Update: Mold & Winter Moisture Issues

by ziggy on February 18, 2011 -- 12 comments -- Follow

I have some new insights into my winter moisture conundrum. I have a new theory, but I am trying to make complete sense of factors at play here. Currently, I think it is indeed a combination of condensation and drainage that are resulting in moisture entering the house.

Read ahead for an update.


Condensation must be playing some role, but perhaps not the primary role. I thankfully got in touch with Hap Mullenneaux, who lives in an all cob house in Iowa, which is even colder than Missouri. Turns out Hap and his partner Lin have had similar condensation issues in their home:

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.

Perhaps most unfortunate is their solution: to build a superinsulated house and make the cob house a three seasons dwelling. (But perhaps not so unfortunate, because really, cob is just not appropriate for this climate! More on that issue later….)

Anyway, mold does not only appear on the lower half of my walls — it does show up in recessed book shelved and nooks, where the walls are thinner. The walls are simply too damn cold, and condensation collects and mold develops.

But the walls seem inordinately damp at the bottom….

Drainage problems

I think drainage issues are feeding the overall moisture content of the lower walls. There is no way that the wet spots along some portions of the floor along the foundation are from THAT much condensation. It must be either rising damp, or water infiltrating through the foundation. The foundation is definitely not perfectly tight: the urbanite has significant gaps here and there, and the mortar itself is clay/sand, and obviously extremely prone to getting wet and wicking moisture. So is it rising damp, or is it water simply washing through the bottom course?

Drainage problems in action - rising damp or something else?

It seems like water entering the trench should immediately migrate downwards, but could it could be getting the urbanite and mortar damp, which wicks up into the walls.

The daylight drain is working — I have inspected it and saw water exiting the pipe. So I do not think the system is clogged or plugged up somehow.

The area around the house is wet, especially on the east and south. The house does need gutters to prevent all that water spilling off the roof from further saturating the adjacent ground. Perhaps the site needs a curtain drain to move water from upslope away from the foundation. Our soil is extremely heavy and drains very poorly.

The foundation and walls outside are almost never visibly wet (although there was a time during very bad flooding last spring that there was some water gushing directly towards the foundation), so it’s not obvious how the water is getting in…

If I can figure out how to alleviate the drainage issues, I think the overall moisture problems would not be as bad as they are, but there will probably always be issues of condensation unless the house is wrapped in insulation somehow… either way, it’s a lot of work, and the lesson learned is that cob is not appropriate for this climate. (Again, more on that later.)

p.s. Click here for dehumidifier ratings.

Send Me More Updates Like This!

  • I’m a longtime lurker and I’m sorry you’re having to deal with this! I am planning to build a combination cob/straw bale house for myself, somewhere and sometime, hopefully soon!

    I had a thought, do you think it would make a difference if you mixed in generous amounts of dried powdered antifungal herbs with your cob mix? Or mixed it with a plaster to use as a final coat? And then perhaps it could be applied prophylactically on a regular basis. If the medium itself is inhospitable to nasties, perhaps it might slow or retard their growth…thinking out loud! I’m hoping to build in Southern or Central OR, so this is most likely an issue I’ll have too.

    I’ve very much enjoyed all the photos & posts you’ve had over the years, thanks! 🙂

  • Dawson

    Sorry to hear about your issues. About the base of the walls, could you build up the base on the outside so there is a good slope to channel water away? I thought of this because the other day I saw a conventional house being buildt. The house had dirt piled next to the foundation to make a good gradient to move the water away.

    About the moisture in the air that is condensing. Could you make an air intake that would bring in outside air past the stove to heat it, then into the living space? My car defroster works that way, bringing in outside air. For a fan I notice you have one of those heat driven fans on top of your stove, maybe that could be made to draw in air? Don’t have anyway of knowing if that would draw enough air. Good luck.

  • Roger

    Why do you think the climate makes cob unsuitable ?
    Cob houses in the UK from way back and still being built now. I found condensation problems in an old stone cottage we had too. The walls were 18″ thick and held the cold – place was like a cold store when we moved in lol
    It had had central heating put in and the first winter it never got warm and we had black mould on the walls behind furniture etc.

    I’m sure that was about the temprature difference between air and wall. Remember its that there is a significant difference in temprature, rather than the tempratures per se that cause condensation (along with humidity etc.)

    We put back the open fires and the problem largely disapeared (although you could die of exposure in the bathroom !!) The walls acted as a heat sink from the direct contact with the fire and the chimneys helped with ventilation. It took a long time to warm the place up if left empty for any time but once warm it was fine.

    Like I said before, maybe a rocket mass heater is the answer or a cob fireplace built into/onto the wall.

    Warm it up, dry it out and ventilate all in one go.

    Hope you can get this fixed cos its a great looking house.

    Roger :))

  • jill – Not sure. Amazingly, folks at Cob Cottage Co. and elsewhere in OR don’t seem to have many mold problems, which completely baffles me…

    Roger – I think your reply answered your own question. The walls soak up cold just as easily as they do heat, and this climate is TOO cold to overcome those cold temperatures effectively with a wood stove. Unless you are okay with burning lots and lots of wood, but I don’t think that makes much sense. It’s like trying to heat a refrigerator. It’s not efficient.

    Mold is no small concern. It should not exist in anyone’s living quarters. It has severe effects, and worse of all, it’s difficult to know the toll it can have on your health. I don’t want to take any chances, especially now that April has been having strange allergic reactions throughout the winter.

    The rocket stove I did build didn’t function and I’m not terribly interested in trying again. (Although that’s not to say I don’t like them or don’t think they work.) Fireplaces are not appealing — they are definitely not efficient.

  • Mike

    Hi Ziggy,
    Like others I’ve been following your project(s) off and on for about a year, and we send our sympathies from interior B.C. Canada. We started a building (that we keep expanding) about 4 years ago. Other projects, most notably an earthbag root cellar, a market garden business, and two young kids, have delayed completion of the project. Winters where we live fall to -30 degrees C. Really friggin cold, but we’re really dry.

    As we have not yet lived in the cob (hybrid) building, the following are only suggestions.

    Judging by the comments, the photo, and your description, you obviously need to improve drainage and ventilation.

    I would consider digging (when it warms up) a substantial curtain drain, backfilled with tamped gravel all around the building. This will definitely dry things out. One, problem, however, is that it looks like your site wasn’t properly excavated at the beginning. In an area with high humidity and heavy clay soil, a lot of these issues might have been prevented by thoroughly removing the topsoil from the area inside the foundation. Perhaps you did this, but I noticed an older photo of your completed foundation, and the area inside was pretty thick with grass. It seems like this is the case for the balecob kitchen project too. If you were in a position to re-do the floor in the cabin, you could also take measures to prevent moisture from rising up from the ground (ie.drainage layer, vapour barrier).

    Ventilation, especially in a humid area, is probably an equal (if not greater) concern. On our building all of the windows are double-paned sealed screened sliders. If there is any way you can replace existing windows with ones that can open and close, it would be a big help. I was really worried about ventilation in our root cellar, particularly the possibility that mold would grow inside on an earthen plaster, but so far so good. We put in a six inch vent pipe, that can be stuffed when things drop to -20 or below. But again, our drainage is superb. No clay to speak of.

    I totally empathize with you right now. In fact, you have me wondering about one particular wall on our building that could be a problem area. These buildings are so very labour intensive and, in many ways, very experimental. When experiments don’t work out, as is the case with us, it’s pretty demoralizing. The main thing is that you do not live in the building right now, if at all possible. The steps you will need to take are probably not winter-friendly projects. If you can crash somewhere else, revisit things come Spring.

    Oh, and as for the buildings on the Northwest Coast and England, the climates are indeed wet but not particularly cold.

  • Sean

    I have followed your progress since you started building your home and would just like to say that your experimentation and journey are an inspiration to a lot of people. I have a couple of thoughts about your moisture issues. I think you have a combination of problems. First you have a moisture/condensation issue. Second you have a mold issue. Yes, they are related, but your potential solutions vary depending on what you are fixing. If you fix the moisture/condensation issue then you will also fix the mold issue. But I think fixing the moisture issue is going to be harder and take longer. And since you are living in there, fixing the mold is of more immediate importance.

    In order to fix the mold problem, as you know, means changing the growing environment. Your idea of a lime plaster, with its antifungal properties, sure can’t hurt. Mold requires relative moisture above 60% to grow, optimally much higher to grow well. A lot of people have suggested increasing your ventilation, bringing in more outside air. I am not sure that will do much because in the winter with a fire you already have natural air infiltration from the pull of the air up your chimney. Also once that cold air heats up, its relative humidity drops, due to the fact warm air holds moisture more easily. This is why “conventional houses” are so dry in the winter. What it sounds like is that you are getting areas (bottom of the walls, bookshelves, behind furniture) where the warmer room air is getting trap and is cooling down. When that happens the air’s relative humidity raises and you have the right conditions for mold growth. The easiest and quickest solution is probably just more air circulation (not ventilation). By setting up a fan and moving the air around in your house you will probably prevent most of the mold growth. By having the air moving it never gets a chance to cool off enough to raise its humidity in that spot, thus preventing the proper environment for mold growth. In addition good air circulation will also aid in the evaporation of any condensation, just like cloths dried on the line dry much quicker on a breezy day than a still day. I know you don’t have power and I have seen that you have one of the heat engine fans on the woodstove, but I think a decent sized electric (sorry) fan, say 12 inches or more, would really go a long way to alleviating your mold issue.

    If memory serves you did not have a mold problem last winter when you had the ill-functioning rocket stove. My guess is that last year the house was never warm enough to create dead zones where the warm air could cool and lose its moisture, thus causing mold.

    Again, my memory of your adventures is that you did have mold issues in the summer. In your climate with humid summers and large thermal walls to cool that humid air, that is sort of to be expected. However, the fact that you have moisture issues in both the summer and the winter indicates that you have water infiltration from somewhere. My first look would be the floor, since it is a large earthen floor, but I looked at your old blog posts about the installation and you included a gravel layer. This should act as wicking break. I am not sure but you may need the same thing on the walls of your home. In many strawbale houses, they use a gravel layer held in place with wooden sides as a wicking barrier. That way moisture can leave, but can’t wick up from the ground, ruining the strawbale. You may be having the same problem. Unfortunately, you can’t really fix that now. I think your ideas about digging another drainage trench outside your foundation may help, though.

    Your immediate problem is controlling mold and this post is plenty long already, but I have some more thoughts and ideas that might help, particularly in regards to insulating the outside and adding a solar greenhouse for heat gain. If you would like, just let me know.

    Good luck and I hope for the best for you and April. Again what you are attempting to do it really great and I applaud you for it.

  • simon

    hi ziggy,
    also havin’ followed your blog since it started i can only add a thought based on my trailer-living experiences: these trailers we are using here in germany are often wood-built and experience mold problems regularly, too – mostly because of humid interior air and condensation at cold areas (walls, windows, corners). to avoid it there are often two small ventilation holes built in opposite walls (as high as possible to remove the most humid (warmest) air) which can not be closed – causing a small but ongoing ventilation effect, even though causing more heating energy needs.
    i hope you get this problem faced as you seem to get all the other hassle down in the post.

  • Joe

    THere have been alot of suggestions and ideas given- not sure what will work and not work from all of them- the one thing I do agree with is that if the condition is causing health related concerns you and April should consider getting out of that environment until that is solved.

  • God

    ROGER got something there I think you’re missing here.

    ROGER: “We put back the open fires and the problem largely disapeared… The walls acted as a heat sink from the direct contact with the fire and the chimneys helped with ventilation. It took a long time to warm the place up…once warm it was fine…”

    An “open fire” is different from a wood stove. The heat radiates outwards. You get sunburnt looking at an all-night campfire but you dont with a wood stove.

    SIMON continues with the same point.. “..there are often two small ventilation holes built in opposite walls (as high as possible to remove the most humid (warmest) air) which can not be closed – causing a small but ongoing ventilation effect, even though causing more heating energy needs…”

    AS OTHERS pointed out, moisture is coming through from the outside. Are your french drains lower than the foundation all the way around? Your roof overhangs going far out enough? Moisture breaks between the rubble foundation and the cobs? Constant ventilation to move the air around inside with vents or cracked open windows?


  • Casey

    Have you tried effective microrganisms? Maybe make up a batch of plastic with an EM solution or EM bokashi mixture and coar the walls. The EMs should flip the balance against the mold spores and neutralize them, and you wouldnt have to do this daily.
    It would be an experiment, but just may work. i know they treated mold in new orleans after katrina with EMs.

  • Casey

    That was supposed to say a batch of plaster not plastic. Sorry

  • John Sharp

    I live in and have been renovating an old stone/clay and cob cottage/mud hut.The Walls had been dry lined with aspestos and where soaking wet . So I removed the aspestos and moldy old wall paper and restored with lime mortar and painted with simple hydrated white lime paint. In places the walls are still damp at the base but there is no mold. The parts of the walls that are cob I have rendered over with lime in the same way and they seem to be free from mold.
    I think the issue with the mold may be related to your paint recipie and possibly to to much straw in the cob mix.As a potter I can tell you that clay can sit damp for months and never show mold so the mold has to be either the paint or the organic matter in the cob. So I would render with lime mortar and paint with a lime and clay paint mix.
    As for the weather hear, well they say if you can see North Wales mountains its about to rain and if you can’t see them , o well then its raining already.

Previous post:

Next post: