Fear of water vapor when installing hardwood floor

Q: Our house was built in 1976. I am unsure of how thick the slab on grade is, but my wife is concerned about the emission of water vapor now after cruising the internet as I just started to lay down the hard wood floor. THANK YOU INTERNET, THANK YOU. Before I go any further, I need some re-assurance to ease her worries or a slap on my wrist with re-education to what I have been taught.

My preparation:

1) Carpet in living room and hallway removed to expose the concrete slab. No cracks were visually present and no visual defects with the exception of about 40 years of spill stains that washed right up.

2) Vinyl tiles in kitchen and dining room removed to expose the concrete slab. The tiles were not terribly difficult to remove, but it was quite a job with pry bars and scrappers.

3) 3/4″ tongue and groove OSB was laid down and ram set into the concrete with about 16 to 20 nails per sheet. 2 weeks exposed to allow the moisture levels to equalize to the relative house levels.

4) #15 roofing felt rolled over top of the sub floor.

5) 700 sq ft of reclaimed 3/4″x 2-1/4″ hardwood flooring moved into a neat stack in the living room and allowed to sit for 1 week before installation began. About 80sq ft of floor laid down so far.

Insert Internet fears here and I will now list my findings of readings from different methods of moisture readings.

1) Plastic sheet method – 18″x18″ plastic sheet was laid down on the concrete and taped down around the edges using t-rex tape. To further seal, I sprayed the edges of the tape with hair spray.
*Result – Removed the plastic 24 hours later to reveal visually dry concrete and plastic with no signs of water droplets on the plastic and no color variance between the covered and exposed concrete.

2) Moisture Meter – A moisture meter was then purchased and readings have been taken from every room after re-exposing the concrete in spots.
*Results over a 2 day period:
Doorwall in kitchen – 10.2%
Dining room – 9.8%
Living room – 9.2%
Hallway – 8.9%
Bathroom – 7.6%
Doorwall in kitchen – 10.9%
Dining room – 10%
Living room – 9.6%
Hallway – 9.1%
Bathroom – 8%
Now that I have a new toy, I have been testing the moisture percentage in everything I can. The upper cabinets have a moisture percentage of 10.7%. The OSB sub floor fluctuates between 3.6% and 4.8%. Hardwood floor is 5.9%. Now having the meter, I will do another plastic sheet test and then take a reading directly after pealing the plastic up.

3) My next test is to purchase a hygrometer and then seal it under plastic for a few days and take the RH readings while it is sealed under the clear plastic.

I do not know what the exact RH of the house is because I do not have the hygrometer yet. We do have a humidifier that is turned off during the summer and has recently been turned on for the winter and set to the lower end of the comfort level setting. I have been through a wide variety of the building trades throughout my employment history, but with all my experience and knowledge, my points of the argument does not ease her mind.

Husbands point of the argument:

1) If there were a high abundance of moisture vapor, the kitchen tiles would not have been so hard to pull up.

2) What is the point of laying down 6ml plastic over the concrete when I am going to put thousands of holes in it from the flooring spikes?

3) If there is a problem with cracks in the slab with an abundance of water coming through, your floor covering doesn’t make a difference. your problems are bigger than she realizes and a 6ml sheet of plastic is not a magic layer to protect.

ANY advice on my current situation is more than welcome!

A: I agree with husband. You’ve not seen any evidence of moisture anywhere. Not on the carpet or tile. Not on the OSB. Not with all your tests. I think it is safe to say, your concrete does not have a river running under it.

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