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%.

[iublue]

An update on the moisture "problem".

I actually have begun to consider it a blessing.

Last winter, with the small dehumidifier being emptied 1 1/2 times every day, the RH in the shop was very stable.  Right between 52 and 58%.
Obviously working with wood it is desirable to have a temp and RH controlled environment.
In the summer the AC took care of any excess moisture.

Fast forward to this winter and it gets weird again.
I have only emptied the dehumidifier three times the whole winter!
Once the RH got down to 50%, I thought, good grief am I going to have to buy a humidifier??!!
It never got to that and it is still very stable.

It will be interesting to see what happens in the future.

[MstrCarpenter]

Do you think it's possible that your footings and stem walls were actually the sides of an in-ground pool? Only thing missing would be a bottom that doesn't leak (i.e. a layer of clay).

[jteneyck]

Sounds logical.  The slab dried out some more.  I run my dehumidifier all summer long to keep the shop at 50% RH.  In the winter I need to run a humidifer.  If I didn't, the RH would drop to less than 35%.  It is amazing how fast the RH changes in response to what happens outside.  You would think with most of the basement walls underground that that would dampen the response time to what's happening outside, but that's not the case.  If it's screaming cold one day the humidifier runs hard.  If it's warm the next, it might not run at all.  And the reverse in summer.  

John

[iublue]

Do you think it's possible that your footings and stem walls were actually the sides of an in-ground pool? Only thing missing would be a bottom that doesn't leak (i.e. a layer of clay).

No, the footings and stem walls were put in specifically for this building in the corner of a farm field.

[iublue]

Sounds logical.  The slab dried out some more.  I run my dehumidifier all summer long to keep the shop at 50% RH.  In the winter I need to run a humidifer.  If I didn't, the RH would drop to less than 35%.  It is amazing how fast the RH changes in response to what happens outside.  You would think with most of the basement walls underground that that would dampen the response time to what's happening outside, but that's not the case.  If it's screaming cold one day the humidifier runs hard.  If it's warm the next, it might not run at all.  And the reverse in summer.  

John

John,

As I said, it will be interesting what happens going forward.
Will it get like your basement when I have to introduce water during the winter?
Has it reach an equilibrium and future years will be like this last winter?

I wonder how the outside moisture affects the interior of the shop since all the walls and roof are sprayed with closed cell foam which prevents water transfer.

Almost like a science experiment!

Toney

[MstrCarpenter]

No, the footings and stem walls were put in specifically for this building in the corner of a farm field.

I wasn't suggesting that you actually built on an existing pool; only that if there was no way for water to drain, any rain between the time the walls were poured and the weather tight shell was completed may take a long time to escape. Probably not years though.