Plaster and Siding for Natural Buildings

Dan Chiras has been involved in natural and alternative building since 1994 and lives in an off-grid passive solar/solar electric home in the foothills of the Rockies. His house is built from straw bales, rammed earth tires, and numerous green building materials and is powered entirely by wind and solar energy. Dan is author of The Natural House: A Complete Guide to Healthy, Energy-Efficient, Environmental Homes, published by Chelsea Green; and numerous articles on natural building and sustainable design, which have appeared in Mother Earth News, Natural Home, and The Last Straw. Dan embraces a comprehensive systems approach to building that offers a wide range of benefits to people, the planet, and our economy. He will field general questions on natural building and offers consultation on project design and construction, as well as lectures and workshops on various aspects of natural and sustainable design and construction.

Questions and Answers

Q: Also, we get quite a lot of rain and I am not sure about the type of plaster to use. I like the look of the clay alis but don't know what formula to use due to the humidity.

A: Believe it or not, an earthen plaster should work fine both inside and out, but the exterior walls will need to be protected by overhang. Be sure to install gutters and downspouts so no water will run down the walls. You can use local subsoil to make a plaster, not topsoil, so long as it contains about 5 to 7 percent clay. See the Natural Plasters book for directions on how to make it.

Q: What kind of siding is best for homes in areas prone to wild fires? Why?

A: (Kelly) The best materials to build a house out of in a fire-prone area is something that will not burn, such as all masonry and earthen materials. I would assume from your question that you want to know what to apply to a wood-framed house. In the area that I live in, which has a severe danger of wildfires, many people apply a cement stucco to their exterior. Other possibilities would be stabilized adobe, brick veneers, non-flammable synthetic siding, etc.

Q: I am remodeling a wood built home. I am trying to decide what to do for interior walls (pine vs. sheetrock). I have no idea if sheetrock is treated with anything.

A: (Kelly) As far as I know, sheetrock is not generally treated...it is basically gypsum and paper. It might be better than using pine boards, unless the pine is harvested sustainably.

Q: I live in Central New Jersey and plan to redo the outside of a 1940 house. I would like to use eco friendly siding. I wish it to be low maintenance. Please refer me to a product that will help the environment.

A: You may want to look into Hardi plank siding. It's made from cement and wood fiber (recycled) and is a very durable product!

Q and A: I do not want to use traditional "stucco". I want to use a natural [earthen?] plaster for some exterior walls on a modern/contemporary "Green" environmental residential project I am designing in Southern CA. I also plan on using ICF (cement-wood) wall forms for the most part, so the plaster will have a very rough surface area for application. So what I'm basically looking for is a proven plaster recipe that'll work in the Coastal Southern California region (Orange County), that will look clean, keep from cracking, and generally hold up well. I have seen some programs showing natural plaster for adobes (PBS) and also bought a natural plaster book,.. but most of it seems to be about lots of trial and error, many different ingredients,..etc., without showing a real proven example or recipe. This just confuses me, since natural plaster has been around for how many centuries? There has to be a useable recipe that even someone who just does traditional stucco for a living could follow and apply.

Everyone wants a recipe for natural plaster, but what people don't understand is that the subsoil varies from one place to the next...so a recipe that works at your site may not work on a neighboring site even a half a mile away. Soil clay content and the type of clay varies from one location to another. So, that's why we suggest experimenting with your subsoil to find a suitable recipe. That said, I've found that earthen plasters work pretty well with widely varying ratios of sand, clay, and silt. If you can make a sample that doesn't crack or dust off using your subsoil, it will probably hold up pretty well. You can also consider Clayote, a natural plaster manufactured in New Mexico. It comes in buckets, costing $50 to $60 per five gallons...It's generally used for interior plasters only, however.

I also hear conflicting comments (yours as well) about earthen plaster being able to hold up in rain and needing some sort of an over hang, while others seem to think it should be just fine. Most adobe designs seem to have pretty exposed walls for the most part, so how can that be. Plus, though it does rain here in CA, it's not like other regions and we don't get winters either. Couldn't it be sealed/protected as well? There are natural plant-chemistry products which are used to help seal concrete, even paint and stain it, and isn't natural plaster essentially the same thing?

You do see earthen plasters on some buildings built without overhangs, but they require periodic maintenance -- notably, reapplication of the alis (a protective clay paint) every year or so. Frankly, Darryl, earthen plasters don't hold up to driving rain. I generally recommend that people who are in areas that experience driving rains (who don't want to provide sufficient overhangs) to apply a lime-sand plaster over an earthen plaster. You can seal them with various oils (described in my book) but I wouldn't rely on this for year-round protection, myself, and some people use cactus juice and say that works pretty well to weatherize a plaster. They say it works pretty well, but I have no experience with this myself so I can't say one way or another.

I'm also curious about the ingredients used in natural plasters. Earthen sounds the best to me, with cactus juice (opuntia), clay, sand,..??? It also seems that fibers (plant,..) added to the plaster helps not only strengthen it, but also helps with cracking (think fiber reinforced concrete - Ductal). Personally, I would think to use hemp fibers/batting. It's strong, has long fibers, is rot/mold resistant,..etc. Lime plaster sounds ok, but the fact that it is an eye, skin, lung,.. irritant concerns me. Anyway, I want to find someone who is knowledgeable and experienced in this field, someone that does this for a job. I'd think that there has to be someone in CA who can do this. Any insight/help is sincerely appreciated.

You might check with the California Straw Bale Association to see if you can find someone in your area who knows natural plasters. There aren't too many people who are applying natural plasters commercially, but if anyone would know, the folks from this organization would.

Q: I'm looking to cover some interior plaster walls of an old farmhouse I'm renovating. I'd like to end up with a nice textured, adobe look, and I was hoping I could get some recommendations on possible wall covering ideas. I'd like something that one could easily mix up in small batches, that would use inexpensive or locally-available materials (I have a nice very fine sandy loam soil on site), and that would adhere to the existing plaster wall without falling off easily and making a mess. Any thoughts appreciated!

A: I'd suggest an earthen plaster made from the subsoil on your property. You might want to get a copy of my book, The Natural Plaster Book co-authored with Cedar Rose Guelberth, to learn more about making earthen plasters or attend a workshop. Natural plasters are prepared from subsoil containing varying amounts of clay, sand, and silt. Do not use topsoil. You screen out the large and small rocks add water and voila! you have a plaster. You only need a subsoil with 5 to 7 percent clay to get a good plaster in most locations. You'll need to test your subsoil first, however, to be sure the mix is right. We explain the simple tests you need to conduct in our book.

Q: We have an old building on our farm that is basically structurally sound, but the fiberboard siding has deteriorated. We would like to make the most of this building by applying some earth friendly techniques. We have recently acquired a huge amount of clay from excavating a small pond. Can we apply lath to the fiberboard and plaster over it with earthen plaster? Any suggestions?

A: You could probably apply a clay plaster as you suggest, but I'd have to see the structure to determine whether this would work. It's hard to make a diagnosis such as this over the Internet. The main factor of concern is how exposed the wall is. If there's a good overhang and you don't receive a lot of driving rain, which will wash off an earthen plaster, your idea might work. You might consider an earthen plaster base coat and a lime plaster finish coat...or simply adding lime (about 5 percent) to the earthen plaster to create a more durable surface. As I said, though, I'd want to see the structure before giving you a nod. Do you have any people in your neck of the woods who knows earthen plasters?

Q and A (Kelly): I'm looking at a few different alternative ways of construction. I would imagine that the hay bail homes and adobe homes have similar problems of rain soaking through the stucco or whatever coating you would put on the outside; would this be an accurate assumption?

Both adobe and strawbale homes do best with substantial roof overhangs to protect the walls from too much moisture. Neither of them need to be completely sealed from moisture though...it is best if they are allowed to breath.

In your opinion does the modern day home that most construction companies build have the same problems of water soaking through their exterior walls as well if untreated with paint or another similar coating?

Wood-framed and sheathed homes generally use a material like Tyvec behind the sheathing; it allows some vapor to pass through, but not moisture.

Do the Earth Bag homes use similar coatings on the exterior walls as do the other styles of homes? 

Earthbags do have to be covered with some some of protective coating, I have heard of the use of stucco, papercrete, and earthen plasters. I am an advocate of breathable walls, if at all possible; it is usually healthier for both house and inhabitants.

Q: Is it possible to use lime plaster on a stick-frame home. I am trying to come up with alternative natural siding options. Can you give me some inexpensive examples and the method one would use to apply them? The two-story house that my husband is building all himself is in NW Louisiana and has only plywood sheathing and black paper on the outside.

A: You can apply lime to a stick frame house, the same way cement stucco is applied. I suggest you and your husband contact local stucco subcontracters and ask them what they use as a substrate for cement stucco in your area. Run a few tests first, though, to be sure it works.

Q: What sort of earth-friendly siding do you recommend?

A: I recommend Hardieplank lap siding. It's a mixture of cement and wood fiber and lasts a long, long time.  There are a couple of other similar brands (Maxiplank and Fortra Fiber-Cement siding).

Q: In one of the forums you said that there are sources for formaldehyde-free osb and to write you if we wanted more info. I definitely am looking for this product since I have asthma and chemical sensitivities and am building a healthy home in Southern rural Oregon (Applegate Valley). Can you tell me who makes the formaldehyde-free osb?

A: Weyerhouser and Louisiana Pacific both produce a low-formaldehyde OSB. The James H. Huber Company produces a no-formaldehyde OSB. 

Q: We are having a shell built by Tierra Concrete Homes that we will do all the finish work on. I would like to use an earth plaster on the interior concrete walls. There are several corrections that need to be made in the walls and I think I'll need to build up a fairly thick layer of base coat in order to smooth things out. I'm trying to figure out the best possible adhesion coat and am wondering if your basic wheat paste recipe (from your Natural Plaster book) will be adequate or if I should put a little cement and or plaster into it to create a better transition from the concrete walls to the plaster finish and should this adhesion coat or even the plaster type vary any for different room usages (bathroom vs bedroom)? Any guidance you can offer will be greatly appreciated.

A: Very interesting project. I've applied earthen plaster directly over cement stucco and rather successfully, but the surface was very rough. I found that the earthen plaster stuck very well. I've put natural plaster over small areas of concrete wall (in a NY project) but frankly I am not very confident that the plaster will stick over the long haul. The true test will be whether the plaster is still sticking to the cement 5 or 10 years from now. Applying an earthen plaster right over concrete is going to be challenging. I'm not sure the adhesion coat discussed in my book, The Natural Plaster Book, will work over large expanses. Is there any way you can rough up the surfaces to which you will be applying earthen plaster to give it something to key into?  Maybe you  can insert some wire into your forms or some plastic mesh that can be stripped from the wall? Roughing up the wall would be my first approach. If that's not possible, try the adhesion coat, experimentally. Include lots of coarse sand in the adhesion coat so you have a pretty rough surface. I'd try to minimize the thickness of the base plaster layer, too. And, I'd apply a really sticky base coat (lots of clay). Keep it as thin as possible, and apply the base coat as rough as possible.  Then you can apply a finish coat over that base coat. I would try a section or two to see how it works out, before you plaster the whole house.

Q: I have a client who wants to put an exterior earthen plaster mixture over his log home (exterior application). I have been working with Barry Fuller as my research indicated that papercrete is the best option for this project. The client wants to use clay from his site mixed straw but I am concerned about the difference in expansion factors of the material. We are located in in South Africa, the coastal region. Any comments or suggestions would be great.

A: (Kelly) I can see either earthen plaster or papercrete as possible materials for this job. It is not unusual to put stucco over wooden walls, but usually they lay down a layer of tar paper first, and then use  some sort of stucco mesh, like chicken wire, tacked on the outside before plastering. This should help stabilize any differences in expansion. I would recommend some experimental patches to make sure that the result is acceptable.

C: The papercrete procedure was also to use the chicken wire but no paper underneath, we were to remove the varnish and rough the wood by sanding and sandblasting.

A: (Kelly) My suggestion to use the tar paper under the papercrete was really to protect the wood from getting damp, should the papercrete absorb enough moisture to pass it into the wood. If the wood is already varnished, I would just leave it that way and do the papercrete with mesh directly over it.

Q: We have a wood frame (2x4 that we're making into 2x6) room that we're renovating. We've torn out the interior walls, so we can insulate them. What I'm wondering is how to put on a good earthen plaster after we're done. What base material (plywood etc) over the vapor barrier would you recommend? We were thinking of covering plywood with chicken wire so the plaster sticks. After that do we need two plaster coats?

A: Some people apply earthen plaster directly over drywall. Some of them apply chicken wire to the drywall first to hold the earthen plaster in place; others don't. Both seem to get satisfactory results. Before doing so, you should run some tests to be sure it works. The stickier your plaster, the more likely you'll be successful. You may also be able to use straw board or some such material. Straw board is rough and makes a good base for earthen plaster, or so I am told. I have never used it, but am contemplating this product for a big project I have coming up this Fall. I believe you have to water proof the straw board first, and it is pretty expensive.

Q: We Have developed a whole foods growing system that will use special grow lights (light therapy) in a controlled housing environment completely void of exterior light. Tests have shown vastly increased food production.

What we are looking for is a natural building material with a long building life that is 25'Wx22'Hx96'L with no interior walls or support structures, completely solar powered. That can be covered with an environmental energy paint barrier to aid in energy efficiency. We could use a concrete monolithic structure but are interested in a less expensive & a more sustainable natural material. We have an energy paint that has high reflectivity that will also be used on the interior surface to spread the blue/red spectrum light.

A: Is this building above ground? If so, you might consider rammed earth or strawbale, provided you do a good job of controlling humidity and preventing humidity from entering the walls, especially around windows, doors, light switches, electrical outlets, and other penetrations.

The problem with a natural material is that we usually cover them with earthen plaster, especially on the interior walls, which are protected from the weather. Applying a latex-based paint with a reflective additive is generally not advisable. You could use cement stucco on the inside and then a synthetic stucco finish coat. To that you could apply your paint.

Q: Would an earthen render provide strong enough keys on a plaster and lath building (as a first coat) ?

A: An earthen plaster can definitely have a strong enough key for a first coat particularly if you make a mix with this in mind, and scratch the surface with a plasterer's comb.

Q: I am an Architect currently working in Rajasthan,India. I had certain queries regarding mud plaster and lime plaster done on walls. I wanted to use this technique to renovate an old retail store. The walls are currently done in cement plaster and then painted for all these years. Can mud or lime plaster be done on such walls? Will they not crack after drying up?

A: You can certainly add plaster to your walls. Mud plaster would be the easiest as lime is much more complicated and would likely need a substrate of some kind (wire for example) to allow the lime to adhere well to the wall. Mud plaster simply needs enough binder (sticky clay or an additive like a starch paste) to allow it to stick. If you make sure to apply it in a thin coat(s) (3mm-6mm) it has a better chance of success. Cracking happens due to an imbalance in a recipe most commonly. Cracks show up when the recipe has too much clay and not enough sand or fiber to reduce the cracking. So doing tests to create a strong plaster recipe is the most important step. If the wall has cracks or seams the inherent movement of the building over time (heating and cooling for example) can also cause cracks in the plaster. But if your wall is stable and without cracks your plaster is likely to be crack free too. By the way, you can also make a paint from your mud which is easy to apply with a brush and is thick enough to cover the existing paint but thin enough to prevent cracking.

Q: I have an earthbag home I built in New Mexico. I’m currently working on the finish coat of plaster on the outside. Been reading about lots of different things to help with waterproofing. I want to try adding some fermented cactus juice to my mix. But so many websites all say different things. I have a large plastic garbage can that’s been fermenting about a month now; it smells really really bad, and I’m wondering is this normal?   Did I add too much water? Too much time? Should I have left the lid off? Just wondering if you might have an idea or know of a “recipe“.  

A: It sounds like your mix has gone bad. I have not made this in many years and I know the recipes differ, so I contacted a colleague of mine who does restoration on historic buildings in the southwest and uses the nopal mucilage. Here are his instructions. Nopal Water or Prickly Pear Cactus I usually use 5 gallon buckets filled about half with chopped up cactus to start the juice flowing then fill to the top with water. Leave there for about 3 days. If you have it in buckets for longer it will start smelling very bad and attracting flies. Then remove cactus from the bucket and what you have left is cactus residue. Then pour your nopal water through a fine screen and viola you have clean nopal water. I use it with lime to do a lime wash. Here is another recipe I had on file for a lime /sand plaster using the nopal. Lime/Sand Plaster with Cactus Mucilage as a Stabilizer The mucilaginous juice obtained by soaking the chopped up stems of cholla cactus or the chopped-up pads (nopales) of prickly pear cactus is traditionally used in Mexico to increase the durability of lime-based plasters. An article from DESIGNER/builder magazine (October 1994) describes how a master plasterer from Mexico, Pedro Sanchez, has used this plaster to protect historic adobe churches in New Mexico. He starts the process by soaking an unspecified amount of cholla stems for about six weeks in a covered, sun-warmed, 5 5-gallon [208 liter] drum filled with water. He then mixes together: • 100 pounds [45.4 kilos] of lime (the article specifies type N) • 50 shovels (probably round-nosed) of a coarse sand found locally in a dry streambed, • and, 10 gallons [about 37.8 liters] of the gooey "cactus water". The traditional three coats were then applied to the adobe walls, an approach that should also work directly over bales.

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