Mold Has Reared Its Ugly Head: Winter Moisture Issues

by ziggy on February 14, 2011 -- 25 comments -- Follow

Mold has reared its ugly, ugly head in my home, and I have recently been consumed with attempting to determine a solution to this problem. I ask readers (especially those with experience living in cob houses in cold climates!) to please read ahead and help me to determine the best course of action. Any advice would be dearly appreciated.

Here are all the details fit to print.

I live in northeastern Missouri, where temperatures can get to below 0º. This winter, we’ve had perhaps four below 0º nights thus far. But typically, our winter lows are in the high teens or 20s, with daytime temps. in the 20s or 30s. My house is all cob, with 18″ thick walls. There is no insulation in the floor, foundation, or roof (other than the soil). (I will never build like that again in this climate…!)

I am having  a very big mold/mildew problem on the bottom 15% of the interior walls, particularly on the west and north walls. The walls register as low as 38º towards the bottom of the wall, and the foundation about the same in the mornings. Higher up (at head height) the temperature reads in the mid 50s, and even higher it reads in the 60s. (I tested the wall temperatures with an infrared temperature sensor.)

I believe it is condensation that collects on the face of the cold urbanite foundation and the coldest bottom part of the wall (because of the big temperature difference between that part of the wall and the air inside), and then molds from not being able to dry quickly. Condensation is so great in places that the floor is damp in spots, too. We’ve got the small Morso wood stove and can get the house to about 70º, but most of the walls remain quite cool, of course, even after a day of heat.

Furniture close to the walls on the west and north is also a concern, since airflow is not as great with the bed against the wall, for example.

The mold appears as white fuzz, but in some rare places it is a green mold if I have not been over it with vinegar for some time. The regular task of moving furniture to spray the walls with vinegar to kill the mold is tedious and tiresome. The colder it gets, the worse the problem. I have reason to believe that April”s recent health problems (allergic attacks in the form of swollen lips and eyes) have been the direct result of this moldly living environment, as well, which is a huge concern for us, and more than enough reasons to solve this problem as soon as possible.

My current line of thought is that I must insulate the bottom part of the wall and/or foundation. I imagine it would be better to insulate on the exterior, but all I can imagine is having to build a kind of extended foundation and then somehow build basically another wall (or tapered wall) to insulated the coldest parts of the cob. But with light clay straw or straw bales, I am not sure.

I also wonder about the effectiveness of lime plaster in this situation. Would covering the lower 1/4 of my interior wall in a lime plaster or lime wash prevent mold from developing on the surface of the wall? Would moisture simply condense on the surface, and could I then wipe the lime with a rag if it’s bad enough? Obviously lime plaster would not stop the wall from condensing, but it could stop mold from developing since it is antifungal, I think.

Additionally, we are considering an insulated bale-cob addition on the north of the house for insulating that coldest north wall, but the west wall would still be unprotected and need some kind of insulation.

A greenhouse addition could help to maintain higher indoor temperatures (on sunny days, anyway), but it would doubtful have a big impact on condensation development.

Any advice would be dearly appreciated. I fear my girlfriend’s health is at risk it we continue to live in this environment without resolving the moisture issues.

Also, it is important to note that the humidity is always very high, even in the winter. (Right now it’s around 65% and it can even get higher!) Perhaps we need better ventilation…

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  • Joe

    Mold is an issue of ventilation, temperature and moisture- since you have probably no way to put HVAC system in or hepafilters you have to have a way of removing moisture or preventing it from happening- not sure about your type of construction but here is some information on monolithic domes and preventing mold.

  • brent

    I stumbled on a product called DampRid (calcium chloride). I don’t know how effective it is, but it might be worth giving it a try…

    DampRid (calcium chloride)

    Calcium chloride

  • Do you think the moisture problem may be capillary action from using sand/clay as mortar in your urbanite foundation? You mentioned earlier that you had worms in it last year…

    I think good ventilation could solve a lot of the problem. Problem is, I don’t know how you could do it, sorry. Good Luck Ziggy and April.

  • Daniel

    If you come up with a solution, please post.

    If the underside of the place is accessible, I’m wondering if a sheet of plastic would help at all. I’m also wondering if the problem has anything to do with the roof, in relation to drainage. I imagine you’ve thought of these things.

  • hallie

    Hmmmm … I hate to say this, since it’s the worst possible thing to have to fix, but it sounds like either the siting, or the drainage wasn’t done correctly on the house when it was built, or you have woefully inadequate ventilation in your home now. Ventilation should be possible to fix – it’s tempting to try to keep things shut as tight as possible when you’re in a very cold environment, but improper ventilation makes for an uncomfortable living space, and if the cob in the bed you tore out never completely dried either, it’s probably that inadequate ventilation is still a problem even if the siting and drainage were correctly done.

    Are you hip to the Cob Cottage Company? They’re very warm and gracious folks there, and since they too are living the cob dream (in a far wetter environment – the coast range in Oregon) and have built over and over as they taught workshops in the art, they may have some more practical advice for you. I honestly don’t know what to say – if it’s always damp in there, if there’s condensation constantly, and if there’s serious enough moisture retention that you feel your health is at risk, then your girlfriend is just the canary in the coal mine. That environment isn’t good for anyone’s health, and you’ll be suffering too, though apparently silently. Start looking for solutions to the ventilation problem first – that would be the easiest to solve – and look for other signs that the drainage isn’t working properly.

  • adam

    you don’t have to be a cob expert to know that mold on the lower portion of the wall means moisture is being wicked up into the wall from the surrounding ground. That’s true in any kind of construction.

    You have urbanite footings, and unfortunately concrete is very porous and will suck water straight up. This is why concrete slab foundations are are cast with plastic around them. You also don’t have any kind of gutter system, so all the water from your roof lands on the soil right next to you walls – some of it even splashing onto the bottom of your walls.

    Look at the drainage topography around your house. I’d bet a steak dinner, the flattest areas are to the N and W. You might even see some puddles form in those areas after rain. You need to alter the drainage around your house. Ideally the land around it would slope down 1″ in 16″ for at least 10 ft. in all directions away from your house. Failing that, some sort of drainage ditch that leads to lower ground will help. Also if you could devise some sort of gutter, or at least get more of the rain to dump on the S and E sides, you will see a big improvement.

    You will never be able to ventilate or heat that moisture away. you have to prevent as much as possible from getting there in the first place.

    Best of luck with this. The good news is that dryer walls will also make a warmer house!

  • Victoria

    Ziggy, As other readers have commented, it’s a temperature, moisture and air issue and the accompanying gradients. If you think the source of the moisture is from the ground up (suggesting a drainage issue) you might be able to dig a “sump pit” at the lowest part of your house (outside, preferably). This sump pit would gravimetrically collect the water from the foundation and surrounding area which could then be pumped out. My friend uses a solar powered pump for watering his garden.

  • Anna P.

    I was going to suggest DampRid as well but doubt it would be a total or permanent solution. I definitely think a plaster would help also and was wondering why you hadn’t already plastered either inside or outside? From what I understand, plastering is recommended to reduce erosion on the outer walls.

  • Michael

    Hi Ziggy this is the first time that I have posted a comment however I have been following your progress for about a year & a half now and I would hate to see you throw in the towel on all your hard work. Not to mention you are an inspiration to the urban dwellers stuck on the Gerbal wheel of life. Well I would have to agree with the last post that I read you have a drainage problem water is being soaked into the walls. Two things need to be done one you need to make sure that the water is being routed away from your house this is done by sloping the ground so that the water flows away from your house & second you need to water proof the outside of the house with a lime wash not the inside wall you can do the inside as well however the outside is where the water is coming in. Mold forms when the right enviroment is present mostiure & little light. Take away the right enviroment & the mold will not form. now that being said ventilation is very important as well there is natural moisture in the air and without the means to remove that from the air you will have some mold issues but I am sure I am not telling you something that you don’t already know. I know that you are trying to live w/out electricity however you may want to consider some solar power enough to run a dehumidifier.

  • Roger

    agree with all the other comments re drainage but when you’re done digging drains and adding gutters – assuming you can still move by then :)) – maybe a rocket stove on the worst affected walls, with the duct taken along the base of the walls would help to keep the damp out.
    Did you seal the cob with linseed oil outside ??
    In the meantime get all the heating you’ve got running full belt and keep the doors and windows open all day.
    Good luck and stay healthy
    Roger :))

  • Jeremy

    Long term vinegar will add moisture despite its first action or removing mold. The mold killing ability will only last for a short time until the acidity in the vinegar is neutralized.

    Moisture is your downfall with mold.

    This may be a good way to lower moisture and bring fresh air in to house:

    Your current wood stove may be bringing in more air than you know. Wood stoves have a draft effect which sucks air in to the house to replace the air going up the stove pipe. Some stoves get air from the outside to avoid sucking the air out of your house.

    If you did both:

    Ground-coupled heat exchanger


    Stop the air flow caused by your stoves draft effect

    You may also go a long way towards improvement.

    I agree also with Michael…make sure no water is entering your walls, ceilings and floors

  • Jeremy

    I WROTE:

    Your current wood stove may be bringing in more air than you know. Wood stoves have a draft effect which sucks air in to the house to replace the air going up the stove pipe. Some stoves get air from the outside to avoid sucking the air out of your house.

    Meant to write:

    Some stoves get air DIRECTLY from the outside to avoid sucking the air out of your house.


    the air for the stove is piped from outside in insulated pipes to avoid heat transfer to pre stove contact

  • The earthen buildings I’ve seen in Tennessee (also a very wet environment) all had mold issues. Lime plaster helped, as did modifications to channel water away from the building.

    The previous posters here all made good points. The only thing I didn’t see was a mention of swales and berms. They can help redirect or “sink” rain water running across the ground towards your home.

  • Ok, here are more details:

    The walls are not sealed with linseed oil, inside or out.

    The site of the house is located on a slight NW slope – more to the west than the north, but the water runs in that direction. The wettest areas around the house are the east and south. (There is a little trench and some earthwork on the east to keep most water away from the house on that side.) Water on the west or north does not run towards the house.

    Yes, I need gutters. Rainwater does not fall and touch the walls, however.

    The drainage trench appears to be functional, as there is water flowing from the daylight drain. I checked the past two days. It is unseasonably warm this week, so I will have to check during another cold stretch. I sit here and wonder now: could the exit length of drainage (the ~20 foot stretch of pipe from the house to daylight) be freezing in extreme cold and essentially be backing up the entire trench?

    I still lean towards condensation, based on the very low temperature of the lower walls/foundation inside, and the lack of good airflow in those parts of the house. During times of extreme cold, small patches of ice form on the bottom wall inside!

  • Scott

    My guess is you’re going to have to dig another drainage trench-sounds like the place is wicking up water. It’s possible the drain line is freezing,but not likely(is it underground?). Could it be something’s built a nest in it(or it’s gotten clogged in some way)? Since you don’t have power, it makes ventilation more of a challenge. How thick are the walls? Could you drill vents halfway through from the outside(put screens over them) around the bottom of the house? You’re going to have to move some air,no way around it. Something you might want to consider is getting the stove’s combustion air from outside(maybe with flexible automotive exhaust tubing. Hard to guess without actually seeing the place. Good luck,and don’t give up.

  • Dear April and Ziggy,
    I agree that condensation is feeding the mold. We have excellent drainage around our cob house here in SE Iowa and our cob floor is dry and sealed, but we experienced similar mold problems on the lower part of our cob walls. Our inside plaster is primarily kaolin clay and it made a great growing medium. We scrapped mold and some plaster off the walls, moved or removed furniture to create better air flow and in the summer painted the walls with an alis that is easier to clean than the kaolin plaster.
    Cob walls can take in an amazing amount of vapor and so long as they aren’t sealed on the outside that vapor will migrate out. However, with the winter we have had, the walls are cold enough on the inside for the vapor to condense and breed mold.
    I have been studying super-insulated house techniques. Sealing the house against air infiltration is critical. However, once you make the house tight you have to have an on-going ventilation system. An air to air heat exchanger is often used to bring in fresh air while retaining heat and moderating humidity.
    I don’t know of any short term solutions that you aren’t already doing. Sounds like you might want to stay somewhere else until it is warm enough to open windows.
    We are planning to build a net-zero house next to our cob house and let the cob house be a 3 season home.
    Don’t hesitate to email.
    Best of luck, Hap & Lin Mullenneaux

  • onesojourner

    I disagree with the people saying you have a drainage issue. I have read your entire blog and I have never noticed you mention drainage problems. the problem is the temperature difference between the room and the wall. just like a glass of ice water on a summer day. You really only have 3 options. Insulate the wall, run a dehumidifier off a solar panel and live painfully dry air or build a new house that is properly insulated. If it were me I think I would build cold frames along the edge of the house with a larger green house around them. that would probably keep the walls 10-30 degrees warmer on the south. You could also stack straw bales 2 high all around. You may have issues with the roof not sticking out far enough at that point though.

  • onesojourner

    oh yeah and by the way we have the same issue with door knobs and single pane windows in our house. the collect tons of moisture on them when we have really cold days like we have had this winter. we hit -10 about 9 days ago.

  • Elisa

    Hi Ziggy,
    We built a cob house in B.C Canada, and have been living in it for 12 years. We had a problem with condensation and mold forming on the inside of our cement/rock wall foundation. We solved this problem by insulating the exposed stone ( bottom 1.5 ft of the wall) with rigid foam, covering it with page wire and applying a lime plaster. We also replastered all of our earthen plaster walls with a lime plaster. We used to have a moldy mildew odour where ever our furniture rested against the walls. This changed after we replastered with lime.

  • AdamA

    Hey Zig,
    Elisa and the others who are pointing at condensation are on the right track. Although rigid foam may not be in keeping with your goal, it would probably be the best solution. You could dig away some of the soil from around your foundation and carry the foam down and then out two feet all the way around to insulate the house footprint and cover it with crushed stone and perforated drain pipe. I would cover at least the lower half the outside wall with foam and then earthen plaster on top of wire over that, however if you did the whole wall, you would notice a huge difference inside during heating and cooling season to the good. Plus the foam will keep out outside moisture as well, unlike bale insulation, which will also make your supercool house turn chunky and look like it has an eating disorder. Just be sure to vent the foam in places to allow the cob wall’s natural moisture to vacate, or install a couple roof vents. Mold sucks and is difficult to get rid of, however, I would advocate some various house plants to help the indoor air quality. They can release certain substances which will kill house mold. Research what kinds would be most beneficial. Bottom line, better indoor ventilation and insulate the foundation and walls. The only other thing I can think of in lieu of the foam would be a glassed in green houses or cold frames as others have mentioned to get some solar gain on the mass in the foundation and lower walls to keep it warm enough on the inside to prevent condensation. Of course this probably won’t help much on the north side. Check out Malcolm Wells’ or Rob Roy’s stuff or some of the other books on underground housing for some ideas on dealing with inside moisture problems. The applications may be similar to your situation.

  • AdamA

    As an addendum to the above post. I believe in Rob Roy’s first underground house book from the 80s or 70s, he and his wife Jackie ended up with a condensation problem which I think they dealt with using ventilation. Its been a while since I read it though. He has a website… Maybe he can help with some ideas too. He’s a pretty sharp dude with this stuff.

  • nico morris

    combining a solar chimney or two for ventilation and heat at least when the sun is shining.??
    sounds like perhaps you mortared your foundation? losing the capillary break
    that would be supplied by the air space within.(You can still plaster the outside of your foundation I recommend cement…no insulation and cob in your climate is a refrigerator.
    Use it for a goat shed and build a new one with insulated floor and north walls.
    If you DO decide to straw bale the north side,remember the bails need the same correct foundation and cob covering ALL over to not become compost heaps.

    Nico Cob builder in California
    Oh sodium silicate as a hardener in your inside plaster will not mildew
    oil will, flour paste sucks, cactus juice mildews to.

    get some small computer fans 12 v or 110 v and place them where they will circulate a lot of air where you need it,they are quiet,and you can run them when you are not home if the sound bothers you.

    The Lime plaster is a good idea. although I fail to see where Lime is any better than cement E.wize . some say it “breathes” better I doubt it.
    It’s a hassle to work with.

    Good Luck! Nico

  • smudla

    Problem is in big realy big mistake in construction of base of building. There was used earth with sand as mortar and absolutely nothing like hydrobarier between cob walls and foundantions ….
    And other mistake is this: you dont have roled roof edge to create chanels to direct rainwater form roof to few spots (2-4 where you than colect raniwater into tank/bazen/little pond or simply redirect it in pipe far from house….
    This two factor leads to situation that earth under your house is realy wet and suck water into walls … destructable for stifnes of lower part of wall (Freezeing and defrosting will lead to failure of wall (only question of time if you wil not correct this mistake.

    Condensation problem si secondary and solvable … you must make outside insulation adidional(breathable …. not with polystyrene foam but with rockwool or similar material( thicknes 15-20cm minumum), and insulation of roof(from the insdide of interior … to reasembly roof it will by realy to much work waisted.
    Paint interior walls with lime (white thing) will help to improve surface properties …and try to dry whole inside of house during summer as much as posible.

    good luck. Nice house but you will have to work to keep it habitable in long term.

  • Tempests

    Did you think to use a vapor barrier under your building? The earth is constantly leaching water into your house where it collects on the cool lower part of your wall. You also give off large amounts of water too. You will need ventilation and some form of ground vapor barrier to eliminate the problem.

  • Mozaik

    Hi Ziggy!

    First a warm thanks for the blog, really appreciated!

    I feel for you with your mold problem, i am myself buiding my cob house now, and understand how you must feel about this…

    Onto solutions now:

    I agree with Smudla, the foundations are sucking ground moisture into the cob like a sponge: gutters and hydroinsulating+drainage might help, but since it’s lots of work for an uncertain result, here are two affordable solutions:

    -lime plaster + limewash the inside. I’d be surprised if the mold survives this, and you can limewash the inside once a year for 2$.

    -insulate the roof, that way more heat will reach the bottom of the walls, + you’ll feel cozier. I know you probably want to admire your reciprocal frame, but that probably won’t be possible.
    An easy way to retrofit roof insulation: dig in your cob walls to fit in horizontal beams at desired height, tongue and groove above that, vapor barrier, and whatever insulating material you choose.

    Hope that helps! Good luck!

    PS to all future selfbuilders: speak and mostly listen to professionnals before starting, it won’t cost you but it will probably save you. And don’t think it’s as simple and care-free as some cob buiding books could let you think.

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