Earthen plaster over old concrete foundation?

I'm finally getting around to finishing the flood cleanup from a broken pipe in Feb. (I've got a lot on my plate.) In Feb I removed the drywall in the basement and discovered that the painted foundation wall is bubbling and the concrete is crumbling behind the bubbles. I have now removed the stud walls entirely with the intention of stripping the paint and plastering the concrete. There are also a couple cracks I'd like to keep an eye on so covering it with another stud wall isn't the solution. The surface of the concrete is quite rough and unpleasantly irregular so it will need some sort of coating to make it a nice place to live in (it was and will be the kids' bedroom). The exterior of the 95 year old foundation (obviously) has no waterproofing (other than our heavy clay soil) so the plaster must be breathable to allow the small amount of moisture that seeps through to evaporate into the room. The solution must also be dirt cheap. We're living in poverty for the foreseeable future and free is the best price though I already bought and mixed up some hydrated lime in anticipation of needing a more durable surface to hold up to my kids. I've never worked with cob or earthen plasters but I'm fairly experienced with traditional lath and plaster as I've done a bit of historically sensitive restoration work on the main floor.

The solution I'm hoping will be appropriate is to cob over the walls using clay from the backyard. The straw will hopefully take a bit of the edge off the winter cold coming through. Then finish with either a lime wash or lime plaster as deemed necessary for durability. Will the cob stick to the concrete or will I need to screw up some metal lath? Most of the surface is pretty rough with 2-3" deep crevices scattered around the wall a few patches of remarkably smooth (but far from troweled smooth) surface. (Remember, this foundation was poured in 1919.)

I also have a couple interior stud walls that need a new skin on it. Since I hate drywall with a passion (and can only fit skinny strips of it into the basement, multiplying the most detestable part of the work), I'd like to use the cob and plaster for that too. Would wood lath nailed to the studs be adequate to hold the cob or should I improvise a wattle between the studs? I don't want to add much more thickness to those walls than what was there to begin with but a little extra beef in the wall would hold heat better as it does get a bit of sun in the winter.

Does the cow manure really make that big a difference in durability? How fresh does it really have to be. That stuff is hard to find in the city and will require some logistical planning to retrieve it from a friend's farm.

Am I deliriously naive or is this a reasonable plan? Other suggestions/improvements? Is there an equally cheap solution that would be faster? Winter is coming and I'd like the kids to be settled into their improved space before it gets too terribly cold (by Thanksgiving-ish at the latest).

Thanks in advance for the feedback. (c:

Hi Tim,

I commend you for removing all the "coverings" on the walls to do a better job of assessment and the remedial work to come. OPC...especially modern versions of it...are generally a nasty lot of work in my experience...When it begins to fail there is little that can be done. OPC is not the "magic solution" to so much as, so many believe it to be...and has very limiting parameters of actually being done well that most that do mix and use it...do not...do it well...which is what you seem to be now facing...

I can say before asking some questions I need to further help...clay plasters over OPC is a NO GO...it is not a matter of if...but when it will fail...

If this is to be "sleeping space" of your children, I agree you will need so source of finish, but "waterproofing" concrete is nothing more than a "band aid" over a "bad infection." You may not see the rot...and it may go on for years...nevertheless...the decay will continue until the moisture issues and drainage is dealt with. I have seen this so many time in my career that it saddens me each time I come across another case...I have some in my own area that insist on every "quick fix" and "waterproofing-tanking" method they can find (or get sold to them.) If they had just "bitten the bullet," and dealt with installing proper drainage they would be done and not still spending money...Some now have health issues from the mold and/or chemicals used in the "tanking solutions."

If you have any type of routine high moisture/condensation, and/or flooding in that basement your only choice is proper drainage...

Concrete (OPC) in general facilitate moisture migration effectively, and allowing that back into the house...even in "small amounts," as you have suggested is the least desirable solution possible. There of course will be some...but this should be relegated to a minimum. As for it being "dirt cheap," it will all depend on your desire to do a lot of hand digging and/or other invasive means to facilitate better water management around the foundation.

In the interim...a lime wash (several coats) is going to be your very least expensive solution and perhaps do to fiscal burdens your only one at this time...

With a circa of 1919 I am not sure if this is all concrete and not roughly parged stone? Either way if you have your heart set on cobb work...you are going to have to put at least three coats of lime plaster on the walls...then...create a minimum of a 70mm dead air space with more studwork and fill in with a "light straw clay" cobbing method to clay plaster over...This will give you the "affect" you are looking for, add some thermal resistance to the "dank and cold" while still allowing the concrete to "sweat" without effecting too terribly the bedrooms.

I must suggest that this space is much better suited for a "root cellar" than a living space from the sounds of it...?

Does the cow manure really make that big a difference in durability?

In some forms and application...absolutely...for interior work not so much, as it is a binder for mainly exterior work (but not always.)

I will do my best to address any and all your questions...

Regards,

j

I too have wondered if coving concrete with clay is a good idea or not, my intuition tells me it's not because of the different properties of the two materials.

In an older house I once lived in I observed that the concrete foundation was crumbly in many places where it had been plastered over and painted but when I removed a fuel oil tank I found the uncoated wall behind it to be rock solid still.

For my current house building project I will be using a product called thorocoat or thoroseal (two different formulas) that is supposed to build up to a "nice" finishes surface but still allow vapor to leave to concrete. In Tim's situation it might not be the answer though because of its expense and that it provides no insulation, which in my case is on the outside of the wall.

Jay are you familiar with these products? I've sort of painted myself into the OPC corner and just want something that will work as good as the concrete and keep the mass in contact with the air of the living space.

And by "70mm dead air space" do you mean this gets filled with light clay straw or the studs are 70mm away from the wall and they get the light clay straw and the air space remains between the studs and the wall?

Hi Scott,

OPC is only as good as the original poor...unfortunately, most are not what they could be...and hardly any are as good as we had being made 2000 years ago...

Jay are you familiar with these products? I've sort of painted myself into the OPC corner and just want something that will work as good as the concrete and keep the mass in contact with the air of the living space.

Yes, and I don't care for either of them....I would rather use a limewash or if I must a traditional mineral paint. If it is something like a zoo enclosure we would use a elastomeric, or one of the dedicated epoxies...and these have there challenges too over concrete.

One of the countries leading authorities on OPC and portlands, as well as, natural pozzolans is a collegue of mine...Mike Edison. One of his products that may be of interest to you is silane-siloxane

And by "70mm dead air space" do you mean this gets filled with light clay straw or the studs are 70mm away from the wall and they get the light clay straw and the air space remains between the studs and the wall?

Sorry I wasn't clearer on that...this 70mm is a "dead air space" as a mitigation zone for condensed and permeated moisture from the OPC. The framework would take the light straw claw and/or other natural finishes for the living space.

Regards,

j

Thanks Jay,

So, cob and concrete don't get along? That's exactly why I asked before doing. (The forum on earthen floors seems to indicate that they get along reasonable well in horizontal applications. Since the Saltio tile in there doesn't extend to edge of the room and is oriented to the frame walls I removed and are about 10 degrees off from the foundation walls and is frustratingly irregular surface for building with blocks or wooden train layouts, my plan is to chip it out and go with earth.)

I'm assuming that a lime plaster (with sand) would work over the OPC just as well as the wash. I'd post a picture but the kids are sleeping in the next room and I don't want to rummage around. The surface really is grossly craggy (mostly an original feature from a thick mix and little effort to eliminate air pockets against the form). Unless it was common practice to parge stone using forms, the foundation is definitely concrete. Another blemish I'd like to hide is from the occasional gap between form boards leaving a bulging monolithic vein across a portion of the wall. It's going to take a lot more than a few lime washes to smooth that out.

I'm not too worried about moisture entering the room through the wall. The air here is pretty dry. Doesn't get above 50% humidity in the summer (unless its actively raining) and 10-15% is common in winter. Any water that makes it to the surface will be a welcome and almost instant addition to the interior air. My main concern is getting the paint off that is sealing the water into the wall and making it look nice without resealing it.

Any recommendations on creating a bit more mass in the interior walls? Straight cob? framed with cob between? wattle and daub? Forget it?

Thanks,
Tim

Tim I'm sure you will get a response from Jay in short order -but the usual questions about improving your drainage is in order.

As crazy as it first sounds in order to build big houses on small lots the house has to have a minimum setback from the property line
SO the amount of roof overhang gets trimmed back to the minimum and beyond what code enforcement officer is going to get on a
ladder and measure the distance !

Latter on all future jobs the building contractor cheats the future homeowner of minimum foundation security !

Often NO drainage tile is put in at the foundations walls footers ! And yes this has been going on for a long time, long enough for D.I.Y.
home builders to perpetuate the mistakes !Have you checked for drainage tile outlets on your property?

Are your neighbors having problems too! Do your storm drains flood during/after downpours?

What shape are your gutters and downspouts in! Do the down spouts lead several feet away from the foundation?

Is the landscaping carefully contoured to promote the drainage of surface water away from the house !

For the Good of the Craft ! Big AL

Hi Allen. Strictly speaking my questions are directed at anyone who cares to give an informed answer.

I'd remind you that the house is 95 years old. There were no such codes to cheat, only the homeowner. The garage (built at the same time) is evidence as the foundation sits 6 inches past the back property line and the north eave extends over my neighbor's.

The eves are beautifully sized for sun exposure. The basement windows are in 24 hour shade around the summer solstice and the winter sun hits an inch above the top of the south windows. It smacks of thoughtful construction.

Frankly, aside from some settling, the main house and it's foundation have stood sound for the large majority of its 95 years. The crumbling is relatively new and related to the obviously relatively recent latex paint on the walls.

I have personally recontoured the grade around the house for maximum drainage. There is no drainage system at the bottom of the foundation. It sits several very below the elevation of utility services and there is no sump. As I eluded to in my first post, a frozen pipe was the source of my flood last Feb and moisture is otherwise not an issue. We sleep down there every night. If there were moisture/water infiltration issues, I wouldn't use it as a bedroom. Nor would I pitch a tent in a dry stream bed. If it occasionally gets water, I'm not sleeping there.

I would love to have the time and money to prevent water from getting to the foundation from the outside but disturbing the heavy clay subsoil would cause more problems than could easily be addressed. (The definition of easy being anything beyond digging down to the footer with a shovel to waterproof the concrete and install drainage below the footer.) However, now is not the time for me to think about or deal with that particular issue issue.

Tim, it looks like we have very similar projects.
I am currently restoring an 87 year old home with a problematic monolithic concrete foundation, poorly poured utilizing 12" wide slip forms. The entire basement would turn into a pond, but even so the lime plaster held to the cement wonderfully, unless fully saturated and then came off in sheets.
We tore up the concrete drive uphill from the house, broke it into pieces and laid it as the base for a gravel drive. We restored proper grade, repaired the leaking roof and all run-off is diverted to a pond. We have had no problems yet, but have not been through a winter, so we'll see.
I debated using light straw clay infill(this is actually a much better system, but more labor intensive and I was not sure how to get a good pack with out a back to the stud wall), but decided to stud frame, fill bays with Roxul, install DB+ smart membrane on the inside and drywall. The drywall is covered with gypsum plaster and lime washed as this was the way the original builder finished the lath and lime render. This gives a breathable/durable wall that will not have moisture build-up.

Bill,
Yes it sounds familiar. Though, my basement appears to have been an unfinished cellar for much of its life. The paint predates us but is fairly recent and there's no evidence of previous interior finishing. I added a couple walls when we first moved in. I put up a little drywall but had to cut the sheets into such small pieces it was mostly joints and there is nothing worse than taping mudding and sanding those seams. I did the rest in a commercially available plaster over wood lath and put a skim coat over the drywall to make it all match. At the time, I was surprised to find that the modern plaster dried rock hard unlike the original plaster on the main floor (I now know that the commercial plaster included cement and gypsum). The old lime plaster is much easier to work with and had a longer working time. In the interest of reducing the risk of injury to kids or animals, I opted for hydrated lime but I'm certain the original stuff was prepared from quicklime (which was tempting to eliminate the 6+ week rest). Oh, well.

I won't be insulating the basement as it is time consuming, costly, and I can do that effectively from the outside. Eventually, I will turn my attention to the outside and prepare a waterproof barrier that encompasses several feet of surrounding soil as well as a bit of thermal battery, in which case I will want more efficient heat transfer through the wall anyway.

I think I'm rambling.

So, unless gravely cautioned otherwise (according to my limiting parameters), I'll directly lime plaster the foundation wall and do a finish coat over some kind of cob for the center wall to store a bit more winter sun love. I'm still puzzling over the this wall which sperates the bedroom from the furnace. I don't want to finish the furnace side but,I do want a little mass to store some heat given of from the (pretty inefficient) furnace, but I don't want to answer a lot of questions from an inspector on the of chance that I opt for a more conventional replacement someday. (I'd much prefer to replace it with RMH but of something happens to it before I have a proven RMH in place, I'll want something fast. If I can do something through the proper channels I do. As for finishing the basement, it's 1 inch too short to be grandfathered in by the city as a legal basement. Technically it's a crawl space and can't get s permit to finish it of I wanted to.

Damn, I'm rambling again.

Hey Tim, et al,

If I miss anything...forgive me, and ask again...

I do want a little mass to store some heat given off from the (pretty inefficient) furnace, but I don't want to answer a lot of questions from an inspector on the of chance that I opt for a more conventional replacement someday.

I sure if I was there I would have better (more clear) directives to share...sorry. In the interim...you could build a traditional Kubbhus style wall.

Some framework, cordwood, and cobb....very fast to build...and massive....If you create a viable air layer and do not mind loosing some "square footage" this may even be a wall treatment to consider...

Regards,

j

I've been looking thru the threads, and perhaps someone here has an answer for me regards limewash over conventionl, fresh gypsum plaster skim

I have a 300 year old stone cottage, with various layers of post 1950 conventional cement external pointing and interior paint. As long as there's ambient heat the building is fine, if not it gets mouldy quickly.

Anyhow for better or worse, I just gave in to government scheme to have the house interanlly insulated. They have used rock wool in a kind of woodchip/polystyrene stud frame, followed by plaster board and they are now doing a gypsum skim coat.

Bill Bradbury wrote this some yrs ago on permies https://permies.com/t/43317/Bathroom-Remodel-Retrofitting-Natural-Materials

"Gypsum is a great plaster that often gets overlooked, because that is what drywall is made from. It is still a great natural mineral plaster. I like to then apply a lime wash"

I'm aware the fresh plaster needs to dry to a pale finish and then have at least one coat of watered down paint, before a finish coat. My question is: can I use a limewash onto the fresh gypsum skim (over conventional plasterboard)?

Would I need to let the plaster dry completely first, as is the conventional route, or because I'm using limewash is slightly damp better?

And following the limewash, can I then use conventional paint (a pity I know, just loads of pots of it here when we moved in), or better to just keep up the limewash layers in the way we would do with a lime plaster wall?

Really hope someone can offer insight before the gypsum coat dries!
Best wishes from Wet and Windy West Wales, UK
Jasmine

I'd like to add my own request for feedback into this mix.

I have a 1856 home with a limestone foundation. The main house has a walk-in basement with low ceilings but servicable area that someone many years ago limewashed the interior walls. It needs redoing.

The exterior of the house was unfortunately cement stuccoed. This is my current task

I want to lime plaster over the very rough cement stucco. The stucco looks to have been finished with a heavy sponge and has peaks and valleys of "daubing" that are apx .5-1 inch in depth or height. It is extremely rough. I'm hoping to lime plaster over it but would love to have some serious people verify my approach.  Because of the roughness I'm hoping to be able to plaster right onto it after pressure washing it.

Thoughts, feedback, stern talking to?