I'm leaning heavily into building a significant proportion underground.  Being in FL, the issue of controlling moisture seems paramount when building underground.  (Yes, it's possible to build underground in FL.  The properties I'm looking at are about 150' MSL, with some hillside to dig into.)  Because I intend for my property to be 100% water self-sufficient from natural sources (no city water), it seems like one problem area could be a solution elsewhere.  I've tried searching for water-scavenging technologies, but nothing seems to fit what I have in mind: Capture and store water that naturally wants to condense and/or seep into underground rooms, dehumidifying the rooms in the process.  I already plan to have cistern(s) for water storage and a treatment system for potability.  In order to steer my thoughts in the right direction, I need a little educating:

1. What proportion of the humidity problem in underground structures is due to condensation from the air inside the structure vice wicking/seepage from the soil through the walls?  If one is overwhelmingly more significant as a source, then I need to think about modalities to capture water that work with that source.  I expect that seepage is highly variable based on the materials used in construction; so for this discussion, let's assume I go with ICF.

2. Are there any good resources that might be specific to underground construction in the FL panhandle or its soil types, especially as it relates to moisture control?  Web searches have not been productive.

3. What do folks think about the idea of building a multilayer wall that actually encourages water to seep through from the soil so that it can be trapped in the interwall compartment and funneled to the cistern?  A clever use of materials with varying permeabilities, perhaps helped by applying a highly polar surface coating, should establish an osmotic gradient that can drive water to the collection compartment in the wall.

That seems a bit like allowing the roof to leak, and dealing with the water once it's inside. It's easier to gather the water before it gets in. My ICF basement has waterproofing, plus dimple board to encourage the water to drain down rather than sit against the wall. The basement is backfilled with gravel, and there's a drain system around the perimeter to carry the water away. It would be simple to gather the water from the drainage system. That being said, I've looked at our drain system, and there has never been any water in it. Our soil is clay, so it doesn't drain well, it's pitched away from the foundation, and I have large overhangs on the roof plus gutters. As a result of all of this, I do have a dry basement. The air is still humid, and I run a dehumidifier from time to time. During humid weather, it produces maybe a gallon or so of water a day; not much, but something. I would think it would be more effective to harvest water from more traditional sources- wells and rainwater storage.

I agree, I would collect the water OUTSDE the basement wall. With a sealed wall, moisture inside is most likely condensing from the air. With the insulating properties of ICF you may have less condensation inside the wall, and only have what a de-humidifier could pull from the air itself.

Not sure where you are planning on this project, but have you discussed it with the local building department? In many places, the groundwater does not belong to the home/property owner, but the state department of ecology/resources. Under these conditions you have no right or claim on it and cannot access it without a permit for something such as a well... In fact you must typically provide a drainage plan that channels your roof water back into the groundwater system... If there is an available city/county water supply system, I highly doubt you can pull a well water permit, as they need the customers on their system to distribute the costs of the system...

Good Luck...

How ever energy efficient an underground building is, you can build an above ground one that is more energy efficient. So take a good look at building above ground and then putting in a ground loop (or open loop well for potable water too) if you want to extract heat/cold from the ground. Probably cheaper with fewer hassles/leaks.

Posted By ronmar on 02 Aug 2016 10:03 AM
I agree, I would collect the water OUTSDE the basement wall. With a sealed wall, moisture inside is most likely condensing from the air. With the insulating properties of ICF you may have less condensation inside the wall, and only have what a de-humidifier could pull from the air itself.

I sounds like it still boils down to the first question in my OP: what is the primary source of moisture in a basement? Air or surrounding soil?

Not sure where you are planning on this project, but have you discussed it with the local building department? In many places, the groundwater does not belong to the home/property owner, but the state department of ecology/resources. Under these conditions you have no right or claim on it and cannot access it without a permit for something such as a well... In fact you must typically provide a drainage plan that channels your roof water back into the groundwater system... If there is an available city/county water supply system, I highly doubt you can pull a well water permit, as they need the customers on their system to distribute the costs of the system...

I've only heard of such laws out west, where no one seems to own the right to do anything on their own land.  But I just did a quick search of the county ordinances and I did find a requirement to connect to sounty sewer and water if they are available at your property.  I know with 100% certainty that sewer is not available.  Water may or may not be available, depending on the parcel of land I go with.  I see no restrictions on wells, rainwater collection, etc.  If I had to connect to the county water supply, it wouldn't be that big of a deal.  The base monthly fee is quite low, and I shouldn't incur any additional consumption charges.

The idea of being prohibited from collecting your roof water is absurd.  The government should be requiring all new construction to capture and reuse rainwater!  Or at least establish an exponential price curve on municipal water so that the incentive to conserve and reuse water is overwhelming.

Well in places where the groundwater is the property of the DOE, that groundwater comes from the rainfall, so they own the rain as well, and want it replaced where it will go back below ground from where your roof illegally diverted it...

I agree with you. I think as soon as they can get away with it, they will be taxing us for the sunshine we use, or per the SQ/FT of the shadow your house casts...

In a properly constructed basement, any moisture would come from the air. As I mentioned, I've never seen any water in my drain system, so I apparently wouldn't get any water even if the walls weren't sealed.

Posted By jdebree on 03 Aug 2016 05:04 AM
In a properly constructed basement, any moisture would come from the air. As I mentioned, I've never seen any water in my drain system, so I apparently wouldn't get any water even if the walls weren't sealed.

When I started reading about subterranean building, there was an overwhelming amount of concern for moisture control, both from the air and through the wall contacting soil.  As I've talked to a few masonry/concrete contractors, the impression that I now have is that the soil is a negligible factor and that air humidity is the only thing to worry about.

I'm now leaning toward using ICF as the outer, load-bearing wall with reclaimed Chicago brick as the interior wall (to be exposed).  Between the two walls, I will probably have a ~3ft wide passageway, where all my conduit and physical plant stuff will run. This will allow me to inspect/maintain/upgrade easily over the next 50 years.  It will also give me a space that I can use as a plenum, or run a plenum through.

There are certainly situations where ground water is significant; at my mother's house in CT, there was an actual stream that ran through her basement. If you are down into the water table, you'll have plenty of water to deal with, but ordinarily, that's not the case.

Yea, it is usually the basements that are entirely below grade that usually encounter more moisture. Typically not so much when building into a hillside, but water is where you find it. Rented a house in Upstate NY that had a steady flow into the drainage system in the below ground finished basement. The sump pump was a necessity in that house(owner had 2 installed) or the basement would flood eventually. We also ran a de-humidifier year round with it's discharge plumbed to the sump as well. Nice house, but the basement always had that musty smell... I think one of it's biggest failings was that it had no installed dedicated ventilation.

My neighbor had to dig all around the outside of the basement walls to add/repair drains (they eventually clog) and add an autostart NG generator to fix basement water problems. Would have been cheaper to build on a slab and add more sq feet above grade (mostly garage space for storage).

If you want to scavenge condensation, I think you'd be much better off doing that with a dehumidifier or an A/C, rather than with condensation on the walls. I imagine that in Florida that could be a significant source of water, just due to the moisture that ventilation air brings in.

In principal, the condensate is distilled water and could therefore be pure, but in practice, the coils collect dust as well as moisture, and then mold grows on the dust, and the result can be nasty. So I'd think it easier to use that for flushing toilets and the like than as drinking water. But you might be able to find or design a system that would keep the condensate clean enough to drink.

What proportion of the humidity problem in underground structures is due to condensation from the air inside the structure vice wicking/seepage from the soil through the walls? 

I have seen old houses with field stone foundations.  With that construction water can run right in.  With any more modern foundation I would expect the walls to pass little water.  My rule of thumb is that I do not want liquid water in contact with the structure of the house, because the water will eat it.


What do folks think about the idea of building a multilayer wall that actually encourages water to seep through from the soil so that it can be trapped in the interwall compartment and funneled to the cistern?

I think that you are running well ahead of the current state of construction technology.  It should be possible to do, but there are probably more ways to do it wrong than do it right.  If it was me, I would use proven techniques to build my home, and do test walls in some sort of outbuilding that I could sacrifice it it failed.  You could do each wall slightly differently, then race them.  I also have concerns about the potability of water that has seeped through various building materials or slowly made its way down a basement wall, though I wouldn't hestitate to use it for grey water.

Jonr:
How ever energy efficient an underground building is, you can build an above ground one that is more energy efficient. So take a good look at building above ground and then putting in a ground loop (or open loop well) if you want to extract heat/cold from the ground. Probably cheaper with fewer hassles/leaks.

By going down into the ground he gives the property a built in hurricane shelter.  It will also maintain its temperature with less use of electricity.