, moisture control at the exterior, vapor barrier at the crawlspace floor, insulation at water pipes and possibly at floor with vapor barrier turned toward the interior(if cold climate), air and vapor sealed at the interior surfaces.

Vented to the interior, needed - air and vapor sealed to the exterior, conditioned air provided to interior, vents open to the interior, insulation at the walls(A good setup is rigid taped at seams and batts to increase the R-value), vapor barrier at the floor and overlapped up the walls.

The area of the country will change what may be needed, but that gives you a rough idea.** That is all just a visual inspection, no gadgets needed**. Most of the time the difference between 60-65% humidity or higher(noticeably damp) and a more proper 30-50% humidity is easily picked up by your nose and skin. It may just take some experience to tell the difference. Good luck.

I would not worry too much about determining moisture content during a home inspection, because it is not in the SOP. Unless you are confirming a water leak.


IICRC S500 says the moisture of the wood should not be above 16%.

CDC recommends humidity below 50%.

Nice summary.
Where is the proper placement of a vapor barrier at the floor level of a crawlspace?
Attached is a photo of a 40+ structure in NW Michigan, cold climate, dry dirt floor despite being on a lake roughly 4’ above lake level.
As can be seen, a plastic vapor barrier was laid over the joist, covered by a layer of felt paper, plywood and hardwood flooring. Fiberglass insulation in joist spaces with kraft face stapled to the bottom of joist. It appears a natural gas forced air heating system with supply & return ductwork was added 23 years ago.
The photo shows the insulation sagging & pulled down, generally throughout the floor structure and the vapor barrier and felt paper have been cut as can be seen.
My question is why would anyone do this. There is little evidence of moisture problems that I could see that would warrant such activity. Could the double vapor barrier be a problem?
Anybody have any input?

The double vapor barrier would have little affect on the crawl if it is properly ventilated. It may have a detrimental affect on the subfloor if there was any kind of leak from above. Perhaps that is why someone cut it open, to allow drying out after a leak or spill. Haven’t ever seen it installed that way; it is odd.

It may vary based on the area of the country, but in the central Midwest, the floor of the crawl is the best location for the vapor barrier. Prevent the moisture from reaching cellulose based materials and have a way to dry towards the interior if it does enter.

For 40+ years the picture looks like it’s one dry crawl. Just to add to Cameron’s comments, I always recommend running the vapor barrier up the walls & attaching it to the sill, this will keep any moisture that penetrates the wall from getting into the crawl as well.

IMO the best way to go for any crawl is to encapsulate it & condition it.

I don’t take measurements of crawls as it can and will change if it’s a vented and not an encapsulated & conditioned crawl.

For me, what I smell is usually the first sign of any problems, then if I see signs of moisture or moisture damage I report as such.

Something I learned long ago, The crawl is part of the house, what’s in the crawl will get into the house.

lol smell in a crawl here can tell you a lot . mostly dead rotten cats, possums . unless you under there with Sean after eating at Shoneys then that’s another story