Earthen floor and mould

I have a cob (clay/straw) floor around 4" thick which was laid about 10 years ago. It was sealed with hot boiled linseed oil and then finished with bees wax. When the house was being used as a weekender, I noticed mould developing on the floor under the mats and furniture. I figured it was moisture coming up from the subsoil as I neglected to put a plastic membrane down before laying the floor. It didn't worry us in the early years but now that we live here full time I am concerned about the impact of the mould from a health perspective. We recently re-waxed the floor and the mould problem seems to have gotten worse. Digging it up would be a last resort as it is really a work of art which took weeks to lay. I thought about sealing it with some water proof sealant but not sure this would work? Any ideas?

Hi Paul,

Welcome to Permies!

Well, it sounds more like a finishing issue as much as not laying down plastic. If a foundation is well built and proper gravel layers and drainage installed, plastic should not be a "must have." Even with this done, in some regions, I have seen mold form. It is almost always the finish (and/or to much manure and "stuff" added to the cobb floor like pig or cow blood.)

Waxes can contribute to an issue as well as they are the only treatment of surfaces (even over epoxies and 20 coats of oil/varnish) that can actually seal a surface 100%. The other is traditional red and black Asian lacquer finishes (which few know how to do or could afford the time and material.) Wax is also easy to scuff and wear out, so typically it is not recommended (by me) as a dependable finish. Use it, but with the understanding of its low durability and likelihood of trapping moisture if that is an issue with a project.

Now, without being there, I can't really tell if you have a "moisture migration" issue, or if you just used bad flax oil, have "stuff" in your clay floor, or some other issue. I am guessing that you did not add salts or use a flax oil with a mould inhibitor or UV. stabilizer? These can greatly decrease the issue as well (or just plane stop it.) The addition of borax, or borax wash may also help you get this under control. The problem with salt additives (or others) is without experience in their use you may get some efflorescence

I am in the process of getting a list together for folks here of "appropriate" product manufactures that I am friends with, but you must promise to report back the results (good or bad.)

Also, if you have other questions of me, please post them as a 1.2.3. format to make it faster for me to get back to you. I can't always read through everything.

Good luck,

j

Since you spell mold with a 'u' in it, I take it you are from the UK. It probably is that moisture coming up from the subsoil. Have you been having flooding problems in your area?

Fungi are a natural component of soil, and since you use that soil to make dirt, the dirt floor is going to have spores in it that can come to life once it becomes moist enough for them. The short fix is to dry things out. Maybe purchase a dehumidifier and run it. Move the mats and the furniture so that the air can circulate and dry things out. The wax may have worsened the problem because fungi look on wax as food; you put food on top of them and they already had moisture under them -- what a nice picnic you set for them!

A judicious use of bleach might help out. It is the one thing that can kill fungi on the surface. If you go to seal or wax the floor again, give it a good mop down with a bleach solution first. That will kill surface mold and it will take them considerably longer to come back from that kind of abuse.

Hi John E, et al,

I agree with the "drying out," part as if a home is "too moist" you will continue to have issues no matter what you do.

Bleaching is an option (similar to what I said about using a borax wash) however I have seen some surface degradation from bleaches being used as chlorines tend to be really harsh even if diluted to 10%.

When you have applied these yourself to waxed and oiled clay plasters and cobbs, how do you avoid splashing and other issues of natural finish degradation. Have you ever used a borax wash, mould inhibitor additive before? What are your views on the different forms of flax or tung oil finishes?

This is a great conversation to share them in, and giving me good ideas that I should write about in the new section "finishes."

Regards,

j

Thanks John and Jay C for your insights.

We actually live on the coast in the south east of Australia which has a temperate climate. As a bit of background, the dwelling is load bearing strawbale rendered with mud/chaff and finished with sand/lime.

You have both raised issues which are relevant to my situation. With the moisture, I have noticed that after periods of consistent rain, parts of the floor darken and actually soften. So despite concrete footings 2 feet wide and 2 feet deep, the moisture is still making its way up to floor level. This is just in random patches, not widespread. Since moving here I have improved the drainage around the building and weather has been somewhat drier so no more soft patches.

The mould on the other hand occurs wherever the surface is covered with mats, furniture or under anything that sits directly on the floor for more than a few days. I have also noticed that the floor is still somewhat dry where the mould is growing. There appears to be no correlation between the previous moist patches and where the mould occurs.

I never considered that the problem maybe related to the cob floor mix used or finishing. The floor was actually just a mix of clay and straw only, which was done by hand in a bath tub. There were no other additives such as cow/pig blood or dung. The initial sealing was done with pure flax oil and then successive coats of flax oil and turpentine. No additives such as salts or mould inhibitor were added during the sealing process. The final wax finish was a mixture of bees wax and turpentine. One thing was that whilst the literature recommended boiled flax oil I could not always get this and often used raw flax oil.

Seems like the issue needs to be addressed from both a moisture and finish perspective. I figure the best way to minimise the moisture problem is to have the best possible drainage around the building supported by the dehumidifier inside. Will also do a test area using Jay C's suggestion of borax wash with mould inhibitor additive. Not sure how this will work over the waterproof wax finish but will give it a go and report back.

Thanks so much to you both for sharing your knowledge.

Regards,
Paul

Does the property/land have a hardpan? Mine does and when it rains the water can't all go down so it goes sideways and/or builds up. If you have a hardpan, you might try subsoiling the area around the house. Keep working on that drainage. Maybe you could lay plastic from the foundation out a couple meters, creating an umbrella. If it's feasible.

Hi Paul, et al,

First, or those recommending plastic...I must counter that and state that plastic is not the solution 95 plus percent of the time. Plastic is the "go to" material for so many things, and at best is a "band aid" to replace better attention to detail. Humans have been building and living in dry warm homes for millenia before plastics cam about. The issue is good drainage. Two feet or 20 feet of concrete (nasty stuff with its own issues) is not going to counter improper layout and application of drainage modalities.

One thing was that whilst the literature recommended boiled flax oil I could not always get this and often used raw flax oil.

I can probably claim with about 90% assurity this is your "food" for the mold mycelium. You cannot really use raw flax oil in this application. Even though it is a drying oil (unlike other types of oil) it is a food source for mold unless treated or dealt with in other ways. I have been trying to get one of my contacts to export to the "Down Under" for a while now (Paul you may be the guy) for there natrual finish products. The market down there is wide open.

It sounds like you still have some drainage remediation to do, and possible stripping of floors down to raw clay (the best you can) and strarting anew with proper matierals and methods.

Regards,

j

Jay C. White Cloud wrote:
I can probably claim with about 90% assurity this is your "food" for the mold mycelium. You cannot really use raw flax oil in this application. Even though it is a drying oil (unlike other types of oil) it is a food source for mold unless treated or dealt with in other ways.

I'll add the other 10% to Jay's assurance and make it a certainty. Whether it was raw or boiled, the flaxseed oil is not going to "dry" on an indoor floor like it would out in the sun. The "drying" of this oil is actually a polymerization reaction, and the more it polymerizes, the less there is for the mold to attack.

Two ways you can get linseed or flax oil to hurry up and polymerize on an indoor floor: (1) mirrors. Or UV lights. Strong UV radiation, either direct from the sun or from a UV lamp, with bust open the chemical bonds that need to break for polymerization to get started. And (2) chemical oxidants like benzoyl peroxide (acne medicine). They are also effective at opening the bonds and initiating polymerization. Swab it on and let it sit for a couple of days, and your floor shouldn't break out with mold pimples.

I will give a warning that there are studies suggesting benzoyl peroxide as being carcinogenic, and over exposure was the root cause. Heat also will accelerate the polymerization of most of the drying oils.

Flax oil is an autoxidize in that oxygen is the primary catalyst, while the UV radiation can also accelerate the formation of peroxides and hydroperoxides...from the fatty acids? (help me here John E. the "chemistry water" is getting too deep for me...)

Jay C. White Cloud wrote:
Flax oil is an autoxidize in that oxygen is the primary catalyst, while the UV radiation can also accelerate the formation of peroxides and hydroperoxides...from the fatty acids?

Flax oil only "auto-oxides" because our atmosphere is 20% oxygen. Even still, that oxygen needs an energy (light, heat) or chemical (peroxides, hydroperoxides) kick to open up the flax oil molecule and get things started. If you live in a place like Los Angeles, where ozone is a common air pollutant, that will also do. Ozone is oxygen on steroids and is quite capable of opening a flax oil molecule on its own.

I've been dealing with mold as well, although not on a cob floor, and i'm finding that essential oils (i'm using rosemary and lavender work very well in stopping mold growth

fungi are quite adaptable and nothing works 100% but it might work for you and whatever else it does smell nice too.

i would try rubbing the areas with essential oil mixed with alcohol and dry out the air as much as possible

Hi Paul,

I think it would be wise to figure out ways to prevent moisture from building up in the soil under your floor. You can seal it all you want, but until you eliminate the water seeping in under your home, you'll never solve it. In other words, let's go to the root cause, and address it, and not put a band aid on it, treating the symptom better sealer.

What can you do? I don't know the specifics of your home, so here's a list of things to consider and/or do if you haven't already done so: (1) be sure to install gutters and drain the roof water 10 to 20 feet away from your home by attaching hose or pipe to your down spouts, (2) dig up around your foundation and install a French drain, (3) be sure that the soil around your home slopes away from the house everywhere, so water naturally flows away from the building and not under it, and (4) if your home is on a slope, you may have to install a berm to move surface moisture (rainwater runoff) away from the building. Try these things and see if they help alleviate the underlying root cause.

Hope these help.

Dan

I haven't seen anyone say this so I will ask, "Have you installed a vapor barrier?". All the earthen floors installed here (Pacific Northwest, USA) have vapor barriers. These prevent water vapor from rising through your floor layers. Is your floor directly connected to the ground? If it is you might need to rip up the cob to install a proper barrier. Otherwise you can install under the house by crawling under the foundation.

Hi Adam, et al,

Please don't take this as criticism, your thinking is logical, and also based on what could be called a "normative constructure behavior of modernity." In other words...its a habit based more on, "everyone does it," then "why do we do it?" and is there an alternative...which of course there was and is.

Humans have been constructing dry earthen forms of architecture and other structures for thousands of years without "vapor barriers" and they still do in many less "modernized" area of the world that build naturally with earth, or have unbroken "knowledge lineages" that go back millennia, as you can still find in the Middle East, Africa, and Asia. So the question isn't do we need to add more plastic and modernity to the equation (not a permies solution) yet rather...what did our and current successful users do to accomplish this. The answer is as Dan suggested....DRAINAGE...and the many forms this can take. From roof (which is part of the drainage system) to the way the foundation is designed (what with?) and constructed.


As I get into this post, I realize that it could get rather detailed, with lots of info, so I think I will just post a link to a new thread here.

Cant help but put in my modern and normative veiwpoint in that plastic installed in subgrade new construction is meant for vapor not bulk water. The layer of gravel below the plastic is meant as the capillary break and means of bulk water drainage.

Hi Brian, et al.

I have added this to more post of late than normal...yet have not placed it directly in correlation to such a post as the one above.

Any of us that have worked in the construction field understands the "intent" of the plastic sheeting that has been "creeping" into construction methodologies over the last 40 years. This is the "habit" that has formed, yet the "intent" to begin with was not thoroughly understood or agreed upon by many in the building science field.

Gravel does indeed mitigate the both liquid and vapor effectively...and I would suggest more so in some regards than plastics does when it works in concert with other "natural" parts of the system.

So...I will state it directly once again...if the buildings in Venice, Italy, mills in and next to water around the globe, etc have the same level of ambient humidity as the modern building with plastic "vapor barriers" (yet not the issues of building sickness, the need for additional mechanical augmentation, etc) then why does a builder focused on "natural/permaculture" based structures need to install them? These building work...work well...and have for over 1000 years in some cases...

I would also state...again...I have spent an exorbitant amount of time "fixing" many structures with these allegedly "necessary" vapor barriers (be it Tyvek, plastic sheeting or the related) in walls/floors/ceilings for issues of moisture build up, mold, pest issues, decomposing siding and structural materials. Then only to have another contractor suggest more of the same and then additional technology to try to "make it work," with all types of mechanicals...and more technology.

I say...go for it...if you chose to, yet this is not the goals of "permaculture architecture." We are not trying for more technological dependence with its encumbered "moving parts" that must be monitored, maintained and repaired, nor more modern materials like plastics and chemicals, when you can achieve sustainable architecture without them...It just does not make any sense to me to keep suggesting products of modernity, or insisting that these are better systems when the traditional/natural ones aren't broken...just not well understood by most contractors and designers...(not as profitable usually for them either) especially when the goals are less technology dependance, less chemical exposure, and a more natural way of living overall....

Regards,

j

Would love to hear more about how gravel can stop water in its vapor form.

Hmmm...did I say it stops it?

If I did someplace then I stand corrected.

What I meant was: Gravel does indeed mitigate both liquid and vapor effectively...and I would suggest more so in some regards than plastics does, and for a very long duration, when it works in concert with other "natural" parts of a building system.

Regards,

j

I reckon it’s the concrete slab that is wicking the moisture up into the cob floor and breeding and feeding the mould. I reckon the long therm permanent solution is replacing the slab with gravel and or stone which means starting again. It’s a known fact that concrete slabs wick water up from the ground especially if they are on low ground. You could try to dig a French drain all the way around the slab deeper than the slab to drain all water away from the slab that is doing the wicking.

Jay C. White Cloud wrote:Hi Paul,
I am guessing that you did not add salts or use a flax oil with a mould inhibitor or UV. stabilizer?  These can greatly decrease the issue as well (or just plane stop it.)  The addition of borax, or borax wash may also help you get this under control.  
j

Do you recommend and how much per sq foot Borax / Boron to be added while mixing the floor?

Is that straight boron or 20.5% boron / Borox?