A question about limestone walls in basements

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So had a client reach out to me asking to verify something their realtor told them, and I wanted to tap y’all’s knowledge as I am not a mason.

There was evidence of moisture on the walls in the basement, and the walls were of limestone. The realtor told my client that the owners had a dehumidifier in the basement to help with moisture and it sucked moisture through the walls. Now I know limestone is porous, and in theory I suppose it would be possible to pull moisture through it, but it sounded like a load of horse manure to me. Theoretically possible, maybe, but not likely. But I figured I would tap y’all’s knowledge.

Thank you in advance!

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Yeah, I think I’m with you… doesn’t sound right to me either. I don’t have any specific experience with limestone but just the concept being sold doesn’t add up. A dehumidifier doesn’t “pull” things. It just removes some of the moisture from the air. With there being no air exchange to the outside (as in, the dehumidifier just cycles existing air through it) I don’t see how what they are saying is possible.

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A dehumidifier would need to create a negative pressure zone to be able to ‘suck’ anything more than water vapor out of the air. That just isn’t going to happen! At best, removing the dampness from the air may cause moisture to migrate back into the dried out zone, but again, that is likely on so minimal of a scale it’s ridiculous to even consider it as a significant contributor to an issue.

Hopefully @dandersen will chime in here as I consider him an expert in this field.

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You have to admit realtors come up with some interesting stories…

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Agreed.

“On average, only a fraction of the molecules in a liquid have enough heat energy to escape from the liquid. The evaporation will continue until an equilibrium is reached when the evaporation of the liquid is equal to its condensation. In an enclosed environment, a liquid will evaporate until the surrounding air is saturated.”

In other words, without the dehumidifier the liquid would just saturate the stone then ooze, seep or condensate provided the surrounding gas(air) is saturated. With the dehumidifier, it evaporates.

What I would expect in this scenario is a good bit of visible efflorescence (or salts/minerals) left behind on the surface as the water evaporates.

My Conclusion: Realtor is mistaken

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Agents are such bullsh** ers. Any lie to make a sale, just make it up as you go along.

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First, I am not an expert in geology nor did I stay at Holiday Inn Express… :innocent:
However I worked several years with an old Army buddy, who is a Geologist and Tennessee is all about limestone.

As for being porous, limestone is considered to be porous, however it is less porous than any other material used for foundations. Most limestone leakage is from rock fractures and soluble due to acidic water in the aquifer causing erosion of the rock over years of exposure.

As for a dehumidifier “pulling water through the wall”, yes it does!
Air pressure is not the driving factor in this case, it is vapor pressure.

The vapour pressure of water is the pressure exerted by molecules of water vapor in gaseous form

The dehumidifier increases the presssure differential and disrupts it’s equilibrium.

The saturation vapour pressure is the pressure at which water vapour is in thermodynamic equilibrium with its condensed state.

You are likely to see the presence of efflorescence from this occurrence.

Efflorescence is a whitish crystalline deposit that manifests on the surface of limestone. These unsightly salt deposits are for the most part, water-soluble salts that come from various possible sources. For efflorescence to occur there must be water present to dissolve and transport the salts through the stone.

Robert- BTW I stood in front of that tree in your pic during a field trip. :wink:

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So actually having a dehumidifier in a basement may not always be such a good idea.

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I cannot find anywhere where evaporation increases capillary flow. For evaporation to occur, there must be surface moisture. The dehumidifier is not going to draw it up from the soil, stone etc. Capillary action, temperature, or pressure must force the moisture up to the surface (or pores in the surface) before evaporation will even happen.

So, the question is will evaporation break the surface tension to allow for more water to flow? Maybe, but other forces are much more predominant.

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Not according to David’s explanation. ( dehumidifier “pulling water through the wall” ).
At least that’s how I understood it.

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Apparently we have a difference in opinion.

Lets say you have a wet soil against a brick wall. The water on the wet side is not going to wait for evaporation to move to the surface. Capillary action is going to get it there first.

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It’s not so much the dehumidifier pulling, but the effect of the vapor pressure the dehumidifier creates as David mentioned. Try this experiment - dip a paper towel halfway in a glass of water. What happens, the vapor pressure pushes the moisture up the towel to the dry area.

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Not true. Capillary action is what your describing.

Capillary action is important for moving water (and all of the things that are dissolved in it) around. It is defined as the movement of water within the spaces of a porous material due to the forces of adhesion, cohesion, and surface tension .

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I disagree, that is capillary action. Once again, I have yet to find where evaporation is a force behind capillary action. Sure, evaporation my help prevent saturation but it all occurs at the surface. Capillary action brings it to the surface, evaporation removes it.

“Capillary action occurs when the adhesion to the walls is stronger than the cohesive forces between the liquid molecules. The height to which capillary action will take water in a uniform circular tube (picture below) is limited by surface tension and, of course, gravity.”

image
image340×275 69.8 KB

This article verifies the paper towel is exhibiting capillary action.

Capillary Action and Water | U.S. Geological Survey.

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I agree Brian your right. Good point. I was actually thinking capillary, as the moisture is attracted to the dry. If you did the same test with a wet paper towel, basically the water is going to move much less. Stone is similar. Perhaps we have capillary and vapor pressure :slight_smile:

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Good conversation guys. :+1:

You are all correct, but you are talking about different things.

Fluids are in the liquid state or vapor state.
In the liquid state capillary action occurs. As Bob posted from USGS, it is about the movement of water (liquids) due to the forces of adhesion, cohesion, and surface tension.

In the vapor state, moisture moves due to pressure differentials (Delta P).
This differential is addressed by Psycrometrics (HVAC Stuff etc). Just as higher temperature always moves towards to lower temperatures, higher pressure moves to lower pressure. Also keep in mind that very moist humid air is lighter than air without as much moisture.

Be aware that what was posted here from the web about the state of moisture/vapor pressure is about that happens in a closed container (like HVAC systems). In an open container the molecules in the gaseous phase will just fly off and the equilibrium talked about here would never be reached.

Yes, that is exactly what is happening. Someone mentioned “pushing” water. It’s more like sucking.

So, lets forget about everything discussed so far, and focus on an “Open Container” situation. When liquid and vapor are in a closed container, with a stable temperature/pressure, molecules are changing state from liquid to vapor and back continuously, and at the same rate. When liquid evaporates it gets colder. Being colder it simultaneously condenses. This is not the case in this thread.

“It sucks” :scream:
We said high temp always moves to cooler temp. Well, the dehumidifier makes it hotter in the crawlspace. :thinking: This heat allows liquid to evaporate faster than condense, breaking the equilibrium. The dehumidifier makes a cold coil through refrigeration and condenses the vapor and moves it out of the CS. What is removed, must be replaced. This basically creates a vapor vacuum in the CS (to make it simple) which changes the surface tension allowing more liquid to change state to a vapor. What does this do? It causes the CS side of the foundation to give up heat and become colder. As we said, heat moves to cold. When the inside surface is colder than the outside, movement will be inwards. Again a Delta P is created, liquid turns vapor, and liquid will replace itself. Keep in mind moisture is heat (970 Btu/Lb).

So looking at this chain reaction, moisture is actually being pulled into the CS by the psycrometric changes the dehumidifier created.

For you guys who deal with Mold, if you have mold in a CS where there is a dehumidifier present, you may notice that there is a pattern to the mold throughout the CS. Mold will grow and create a pattern away from the dehumidifier location. This is from moisture entering the CS where it is cool enough to condense moisture from the outdoor Air entering the space. Mold may be on just one side of a joist, or there may be more growth in one area over another etc. If you are in the mid-south, this is very evident.

[quote=“Christopher Currins, CMI, post:8, topic:221269, username:ccurrins”]
So actually having a dehumidifier in a basement may not always be such a good idea.
[/quote] :+1:

This can be a case of “A little can be good, but more is not better.”
We want to control moisture in the space at acceptable levels without pulling all kinds of moisture through the building envelope where moisture will really cause the water damage. If the CS becomes dryer than the interior of the house, where do you suspect moisture will be pulled from? There is an Air Conditioner up there operating below the dew point temp of the outdoor air. Supply ducts are located around windows that are primary leakage points. What happens on the inside of the wall there?

We have a problem in the Nashville area where building codes are requiring improper sealed crawlspace and dehumidifiers are installed. Mold in new construction…

As for mediation; psycrometric testing can identify this condition.

Hope this helps.

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How do the codes require improperly sealed crawlspaces? Where can I look up these exact codes?

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This is from IRC 2018

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What is? are you pulling Roy’s thread on moi, can you be just a bit more specific, what exactly are you referring to? I guess a code citation would be helpful. Simply making a claim that code requires something, well, we can all do that.

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I was just siting the code for underfloor spaces R408