Moisture problem in new shop!

[Snipe Hunter]

I'll test the pin meter on my finger to make sure it's working. If it beeps and the light turns red, I'm drinking enough water. I'm looking for anomalies. Normal in an unconditioned space should be somewhere between 0 and 7%. The floor might be a couple % higher as it's the coolest part of the room. I suspect you'll see readings a lot higher judging by the amount of water the dehumidifier is extracting from the air. Remember that a dehumidifier is really just drying the air which would allow more water vapor to pull up through the floor from below. It isn't really doing much aside from speeding up moisture intrusion. Dehumidifiers are great for removing moisture in the air caused by the basement air being cooler than the rest of the house, making all the humidity in the house travel to the cool basement air. They really don't to much, if anything for moisture intrusion. As soon as you dry the air, it absorbs more moisture from the source. Like a wet paper towel can only reach 100% MC. If you dry the towel, it will pull in more moisture again.

[iublue]

I know you didn't ask this question of me but I think it's a poor test. It would not tell you the moisture content of the slab. To get moisture on the film, you would need to seal the edges to the concrete. You would also need to cover the entire floor to get a real clear picture. Using a moisture detector, I find huge differences in MC all over the place, I can see 30% (very high) and move the meter 2 feet and find 7% (quite low). The reason home inspectors use 19% as the critical MC is because mold won't grow well below 19% and the organisms causing wood decay will be much less active below 19%. 19% is still very high. As far as MC goes in a dwelling, anything above about 7% is a bit high but manageable with de-humidification. A radon fan and a proper vapor barrier will usually be closer to 0. But I'll still see 5-7% in the walls and 2-4% on the floor often enough. Especially in the humid summer months but that can also be attributed to condensation and moisture in the soil behind the walls. The idea is to stop the moisture intrusion, not remove it after it comes in. Removing it is always more difficult. In a home, a french rain is the last resort and a lot cheaper than digging up around the foundation, installing a drain tile system and a pump and re-sealing the walls.

The reason I recommend a radon fan is because I almost never find high MC in a basement floor with an operating radon fan with the exception of very old homes where the crushed stone under the slab is either non existent or water table is high enough to fill the voids between the stone or the voids have silted in or a sump pump has failed... any reason where the air gaps ave filled with soil or water.

I got the meter and made measurements are many different places all over the slab.

Readings were about 2% apart with the high at 15.8 and a low of 13.5.  I didn't average them out but more readings were towards the low end so I would think the average would be around 14 - 14.5%.
So higher than optimal but not into your very high range.

Thanks for the information.

[Snipe Hunter]

I got the meter and made measurements are many different places all over the slab.

Readings were about 2% apart with the high at 15.8 and a low of 13.5.  I didn't average them out but more readings were towards the low end so I would think the average would be around 14 - 14.5%.
So higher than optimal but not into your very high range.

Thanks for the information.

Not high enough to grow mold (19%) but 14-15% is still high. Your concrete is 14 to 15% water. That's a lot of water. Normal would be around 4-5, maybe 7%.