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Passive dehumidification: how to control mold off-grid

 
Posts: 35
Location: The Ozarks
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In many climates, both conventional and earthen homes will have moisture and mold issues without a dehumidifier. In conventional homes, that's part of the HVAC system. In earthen homes, where there is often no need for a heating and air system, there will still be a need for a dehumidifier in many climates. It's not the earth-berming that makes a home moldy...it's just that the dehumidifier is thrown out with the now unnecessary HVAC system.

I know it seems simple to say, "well, just get a dehumidifier!" But surely there's a simpler, more natural solution to the problem of excess moisture in the living space.

Is there any method of passive dehumidification you know of?
Old-timey solutions? Negative/positive ion theories? Magical sponge that soaks up moisture from the air that you can wring outside and hang to dry?

I'd love to hear your thoughts!

 
Posts: 331
Location: S. Ontario Canada
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Our cabin feels damp and chilly when arriving but after a day with the stove going and doors opening and closing it's comfortable.
A wood stove produces such dry heat that it's a must to have a bucket of water simmering away almost all the time in the winter months. I'm sure that would help.
 
steward
Posts: 2719
Location: Maine (zone 5)
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There is a moisture absorbent product called DampRid,  which is made from calcium chloride, a salt.  I don't know to what degree you could reuse such a thing but I bet you could probably buy the salts and make your own with a cotton or wool bag.  Then if it's safe to do so, heat the salt outside to drive off the moisture.  Then... reuse again.  Of course this is energy intensive in the drying process but maybe worth looking into if all you need is the salt and a place to dry it once it's saturated.  Maybe setting it in the sun on a black surface would help, but I'd bet it's going to require something like a steal sheet pan and a bed of hot coals to get it really dry.

Calcium chloride

 
Posts: 134
Location: Zone 4b at 1000m, post glacial soil...British Columbia
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Ah yes, calcium chloride is what they spray on the roads around here in summer, to keep the dust down.  It does do that, by drawing moisture to it.  Like slug snot to drive on after a rain, and sprays up on your vehicle, where it remains and attracts moisture to your vehicle's underparts as well.  Also used in hardening cheeses like cheddar; presumably purpose-made and not from the back of the highways maintenance truck.  

How about silica gel, the kind that comes in little packets with things like new shoes, that say Do Not Eat?  Their purpose is to draw humidity and spare whatever they're with.  Can this be gotten in larger quantities?  Has it a second life?
 
pollinator
Posts: 240
Location: Worcestershire, England
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If you have the space to put it you can use charcoal as demonstrated at around 7 mins in this video. They seem to be using 1500Kg of the stuff! They put under the floorboards but sadly it doesn't indicate the size of the house.

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This is a critically important topic. Mold can ruin houses and high humidity can make a living space horribly uncomfortable. I am happy others are looking into this topic. Thank you to all who contribute.

I know that many different materials can act as humidity buffers due to their porous nature. I am trying to find a very good website on the topic. I will report back once I find it.
 
Joe harrington
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pollinator
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Very important topic to bring up. Without an HVAC system your correct issues with humidity could be pretty bad, be it an earth sheltered home or a standard stick frame home.

I like the charcoal suggestion Henry offered, since one can make their own charcoal rather than needing to buy it. Weird how the video he linked really is only about 15 min long, but repeats itself. Interesting video though and I was a little disappointed it wasn't actually as long as it looked like it was, I was wanting more about charcoal and it's uses.

On the topic of options for dehumidifying earth sheltered homes, of course something important to design into the home itself is air flow. Making sure you don't have places where humid air can stagnate and collect. You want a good flow of air moving through the house, especially when you open up windows. The circulation of the air flow or lack of it can be a huge factor in humidity issues for a home.

Of course also good thing to remember, bathroom and stove top venting. These high humidity creating points in homes need some good venting to help keep them from adding too much humidity to the home even in standard homes with HVAC. So in a home without, even more critical.
 
Henry Jabel
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Location: Worcestershire, England
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Devin Lavign wrote:
I like the charcoal suggestion Henry offered, since one can make their own charcoal rather than needing to buy it. Weird how the video he linked really is only about 15 min long, but repeats itself. Interesting video though and I was a little disappointed it wasn't actually as long as it looked like it was, I was wanting more about charcoal and it's uses.



I was only looking for that bit and that was the first one I found. I probably should have posted the full version posted by NHK instead:

 
Devin Lavign
pollinator
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No worries Henry, I actually went and watched the full version, as well as their episode on bamboo, and plan to check out more of their videos.

I had already planned on doing some charcoal making eventually on my homestead, as I want to create a blacksmith forge. As well as just charcoal has many uses.

But seeing the bags used to help control humidity, I might be doing charcoal sooner than I planned. As I like the idea for use in an underground home. And I am planning to build an earthsheltered home, so it is a worthwhile consideration to put into place before I start building. The video showed they had planned space under the floors for the bags of charcoal. So I would need to similarly preplan where to place such bags.
 
Posts: 73
Location: Ant Village
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I've been thinking about this for a while now as well.

I think it's important to understand that warmer air can hold more moisture. When warm air with high moisture content cools down, then that excess moisture has to go somewhere. In your house that cooling of warm moist air will condensate on everything.

In the summer, there seems to be a few good ways to deal with this:

1. Make sure to have a lot of good air flow. Open windows/doors and have fans and such. You don't want to allow the warm moist air from outside to stick around too long in your house and cool down to start condensating on all your stuff.

2. Plant shady trees all around your house. The shade will protect your house from the sun and keep the immediate area around your house much cooler, therefore the air coming into your house will have less moisture as it would have condensated on the trees before it made it into your house.

3. This is completely opposite of number 1. Make sure that you only allow dehumidified air into your house. This seems to be the idea behind the Earthships. They only draw in outside air that has been cooled and dehumidified through a pipe that goes through a large earth bank. They create the draft by having passive solar ovens on top of the greenhouses on the front of their houses.
 
Posts: 49
Location: Mid-Missouri
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This topic has been on my radar for quite some time now and I have been doing a lot of research on it. The OP indicates they are in the Ozarks. I am in central Missouri, on the northern edge of the Ozarks so basically the same climate. While I don't have any personal experience with it I can share what I have learned so far on my journey.

I was fortunate enough to visit Wheaton labs in 2015 and took a tour of both Cooper Cabin and Allerton Abbey. The day we went up there it was 104 degrees outside and I was very impressed with the "air conditioning" of those two buildings. I thought to myself that even if these buildings still required supplemental heating in winter, this "air conditioning" feature was worth the effort to bury the building in earth. However the climate in the mountains of western Montana is much drier than that of Missouri. We have very humid, muggy summers here and if I built a wofati just like one of those at the lab, I can only imagine the amount of condensation dripping from the walls and floors inside. So I too began looking for a way to passively dehumidify the indoor air so that I could have effective off grid air conditioning for my climate.

Earth Tubes: I read about earth tubes in John Hait's book on PAHS. Then I went and visited Abundance Ecovillage in Fairfield, IA just 3 hours drive from my home. The homes built at Abundance are very modern with high end features including hardwood and tile floors, granite counter tops, stainless steel appliances and all the other finishes you would expect in a modern home. But many of the homes here feature earth tubes for cooling purposes. They buried corrugated drain tile, the perforated kind, in the earth around the homes and used a fan to draw air through the tubes and push it into the house. The condensation would drain out of the tubes into the surrounding soil through the perforations in the tubes. I arranged a tour of the village with the developer and asked a lot of questions on how this was working. He said that it worked very well until about the end of August/early September. By that time the soil around the tubes had warmed up to a point that it was no longer effectively cooling. In addition to that issue, this area experienced a drought in 2012 which opened up deep cracks in the heavy clay soil. This allowed radon to enter the earth tubes and be pumped directly into the house. They were able to work around this by pressurizing the tubes and thus preventing the radon from entering but that just added another level of mechanization. The developer told me that solar panels had gotten so cheap and mini split air conditioners had gotten so efficient that they were headed away from the earth tube idea and moving towards a 4000 watt solar array and a mini split AC. Hardly a passive solution, but it does work.

That said, there are some key differences between the earth tubes described in Hait's book and those used at Abundance Ecovillage. First of all, if you look at the picture of earth tubes on the Abundance website link above, they are spaced very close together. Hait describes having these tubes further apart from each other, at least 4 feet from the next tube if I remember correctly. If the tubes were spaced further apart I believe that this would eliminate the issue of the earth around the tubes getting too warm by the end of summer and they should be able to provide cooling and dehumidification for the entire cooling season. In other words we need to optimize, not abandon, this system. But that doesn't take care of the radon issue. Hait's earth tubes are made from solid, smooth wall, pipe. No perforations means that radon, or any other sub soil gasses, can't enter the tubes or your home. So, in order to deal with the condensation that occurs in the pipes Hait's earth tubes feature a constant slope away from the building all the way to daylight, much like a waste water drain. I haven't had the opportunity to meet someone who lives in a house with this type of earth tube but poking around here on permies.com seems to confirm my suspicions. I believe that even with a constantly sloping pipe there will still be enough moisture in those tubes to create mildew issues, so any earth tubes of this style should be built with a way to clean them. I imagine using a rope to pull a large rag soaked with some mildew killing solution through the tube every once in a while would do the trick. I would also be tempted to use potable water pipe, instead of drain pipe, since the potable water pipe is resistant to mildew growth. That type of pipe is more expensive, but still cheaper than a solar array and an air conditioner.

I also visited Dancing Rabbit Ecovillage in northern Missouri which has a very high concentration of owner built, straw bale, homes. And I asked them what they were doing for dehumidification of indoor air. It turns out that many of the residents there run a dehumidifier for about an hour a day and some of them have air conditioners. The houses who don't dehumidify have a musty smell. DR has been around a while and as experienced as they are at living off grid and sustainably, they haven't figured out this one yet. It's a tough issue for those in the midwest and deep south.


I have some different ideas for an "almost passive" dehumidifier but I'll save that for another post. I would love to see more discussion on this topic because I am desperate to build a wofati stye building here in Missouri but really want a workable solution to the moisture issue before building.
 
Posts: 124
Location: Denton, TX United States Zone 8a
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***I have no experience with any of these methods***

That being said I've heard a good deal about the ability of clay and cob (particularly as a stucco) to help regulate humidity. Brief description and video here.

The reviews all seem to suggest that these thingamadoos work pretty well, though I'm skeptical of the claim that they are infinitely renewable.

In my mind the idea expressed by others in this thread and generally on passive and efficient homes- that you heat things not air, then ventilate well- would solve most humidity problems. Maybe everyone else already knew this but I very recently learned that wet/humid air is lighter than dry air. This would seem to suggest that as the air in your home is heated (either by you in the winter or by sunlight/outside temps in the summer), the humidity is more likely to rise and be ventilated outward, so long as you're not determined to heat up that air and then hold onto it.

It's also worth noting, I think, that humidity comes from somewhere- doesn't Paul talk about how dank and wet an underground home can be if the groundwater isn't redirected, like it is in Wofati construction? I'm not sure about the humidity problem with earth tubes, that would be a lot tougher to solve, but as far as humidity in homes not set on top of a concrete plate, I think that would do the trick.
 
Todd McDonald
Posts: 49
Location: Mid-Missouri
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I was impressed by the steam shower video but I think one steamy room for 7 minutes in an otherwise conditioned home is different than whole house 24/7 climate control. If clay and cob alone were enough then the folks at Dancing Rabbit would not have the moisture issues that they do. Most of the houses I toured featured clay plasters on the walls and earthen floors and they still needed to dehumidify.

I would like to echo the points of Davin and Jeremy above and say that design and airflow will also have a lot to do with it. For this reason my current design (on paper) has no closets in the bedrooms. These places collect mildew even in air conditioned houses. I'll use a wardrobe instead. I also think that placement of doors and windows is very important for airflow.

At this point I am leaning toward a multi tiered approach. Using clay plasters and untreated/unsealed wood, using earth tubes, and good design. Even with all this I will be mentally prepared to admit defeat and use a dehumidifier.
 
Kaye Harris
Posts: 35
Location: The Ozarks
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GUYS!

I just ran across something I thought I'd like to share.

UNSLAKED LIME.

See linked video at 49:50.


 
Devin Lavign
pollinator
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Something a bit less permanent, but no less effective for a small space and adds some pleasant aroma to the air. My neighbor has taken to picking wild camomile and other herbs, then drying them and placing them in his trailer to deodorize and absorb humidity. When they rehydrate as they absorb humidity, he takes them out into the sun to dry out and then starts it all over again.

A pretty low tech simple method to do. He hangs some from the ceiling, but also sticks a small planter pot filled with them in his closet to keep the closet smelling nice and mold free.
 
gardener
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Not passive, but one option is to get a heat pump water heater.  I bought a GE model and it does a great job drying my basement air while also cutting my electric bill.  We used to have to run a dehumidifier in the summer before I bought the heat pump water heater, but I haven't had to run it at all since.
 
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This is something I have been concerned with and thought about extensively as I will be building in a climate with hot humid summers, and cold damp winters. I have an idea but won’t be able to test for several years (until I retire baby!!!) but here’s my two cents:

Very similar to cooling tubes, using earth and convection, it would be possible to build a DIY passive dehumidifier. The idea would be to bury a length of metal tubing into the earth (on roughly a 3/1 ratio. For every inch of exposed coils, three inches of piping must be below grade.) which connect to exposed coiled copper, placed vertically, on an indoor wall. Using ethylene glycol, fill the tubing/coils and seal. Back on the  interior wall, build a drip drain under the copper coils. The warmer indoor air would pass over the cold coils causing condensation. Convection would warm the liquid inside the coils creating a temperature difference, which would move and circulate the liquid. Warmer liquid would be pushed up and back into the cold earth, and cold liquid would flow into the coils. It should be able to bring down the humidity of a small sealed room. How long and how many coils you need would be dependent on many, many, factors. To make it an active system, you could have a 12 v fan blowing air at the coils. The more air that passes over them, the more effective it would be. Likewise, a 12 v pump could help circulate the ethylene glycol if convection alone doesn’t exchange warmer/colder liquid fast enough.  Even you used a fan and pump, it would still require less energy than a full blown HVAC system or using standard electric dehumidifiers.
 
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I'm pretty obsessed with this too since I'm in Piedmont NC and conventionally-built houses regularly have to be evacuated and torn down because of mold - black mold is no joke, and it's everywhere here. Air conditioning stops seeming like a luxury product when your books mold through and your shoes start to decompose.

There are some documents about passive dehumidification modeling in Florida using dessicant panels that are dried via solar heat - I'm set up to try it on a pretty small scale this summer, via:

- digital humidity gauge
- activated charcoal pellets (the kind prepared for fish tank filtration)
- kitchen scale
- glass-front box (basically a large picture frame with some air movement holes)

Design:
- Separate charcoal pellets into 2 lots of equal volume
- Take a completely dry starting weight per volume (dry charcoal pellets in the oven at 200 for multiple hours)
- Take a completely saturated starting weight per volume (soak charcoal pellets in water and then drain in a colander)
- Load oven-dried charcoal pellets into the glass-front box, weigh the whole shebang, then weigh several times over the next days/weeks until the weight is stable (keeping a log of day weight + house humidity)
- Take the point at which the weight stabilizes as the charcoal's humidity absorbtion capacity in my house
- Partially vent the box and put it on top of my car in direct sun, see if I can evaporate humidity out of it back down to its starting dry weight.

Happy to report back, as I think this is a pretty big sustainable building hurdle in general - as more of the country gets hot and humid, energy use for AC is only gonna rise.

I will say that even here, a wood stove can dry a house out pretty thoroughly in the winter - but that's depending on the temperature getting low enough to run the stove continuously. Right now the biggest mold events tend to be in the hinge seasons, where it's rainy and 70 degrees with 99% humidity for weeks.

Edited to add: the modeling paper I read for Florida also just straight-up suggested a UV coil to keep the air return from growing mold. I'm not sure how efficient those are, and they're definitely not low-tech/off-grid. The high-labor low-input solution might have to involve cleaning or solarizing your system regularly - I'm sure Earthship people can weigh in there.
 
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Location: In the woods, West Coast USA
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With regard to the DampRid stuff, it's creepy.  It leaves you with a plastic container (already not good) full of slimey liquid that says to flush it down the toilet.  That would wreak havoc with a septic system, and I wouldn't want to know what it would do to a sewer system, just because I try not to pollute.   And if it spills it's just awful to clean up, leaves slime all over everything, and takes a lot of grease-cutting cleaner to get rid of it.

It never worked for me, the humidity gauge never got better, and the crystals turned into liquid anyway.  

 
Cristo Balete
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This post at this site impressed me when I read it.  It talks about using window film and silica gel beads that are at a hobby store.  Since keeping the condensation off the windows is an important part of drying things out, getting something to work on the windows is helpful.


https://permies.com/t/84346/Preventing-condensation-windows-winter

I have not been able to find the beads locally, but I do want to give this a try, especially since the beads can be dried out and reused.
 
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Just recently read this article.

https://www.sciencemag.org/news/2019/09/crystalline-nets-harvest-water-desert-air-turn-carbon-dioxide-liquid-fuel

The system was designed to extract drinking water from arid air.  Perhaps it could also be used to dehumidify air and thereby create a larger demand for the product.

 
pollinator
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My thought is a solar air heater should solve the problem as it heats the air before it gets pumped into the house.
 
I agree. Here's the link: https://richsoil.com/wood-heat.jsp
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