I have a customer who asked me to look at her buckling floor late this summer. I knew right away that it was a moisture issue under the house so I investigated. She had installed a central HVAC to years ago and our summers have been wetter than average. Based on my hydrometer readings the average moisture content of the joist is around 18%, but one beam is very high pegging out at or above 35%. I am trying to determine the best way to dry the space to begin necessary repairs to her framing and floor.

The house is over 100 year old and is pier and beam construction with the accessible crawlspace under the beams varying from 12" to 18". All of my previous experience and research says to use a 6mil or thicker vapor barrier on the ground and be sure that the grade is pushing water away from the house. The problems I'm having with this solution include: the grade of the yard and crawlspace are the even and there is little to no way to change that; the house is less than a mile from the Mississippi River and the ground water seepage often puddles in her yard and the surrounding lots; the soil content has a high level of clay; and a previous contractor put vinyl skirting around the base.

I'm concerned that using the plastic vapor barrier risks collecting water when there is severe seepage or heavy rainfall. Being a low pier and beam structure even taking the skirting off and adding a few fans is not creating enough airflow to dry the space. And almost all of the other builders I've discussed this with are equally puzzled on the best course of action due to the site specific issues. Any advice would be much appreciated.

35% moisture content? This high reading means rot is going to happen!
Cover the dirt with 6 mil plastic sheet. This will allow the existing conditions to taper off. Even if periodic puddling occurs on top of the plastic most of the moisture will be sealed under it which will keep the sub area drier. This is a good first step to moisture mitigation.

Since you can't grade away the water, maybe consider installing a sump pump and basin. You could also go the encapsulated crawl space route whereby you seal everything off and install a dehumidifier to condition the air in the space. That's the more expensive route however. It really depends on how much excess water you are getting. We don't have a lot of crawlspace construction in Canada so this is just from my recollection, not experience. I'd consider contacting an expert in moisture mitigation, like a restoration company.

I'd think about installing gutters and french drains around the house to divert water away.

8 mil black polyethylene plastic over the soil

Gutters/Drains to carry moisture away from the house.

Pete A, there is already rot evident in the beam with the 35% moisture. As for the other advice of gutters and drainage I think that is the best solution, but at this point the project seems to have exceeded the scope of work I'm capable of managing with my small business so I am bringing a bigger contractor. I'll definitely keep updates as things are resolved.

Kris, sorry for reviving this, yet I decided to leave my question here and not to create another topic, especially considering that it's a similar case. OP's client had a massive moisture issue. And in this case the moisture was likely present before install and the floor shrunk.

At any rate. Is installing a hardwood floor over a crawlspace risky? I am thinking about a situation where I KNOW ground water can be very high. Some crawl spaces in this area get soupy under the plastic ground cover in the crawl space and some even flood with inches of water.

So if one were to build in this area on as well a drained site as possible (still likely high ground water) and install a perimeter drain, is it still risky to have a wood floor on the main floor? Would a “total” encapsulation of the crawl space you've mentioned eliminate or help reduce the risk?

I'll be waiting for any replies considering this situation, so thanks in advance, gents!

- Robertha

Others may have more experience than I do, but I would say it's not risky as long as you've thought through and considered all the possible means for moisture to creep into the house from outside.

Aside from dealing the the external factors, you can also prepare on the inside for the worst cases by changing up what product you install. Engineered flooring with a cross-ply backer is going to perform better in high moisture than solid hardwood; you may also want to avoid maple or ash products since those tend to more susceptible than red or white oak. Glue down installation is more expensive than nail down, but you can use a glue product with added moisture protection, which will also help the situation.

At the end of the day, the other options are tile or carpet and depending on the design of the house, that might not be an option. Vinyl is a great compromise, giving you the look of hardwood along with resilience and moisture resistance. There are a lot of nice looking vinyl floors, both drop and click versions, that I would consider in lieu of solid hardwood in this case.

The issue was temporarily resolved using two squirrel cage fans to dry the space and keep continuous airflow, then a couple of months ago the fans stopped and it got bad again in about two week due to our ambient moisture (it's been a very wet year this year with the river staying high into September). The customer finally decided to invest the money to enclose the space and install a dehumidifier. Thanks for all of the input.