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I recently purchased around 600 sq feet of 3 1/4 inch solid hickory from Bruce, which I'm installing over a crawl space. I put down a 6 mil vapor barrier in the crawl space.

I'm measuring the moisture level of my plywood subfloor as the wood has been acclimating about a week. Most readings are right around ~13% with the overall range being 10-13% with some spots measuring as high as 14%. This is during the humid August summer in Kentucky.

The wood still has some time to acclimate. Readings for it at the moment are around 9-10%.

I'm wondering whether I'm in a safe position to install solid hardwood here. I'll be installing this with a #15 felt underlayment. If it matters the bedrooms have 2 1/4" red oak installed (might be original to the house) and those floors are in great condition.

  • Welcome to SE. Are the bedrooms also over the crawl space? – isherwood Sep 1 '16 at 17:12
  • Can't comment on the principal question, but did you consider ventilating the crawl space as insurance? – Aaron Brick Sep 7 '17 at 17:23
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    I had a house with almost identical description, 6 mil in crawl space but was in the northwest where the rainy season is almost as long as the year. I had straight grain oak that was original to the house and it was just on top of the ship lap boards that was the decking. Built in the 40s. Didn't have any problems at all as long as the crawl space itself was well vented. Be sure yours is and I think you are fine with what you have. – Micah Montoya Dec 17 '18 at 18:11
  • What is the thickness of the wood you plan to install? – peinal Oct 26 '19 at 1:40
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    My house was built in 1956. One story rancher over a ~4ft crawl space. Floors are all oak strip over 1x10" (?) diagonal subfloor. Crawlspace does have a 6 mil vapor barrier on the dirt floor, but I have no way of knowing how long that's been there. Anyway, have no issues with any of the flooring. Just had them all refinished 3 years ago and nothing had to be replaced. – SteveSh Dec 21 '19 at 14:14
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I had a similar situation with a house redo with a crawl space. This house is near the coast in Maine and both the contractor and flooring company strongly recommended against using tradional wood flooring on the first floor. They were concerned with cupping of the wood. It got to the point where the flooring company - all they do is wood flooring- would not warranty the work unless we used engineered flooring. It was about $1.5 more per foot but it’s been 3 years and have been very happy

  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know the details of contributing here. – Daniel Griscom Mar 3 at 15:22
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The crawl space must not be more moist than the space above it, or the moisture will migrate through your flooring. You've already placed a vapor barrier in your crawl space, so we assume you have a non-ventilated crawl space. This means you have no foundation vents around the perimeter.

For a sealed crawl space, the vapor barrier must be sealed (i.e. taped) at the overlap seams. Run the vapor barrier up the foundation wall up to, but not over, the sill plates. Roll the vapor barrier around some PVC or wood strips, then screw to the foundation walls with Tapcon fasteners.

For a ventilated crawl space, the vapor barrier on the ground is still a great idea. Be sure you open your foundation vents when the outside temps start to rise.

Wood materials must not be trapped inside a moist cavity. If you were to put a vapor barrier under your new wood floor, you'd trap moisture in your crawl space, which would eventually rot your subfloor and floor system (floor & rim joists, and sill plate).

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Your post reads like you have an understanding of the implications of moisture on a hardwood floor. I think the existing oak floors are a good indication that your project should go OK. The key points that I have to offer are:

Ventilate the crawl space. Covering the ground with plastic is not a substitute for ventilation. Let the flooring, sub-floor and hickory, dry before installing the hardwood. Six percent moisture is a good target for both. I like to let the hardwood spend some time, sticker-stacked with air-space around each board, in the environment that it is to be installed in. Installing hardwood directly from garage storage, to an air-conditioned bedroom, for example, may lead to floor board shrinkage.

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...Assuming you have some sort of flooring that is removable, I would consider using marine plywood and replace that area. That is what we use in extreme wet conditions such as a car wash. The marine plywood withstands water and moisture during all operating hours and is exposed without problems. EDIt: Is this actually in regards to the crawl space if not why is the question worded as such with all the extra info.. Cheers

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    The plywood mill I worked at when we made marine plywood the main difference was the veneer sheets, the sheets used for marine grade were of higher quality small tight knots on the full sized sheets and the core was the same or plugged so there were no voids but it used the same glue and press time. – Ed Beal Dec 22 '17 at 16:35
  • Although the marine plywood patches may not rot, all other floor system components will rot with long term moisture exposure. – ndemarco Dec 15 '19 at 13:57
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A sealed vapor barrier is indicated. More importantly, there should be insulation below the subfloor. If the crawl space cools the floor below the dew point of the air in the room, there will be expansion of the flooring, and there could be condensation.

Rigid foam insulation, cut to size, with suitable R value for the climate is indicated, and recommended. Remember that household preferences and conditions will change. A house with one person in it may be dryer (lower dew point) than one with children, pets, fish and plants. In the dead of winter house with a higher dew point may have condensate or frost accumulating on (relatively) un-insulated surfaces.

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