Binders for Natural Plasters (Part 1)

As their name implies, binders glue the other elements of a plaster together. More than anything else the binder defines the properties of a plaster including strength, permeability, and resistance to weathering. Over thousands of years of natural plastering there are three major binders that have been traditionally used: clay, gypsum, and lime. There’s a lot of variation within all three. Lime in particular is further categorized into hydrated lime, natural hydraulic lime, and natural cement; all of which have very distinct properties and are considered as unique binders. But for simplicity let’s look at the properties and origins of the three major binders first, and then break them down into further categories.The table below summarizes some of the properties of the trinity of natural binders, as well as portland cement and a portland-lime blend (recognizing that portland cement is not a natural plaster, but has been used in natural building).

Binder Vapour Permeability (US Perm/inch) Weathering Resistance Strength Embodied energy
Clay Excellent (18) Poor Weak Low
Gypsum Excellent (18) Very poor Weak Low-Medium
Lime Good (14) Good Strong Medium
Cement-Lime Poor (7-10) Very good Strong but brittle High
Cement Very Poor (1) Very good Strong but brittle Very High

Now let’s look at each binder more closely.


Home made earth plaster (yellow ochre)

Earth plaster pigmented with yellow ochre

Clay is usually considered to be the most ecological of all binders, because it can be dug from the ground and used as-is; or even when it is mined and sold industrially, the energy cost of processing it is typically lower than other binders, which are mined and then heated to relatively high temperatures to achieve their binding properties.  Clay plasters, or earth plasters, are the most vapour permeable of all the natural plasters, and the most flexible – they readily allow humidity to pass through, and adapt to movements of the substrate without cracking. These properties are important when plastering over natural wall systems. Clay also has the interesting property of readily taking in water, but when it is saturated it becomes rather waterproof and prevents further water penetration. However earth plasters trade these virtues for lower strength and erosion resistance – wet clay may be water proof, but under driving rain it will erode relatively quickly. Clay also has very high shrinkage as it dries, so earth plasters are either applied very thin, or contain large amounts of fiber and/or aggregate.

Clay is the product of many thousands of years of erosion of rocks (particularly feldspar), and the deposition of very fine particles, often on ancient lakebeds. Sometimes clay deposits are formed at or near the source rocks, and are relatively pure – these are called primary clays. More often they are transported by water and deposited far away on the beds of ancient lakes, where they are typically a blend of many minerals – these are called secondary clays. Clay is primarily composed of the mineral kaolinite (Al2O3•2SiO2•2H2O) but with widely varying quantities of aluminates and silicates, as well as oxides of iron, calcium, magnesium and many other compounds / impurities. But this doesn’t tell us much about what clay actually is – which is incredibly fine particles that are typically flattened into miniature platelets. It is the interaction of these platelets that gives clay its properties.

The platelets are easily lubricated with a layer of water, so that they stick together very strongly, and yet readily slide over each other – making clay extremely plastic and malleable when wet, yet quite hard when it dries out and this lubricating layer is gone. Clays vary in the size of their particles, and generally clays with very fine particle sizes have high plasticity, and high strength because the clay has a lot of binding power. Unfortunately this goes hand in hand with higher rates of shrinkage. The most extreme example of this is bentonite clay, which has such fine particle size that it behaves quite differently than other clay. Bentonite is typically 10 times finer than any other clay; it can have a surface area of almost 1000 square meters per gram. The very high rates of shrinkage, high plasticity, and extremely low permeability generally make bentonite totally unusable in natural plasters.

On the other hand if you’re using pottery clay, you’ll probably want to use some of the most plastic pottery clays, the ball clays, which are fantastic for mixing with lots of aggregate and fiber to make a strong base coat that can be laid quite thick on the wall. Kaolin clays, which are much less plastic, are often used in thin finish plasters for their desirable white colour and very low silica content – it’s very important to realize that most clay contains significant amounts of silica, so the dust is little safer to breathe than portland cement dust. Clays that lack plasticity (called short clays) will tend to break rather than bending when wet, and generally speaking are less desirable for plaster (but this rule is often broken). When you’re using clays dug up on site, however, they are rarely so pure – your biggest concern will be ratio of clay to non-clay elements in the soil

Evaluating soil types

The most important property of your soil, when assessing it for plaster, is the clay content. Ideally soils used in earth plasters should contain 20-30% clay or more. It may be possible to make a good plaster with as little as 10-20% clay content in your soil, but proceed with caution, particularly at the lower end, because your plaster will be weaker. Particles that are slightly large than clay are called silt, which is often mistaken for clay because it has the same slippery feel when wet – however it lacks the binding properties. Silt can be either benign or very harmful in plaster, depending on how much there is. As a rule It should equal less than 1/4 of the clay content of your soil, so if your clay content is 20% the maximum amount of silt should be 5%; any more than this and there’s a chance of plaster failure. Plasterers will certainly break this rule, but be careful and make a lot of samples  You should aim to use the best soil possible for your plaster, even if it means trucking it in or using bagged clay- plastering a house costs thousands of dollars, so if you’re going to experiment, do it on a shed.

There are several ways to test your soil, you should use all of them and compare the results. You may want to compare soil from a few different places on your site, as it can sometimes vary over short distances.

The ribbon test

Make a ball of clay that is malleable and not-too-wet, not-too-dry; squeeze it between thumb and forefinger to produce a ribbon about 1/8 inch (2-3 mm) thick and less than ½ inch (about 1 cm) wide. Keep pushing the ribbon out to see how long you can extend it before it breaks. A minimum of 1 ½ inches (40 cm) usually indicates at least 20% clay, but to be safe it would be good to have a ribbon double that length. Evaluate the feel as you squeeze it: does it feel smooth and plastic, can you feel sand grains in it? Is the sand fine or coarse, sharp or rounded? Next do the jar test to get even more detailed information about your soil.

The jar test for soil

The jar test for soil texture

The jar test

The jar test is a fairly accurate and easy way to determine your soil type, but it takes time because clay can take up to a week to settle out of suspension. Any jar will work for this test, but a 1 litre mason jar is a nice size. Fill the jar no more than 1/3 full with soil, then top it up to at least ¾ full with water (leave enough air space to get a good shake). Optionally you can add some detergent or salt to help disperse the clay particles. Shake the jar well, until you feel that any soil clumps have broken up. If not using a dispersant, you’ll likely want to let it soak for a day or more, and come back to shake it again. Note that it should be shaken, not stirred; swirling will tend to throw off the accuracy slightly. Have a timer (with an alarm) and a permanent marker handy. Shake for several minutes; start the timer when you stop shaking. After 40 seconds all of the sand component of the soil will have settled out – mark this level on the side of the jar. After 2 hours all of the silt will have settled out, mark another line at this level. Wait until the water starts to look clear (up to a week), at which point all, or nearly all of the clay will have settled out. Measure the height of each layer and divide it by the total height to obtain a percentage by volume of the soil. The measurement of clay will be a little high, because clay continues to compact over time, and even more so when it dries. For more accuracy you may carefully scoop off most of the remaining water and leave the jar to evaporate in the sun for a few days.

Clay plasters are fun, but because of the varying properties of soil, fiber and aggregate it can feel more like art than science.  Different regions can also have very different clay soils, which makes knowledge sharing a little more complicated.

In part 2 we’ll look at gypsum and lime, and talk a little about cement. I’ll also share some links.

Sand for natural plasters

Sand is underrated. It provides the structure of plaster, and the quality of your sand can make the difference between success and failure. So what makes sand good or bad? In general, good plaster sand should be sharp, with a diversity of particle sizes, and clean.

marble sand

  • Sand should be sharp and angular, not worn and rounded. Imagine trying to build any kind of structure out of balls vs blocks, and the reason for this becomes obvious. Unfortunately this means that many natural sands are poor plaster sand. Beach sand in particular should be avoided, because waves have often been rounding the sand grains for thousands of years.
  • Particle size diversity is important to create good structure, and to reduce the amount of binder needed. Imagine a bucket filled with softballs, how many golf balls could you add to the bucket without changing the total volume? Then how many marbles could you add to that? Ideally you’d have a mix of nearly every grain size so that there are few large voids left – this creates a structure that resists movement, and also requires less binder to fill all those voids. Less binder equates to less cracking, and shrinkage cracks are one of the plasterer’s number one enemies.
  • Sand should not, however, contain silt – which is the particle size below sand, slightly coarser than clay. Silt fills the voids in place of the binder, resulting in weak plasters. Clay can also cause serious problems in lime-based plasters. Salt also can lead to plaster failure, as well as causing rusting of metal lath or any other metal used in plaster preparation. So when we talk about sand being clean, we mean free of fine particles, and unwanted salt, chemicals or organic matter.

As a conservative rule the largest particles in your sand should be no more than half the thickness of your plaster, but preferably would be at least one quarter the thickness of your plaster (larger aggregate can provide better structure, resulting in a stronger plaster with less cracking). So if your plaster coat is a half inch, your largest aggregate would ideally be between 1/8 and 1/4 inch. There are several types of sand that are widely available, so when you call a sand yard, or any construction materials supplier, you need only tell them what you want and it will promptly appear at your jobsite… maybe. Unfortunately the definition of sand types allows huge variability (even assuming it is followed correctly), and what you receive on the jobsite will depend on what that supplier carries, or what is locally available. Nevertheless, as a rough guideline the main sand types everyone carries are masonry sand, concrete sand, and (if you’re lucky) stucco sand.

Masonry sand

Masonry sand has a maximum particle size of 3/16 inch (4.75 mm), which could mean that it is a nearly ideal sand for many base coats, and particularly for lime and cement-lime body coats which are typically applied at a depth of about 3/8 inch. Unfortunately, using the ASTM standards for particle size distribution (see Table below), anywhere from 70 to 100% of the sand can be less than 3/64 inch (1.18 mm). Which explains why when we order masonry sand on a jobsite sometimes it’s a nice mix of coarse and fine sand, perfect for a base coat, other times it’s almost 100% fine sand. Worst of all, masonry sand can be 99% rather fine, but with just a few pebbles, making it useless for fine finish coats. With masonry sand you should look before you buy; however it is still often the best choice available for a given plaster. Brick sand can be a synonym for masonry sand, often a relatively fine version of masonry sand.

Masonry sand gradation (ASTM standard)

Sieve Size

Percent Sand Passing Through Sieve

Inch mm Mesh # Natural Sand Manufactured Sand
3/16 4.75 4 100 100
3/32 2.36 8 95 to 100 95 to 100
3/64 1.18 16 70 to 100 70 to 100
0.024 0.6 30 40 to 75 40 to 75
0.012 0.3 50 10 to 35 20 to 40
0.006 0.15 100 2 to 15 10 to 25
0.003 0.075 200 0 to 5 0 to 10

Concrete sand

Concrete sand (technically “fine aggregate”) has a maximum particle size of 3/8 inch (9.5 mm), and five percent should fall between 3/16 and 3/8 inch (4.75 – 9.5 mm). It also must have a variety of mid-size particles, and up to 10% can be finer than 100 mesh. Even though the 3/8 inch maximum is a little more than we’d like for most plasters, concrete sand may be a good choice for a plaster coat that will be applied at ½ inch or more. Large aggregate doesn’t necessarily interfere with a nice finish, as it tends to push to the back. However it makes finishing extremely difficult and frustrating if the largest aggregate is about the same size as the depth of the finish coat.

Stucco sand

Stucco sand is what you probably want for your plaster, the trouble is actually getting it. It’s similar to Masonry sand, with a maximum particle size of 3/16 inch, but is required to have a greater proportion of large particles, and less very fine sand. A large amount of variability is also allowed in stucco sand, but because it is formulated for stucco it’s more likely to have a desirable gradation. Stucco sand is ideal for anything but thin finish coats, however it is far less widely available than masonry or concrete sand.

Stucco sand gradation

Sieve Size

Percent Sand Retained in Sieve

Inch mm Mesh #
3/16 4.75 4 0
3/32 2.36 8 0-10
3/64 1.18 16 10-40
0.024 0.6 30 30-65
0.012 0.3 50 70-90
0.006 0.15 100 95-100
0.003 0.075 200 97-100

Silica sand

Most sand is primarily made of silica, however fine white silica sand is available in 80-100 lb. bags and can be very useful for some finish plasters. Silica sand is most commonly used in earth plasters that are applied in a very thin coat and pigmented (vs painted). Silica sand is available in various mesh sizes – typically mesh sizes between 40 and 100 are used in finish plasters. Forty mesh is almost coarse (but still much finer than window screening, and might be mixed with other mesh sizes to make plasters that apply slightly thicker (up to 1/8 inch, possibly with some fine fiber). One hundred mesh or finer would make an exceptionally fine and smooth finish plaster that would be prone to cracking unless applied in very thin coats. Silica sand can be used for sandblasting, so it can sometimes be found in rental centers, as well as being fairly commonly available in masonry supply and other stores. Silica is very hazardous to breathe, so wear a respirator during mixing to avoid breathing dust from silica sands.

Limestone sands

Marble and calcite are common forms of limestone, and these sands can be used in some of the most exciting finish plasters. Limestone based sands are traditional in many lime finish plasters, especially ones that are highly polished. They are also used in some unpainted earth plasters, where they impart a subtle sparkle. Marble and other fine white limestone-based sands can be hard to find in bags, and since they are usually being used in finish plasters a truck load is probably too much. Marble sand is sometimes used in swimming pools, and other types of stucco, so it may be available at masonry or stucco supply stores. If you have trouble finding it you can try contacting swimming pool plasterers, who might refer you to a supplier, or sell you some sand directly. In the USA marble sand is available online from Limeworks, at a reasonable price for small jobs or learning. You might also try contacting manufacturers to find out who distributes their products. Manufacturers of bagged sands include OMYA (SW350 white stucco sand); Superior Marble in Arizona (Plaster Sand); Universal White Cement (Universal Ultra White Marble Sand); Imersys (40-200 Dry-Ground Marble; Pool Mix). Imersys is one of the largest suppliers of marble sand, they are distributed in Canada by Debro (who sell by the palette = 50 bags). Be aware that the quantities you are looking for are tiny compared to what most companies usually deal with. You can also go directly to marble quarries who may sell (or even give) you some. Typically it needs to be sifted, however, which means it needs to be dry. Even bagged sands often need to be sifted for use in finish plasters.

Sieving you own sand

If you can’t buy the sand you need, you can make it. It’s not unusual to modify sand for use in fine finish plasters, by sieving out the larger particles from a commercially available sand. To sieve your own, you need dry sand (many sands are sold damp), and you need an appropriate screen. First you need to know what mesh size you want. Window screen is typically about 16 mesh. A smaller mesh size can be purchased in the form of splatter guards from the kitchen section of many stores – these might be found in the 20-30 mesh size range. Otherwise, you will likely be shopping online at a specialty supplier.

It’s worth taking the time to make a good setup for sieving sand – this may be as simple as cutting the bottom out of a bucket or plastic bowl and gluing the screen in (construction adhesive is good for this), or it might mean making an angled wood frame. For anything but small, occasional sieving jobs it’s worth investing time in a good setup. Build the frame to sit above a Rubbermaid or whatever bin you find most convenient. Leave the bottom end open for coarse aggregate to exit (hook a bag over the end so that it bags itself –then use it on icy spots in winter, if you have winter where you live). Try out some different angles (you can start with 45) until you find the one that works best for you.

Sand is typically 60-100% silica (and usually closer to 100), so avoid breathing fine dust. This is especially important if you’re sieving it – always wear a respirator to sieve any sand.

A review of the Natural Building Companion

Two years ago I saw a presentation by Ace McArleton, co-author of the Natural Building Companion, and based on that I knew I needed to order the book. For a while it sat on my shelf, alongside a collection of other books about building and natural building. However it has distinguished itself from its companions in the true test – when I have a question, or encounter a problem in my work as a straw bale builder, it is the first book I reach for. I now consider this book to be an essential reference for those planning to build with straw bales, clay-straw or other natural building materials – and while it is useful for the pros, it is still accessible for beginners.

Jacob Deva Racusin and Ace McArleton have spent the last decade working on high performance natural building – marrying the building science of the superinsulation and passive house movements with natural materials. The natural building movement in general has been moving to adopt better practices around managing heat loss, particularly making buildings more air tight, but the Natural Building Companion is the first book to address it comprehensively, and also to offer practical solutions to common problems.

For example, it’s common to couple straw bale construction with timber frame, however the junction of each post or beam with the plaster is likely to become an air leakage problem – movement of the building can even break caulking along these joints when it is used. The best solution to this problem is air fins that run behind the post and several inches under the plaster. Racusin and McArleton discuss proper installation, and share their experiences using different materials as air fins – and the choice of material is critical since the wrong choice can simply displace the crack from the post to the edge of the air fin, or cause other problems.
Bales squared before installation
A controversial topic in the bale building world is the use of cement-lime plasters. A track record in Ontario has shown that cement-lime plastered buildings can work in our climate, if designed properly. However I would agree with Racusin and McArleton that cement-lime plaster is not the best choice in this relatively wet, cool climate – and the authors offer a very good alternative plastering system that is more appropriate for our climate. A lime-stabilized earth base coat with a lime finish coat is a system we learned about from European builders, but we had just plastered our first house with this system when we discovered that there was a wealth of knowledge just across the border in Vermont. The authors have mastered this system and share the details of it in their book.

The treatment of moisture management is also thorough; like everything in this book it starts with design considerations and follows through to building techniques and detailing. The authors have also done extensive testing of their buildings post-construction to see how well they performed over time, while they were being lived in.

Vampire stakes

Vampire stakes

I even learned tricks to improve our bale work, such as squaring bales before installing them, or the use of “vampire stakes” to anchor straw bales to post and beam structures – the most functional and elegant system I have yet seen for doing this.

The book comes with an instructional DVD filmed at Yestermorrow, which shows exactly what they explain in the text, filmed at each step along the way. I’ve only consulted this a few times so far, but each time it clarified something that wasn’t quite translating from the page for me. Beginners to straw bale will likely want to watch it right through.

I have only picked out a handful of examples, but the Natural Building Companion covers every step from design, through construction, to finishing details, and does it all well. They take it from philosophical underpinnings to technical details of the systems they have evolved with their company New Frameworks.

In short, the authors nailed it, they have written a standard for the natural building world. Having used the book, and spent some time with the authors, I would also agree with Andrew C. Gottlieb that these are definitely builders I’d want to work side by side with, for their professionalism, expertise, and knack for having fun.

I would recommend their website and blog as a reference, in particular I’m keeping an eye on the development of their new system that aims to reach passive house standard while using straw bales. Here is a video of Racusin and McArleton discussing high-performance natural building.

The Dumont House

The first time I saw Rob Dumont’s house I was unimpressed. I was visiting an ex-girlfriend in Saskatoon, I mentioned that I was doing some research into sustainable homes, and she said “there’s one near here, we should walk by it.”

It just looked like any other house. The Dumont house is in the colonial revival style, it’s simply built and doesn’t stand out in the neighbourhood, which has a suburban feel to it (though it’s not far from the downtown). I’m used to seeing half million dollar ecohomes, so when you take away the architect and expensive finishes, solariums, thermal mass walls, radiant floors, etc., it’s hardly recognizable as an ecohouse. I arranged to go back later and visit Rob Dumont, who gave me a tour of his home and some other projects he was working on. What initially turned me off about the Dumont House (because it challenged my preconceptions) now makes it one of my favourite sustainable homes.

16 inch double wallWhen Dumont built this house in 1992 it was one of the world’s most highly insulated homes – but the house, like Dumont himself, is understated. Dumont took the double wall system from the Saskatchewan Conservation House, stretched it out to a full 16 inch wall cavity, and filled this space with blown-in cellulose insulation (which is just recycled newspaper with borax added for fire proofing and pest control). There are about 16,000 lbs of cellulose in the house – but what makes this insulation system really special is that the two walls have very little framing between them, so there are far fewer pathways to lose heat through the wall, either through leakage where the insulation doesn’t meet the wood perfectly, or by ‘thermal bridging’ through the wood itself. It may seem obvious, but it needs to be said: wood is way worse insulation than insulation is. A 2×6 stud wall with R20 insulation batts has an overall insulation value of about R13. Rob Dumont’s walls are R60, the attic is R80, and the windows are R5. The whole house is carefully air sealed. It takes less than 1/4 of the energy to heat Dumont’s house that it would for a conventional house. In 2000 Dumont wrote an article describing the energy efficiency features of his home.

Hot water heat-exchangerAs we continued the house tour, Rob showed me some things that really could be added to any existing house. The first was a drain water heat exchanger, just a copper tube wrapped around the shower drain – as hot shower water comes down the drain pipe the cold incoming water in the coil is warmed. “On a shower it will recover about half of the heat that’s otherwise going down the drain,” said Dumont. He quipped that “I’m worried that with the price of copper, in a home invasion someone will steal it.” Still, barring home invasions, the economic payback is pretty quick – if your hot water heater is electric the heat exchanger will pay itself off in 5 or 6 years, with gas maybe double that. Next we looked at the hot water heater itself, which is a standard tank but wrapped with batt insulation and a thermal blanket adding up to about R28.“Without the insulation it loses about 100 watts of heat continuously,” Dumont said, “with the insulation it’s down to about 25 watts.”

Painting of the house by Phil

Because of the simplicity of the Dumont house, it wasn’t expensive to build. The insulation, upgraded windows, and a solar thermal heating system, added about 7% to the building cost. “If I’d put brick on the outside of the house instead of hardboard siding,” said Dumont, “the brick would have cost more than all of the energy conservation features. I’d much rather have an energy-efficient house than a brick house.” In fact, the energy efficiency finished paying for itself in 2008, after 16 years – now it’s turning a profit. And he pointed out other non-monetary benefits – no draftiness, no cold feet, and the nice aesthetic of the deep window ledges.

Rob Dumont, like many of the builders and designers I’ve met, started his work in the 1970′s, and spent some time wandering metaphorically alone in the desert in the 80′s and 90′s. “Society has got a very short attention span,” he said, “there are waves of interest, but mother nature bats last. I started working in the 70′s on the Saskatchewan Conservation House, one had to really keep the faith through a part of the time since, because not many people were very interested. I must admit back in 1973, with the oil shock, I thought the reasonable thing to do would be to change the way we do our houses radically. That was my youthful naivety at the time.” He showed me a book of solar homes that was written in the late 1970′s, a sort of hippie version of what I’m trying to do, and I realized that I’m just the latest emissary of societal interest, something Dumont has seen come and go. I feel like this time it may be different, but I’m not sure. “It’s encouraging,” said Dumont, but “it’s not nearly at the level I’d like it to be. EF Schumacher put it nicely, he said the wind may not always blow but at least we should have our sails up. That’s the way I feel.”

Rob and his wife Phil took me to see a college basketball game, in which the home team, the Huskies, thoroughly trounced the competition (both women’s and men’s teams). I pictured Rob in his younger days playing basketball, fit and idealistic, believing he could change the world. And he did – it just changes very slowly. I wonder, when I look back in another twenty or thirty years, how I will remember this time. As the beginning of real change, or as lost opportunity? All I know is that my visit with Rob Dumont left me more optimistic than when I arrived.

Maintenance and repair of natural plasters

When spaghetti sauce meets unsealed earth plaster, it’s a bad scene. But it’s fixable. Most bad things that happen to natural plasters are repairable. There tends to be a trade off between durability and repairability – an unsealed earth plaster is the easiest plaster to damage, and also the easiest to repair without a trace. Lime plasters can be a little harder, but there are definitely tricks for repairing them. Also the more polished and perfect a plaster is the harder it is to blend in a repair; if there is very little variation in your surface, any blemish is going to draw the eye to it. One of the most polished plasters, tadelakt, is still fairly repairable because there’s so much variation across a wall that your repair is like Waldo, or goldbug, camouflaged by the diversity around it.

Earth plaster repairs

1) Surface marks

  • If you just need to clean a smudge or a pencil mark etc. from unsealed earth plaster, you can remove it with a good quality pencil eraser. If that fails, if it’s a sponge finish try a slightly damp sponge (wet it, then squeeze all of the water out of it) or rag. In this case you don’t want to re-wet the wall much, as it will show a change in texture.
  • If your wall has a trowel finish, you are better off skipping the sponge, and go straight to re-wetting and very light surface scraping, then a light quick pass with a trowel if needed. Spongeing may change the reflectivity of a trowel coat, usually making the area appear lighter, so keep that sponge away from a trowel finish.
  • If this doesn’t work, scrape the stain off with a tool and repair it with the technique described below, for scrapes and dings. With something that penetrates, like marker, you’ll probably need to scrape the plaster off, whereas pencil, and usually crayon, can be erased with an eraser, sponge, or surface scraping alone.

2) Scrapes and dings

  • The key to blending the repair into the existing plaster is to properly rehydrate the wall around the damage before you start. Use a spray bottle on fine mist setting, spray an area a couple of inches around the repair. Mist lightly, try to avoid drips running down the wall. Wait a minute or so, give another light misting,
  • Go away and do something else for five or ten minutes. When you come back mist it again once more, then wait until all the sheen has left the surface of the plaster (maybe 30 seconds).
  • A flexible plastic trowel is ideal at this point, or you could use a small pointing trowel and plastic cut from a yogurt lid. Use a small amount of the earth plaster mix, just enough to fill the damage – if you put on too much, carefully scrape the extra off. Try not to get any plaster on the surrounding wall if you can help it.
  • Now using the plastic trowel or yogurt lid, compress the repaired area once, maybe twice if you need to – don’t overdo this or you will burnish the wall around the repair.
  • If it is a sponge finish you can touch it up with a sponge, very delicately when it is partly set, or wait until is is entirely dry and sponge over the area.
  • If it is a trowel finish, you can improve blending by scraping the surface of both the old and new mud, then retrowelling them. Be gentle. If the plaster is too dry it will burnish when you retrowel it, if too wet it may tear or pull off the wall – in this case finish the repair after a brief drying period.

The attached video of American clay repair will help make all this clear, it’s a little slow to watch because it’s filmed in real time (a bit like watching paint dry).

For large repairs, mist the existing plaster well, then trowel over the damaged area, trying to level carefully to the old mud. The junction will show, but may be blended somewhat with scraping the joint and retrowelling. I find it is then worth letting it dry significantly or entirely, then rewetting old and new mud and either retrowelling if it’s a trowel finish or spongeing a sponge finish. Large repairs require waiting time.

Repairing earth plaster that has been painted is even easier, just fill and compress it as above, then when it’s dry use a damp sponge to wipe any spillover off the surrounding paint. Then touch up the paint, of course.

American Clay have also produced a repair manual that you may find useful.

Lime plaster repairs

Lime plasters are so variable in their composition, and how they are finished, that any advice is going to be a generalization. So consider the following as a fairly basic starting point; and unfortunately you may have to learn from your mistakes, so practice repairs on sample boards first. It’s hard, but not impossible, to make repairs in lime plaster disappear as they do in earth.

1) Porous lime plaster (not painted or waterproofed)

  • Rehydrate the area to be repaired well with a misting bottle (or wet sponge).
  • V-open the edges of the damage area if needed- especially if edges are crumbly.
  • Using some of the lime-sand mix originally used in plastering, fill the the repair, compress, and let it dry.
  • Once it is dry sand it with around 180 grit sand paper to smooth the repair.
  • The same technique can be used in crack filling – generally you would V open the crack using a grout removal tool, backerboard scoring knife, or even a sharpened can-opener, before filling it.

2) Lime plaster with a waterproof surface (e.g. tadelakt) or painted lime plaster

  • Rehydrate and fill with the original mix (as described above for porous plasters), but wipe the excess off the surrounding plaster since it will not stick.
  • Compress the repaired area using a plastic trowel, or a stone in the case of tadelakt. As always, try to be as neat and careful as possible at every stage.
  • This technique can be used to fill large cracks in tadelakt, again it would probably be wise to V large cracks open. Do not open fine cracks, use the techniques described below.


3) Crack filling fine cracks in tadelakt

  • One option, especially for cracks which are raised, is to re-compress the crack with a stone. Re-wet the area first with a dilution of black soap, I find this reduces the risk of scratching or damaging it. Even so, you will probably leave some undesirable marks which could, in the worst case scenario, draw attention to the crack. If the plaster is quite fresh, compression is probably the best way to deal with cracks and some other kinds of damage.
  • I prefer to fill cracks in more fully cured tadelakt using the original mix. For very fine or hairline cracks you’ll want to make a slurry from which you’ve removed all the sand.
  • To do this, use two straight, clean trowels, take a scoop of mix, scrape it down the length of one trowel with the other, pushing most of the mix off the trowel and just leaving a smear.
  • Scrape it again once or twice, discarding whatever is scraped off, until all that’s left is a thin film of plaster with no sand at all. Gather this with the other trowel – you may be surprised at how much there is.
  • Rub this slurry rapidly across the crack using a gloved hand, then wipe off any excess using a clean rag. The cracks will stand out at first, but will quickly fade as the mix dries, and may not be visible at all. Note that this technique only works after soaping, and works even better after waxing.
  • You may wish to reapply black soap and wax over the area after crack filling.

Another alternative to making a slurry, for hairline cracks, is to use whiting instead of sand. However it may be harder to colour match to the original mix.

4) Crack filling cement-lime plasters

  • V out the cracks using a grout removal tool, backerboard scoring knife, or  a sharpened can-opener
  • Wet the cracks using a misting bottle, or paint on a bonding agent.
  • Fill cracks with grout. For most cracks you’ll want sanded grout, hairline cracks may call for unsanded grout.
  • Once the grout is dry, sand the area with a foam sanding block to remove excess.

Planning ahead

  • When you’re at the planning stage of a project you should consider the strengths and weaknesses of different plasters. For example, we avoid unsealed earth plasters in kitchens. Or avoid putting any natural plaster on a corner where it’s likely to get bumped a lot – next to a door threshold, for example. Trim it out with wood if that’s an option, or there may be places in your home where paint or tile is more appropriate than plaster.
  • Plan for a finish you will be able to maintain – most people can learn to repair a troweled plaster, with some dedication, but if a homeowner wants to maintain their own plaster and wants it to be easy, a sponge finish may be the way to go.
  • When you are installing a natural plaster, always save some for repairs later. Most lime plasters can be stored wet, in a mason jar etc., with a little water over the top to prevent air from reaching it. Hydraulic lime must be stored as dry mix. Earth plasters should usually be stored dry, either the original powdered mix, or dehydrate some of the leftover mix.

Ongoing maintenance

  • Avoid contact of oil with any natural plaster. Except, perhaps, oiled earth plasters.
  • Use natural oil-based soaps to clean waterproof plasters or oiled earth plasters. Black soap is the best choice for tadelakt (available here in the US and here in Canada). Use a dilute solution.
  • Wax tadelakt every year or so as needed, if it’s in a wet area. Ryan Chivers, our tadelakt mentor, reccomends Howard’s wax, and I have found it to be good, easy to use, and cheap.
  • Earth plasters need little or no regular maintenance, repair as needed.

Slideshow of natural finish plasters

Biophilic design

At the first stop on a ferry trip down the Alaskan coast, I scrambled up a steep slope through a hemlock forest and my nose came close to the mossy carpet. the smell that greeted me was rich, earthy, it reminded me of a stout beer. I sat down, my back to a tree; a varied thrush sang its long human-sounding whistled notes in the distance. My eyes ran over the green carpet that covered every surface, following the contours of logs and roots, and climbing the lower trunks of trees. Feather moss, fern moss, lanky moss, cat-tail moss, electrified cat tail moss; the names are descriptive of the unique forms. I didn’t know why the varied branching shapes, quilting an irregular pattern across the forest floor, broken here or there by a fallen hemlock twig or cone, brought such a sense of contentment – now I understand more.

As the ferry sailed from Haines the sun was sinking low in the sky, and the snowy peaks of the coast mountains changed from fiery orange to cold blue.
I was on my way to O.U.R. Ecovillage, a community on Vancouver Island, British Columbia, where I wanted to see buildings built out of earth, and meet the people who made them.

At O.U.R. Ecovillage I stayed in a building named the Sanctuary, on the edge of a cedar swamp. I sat for a while on a living cedar log, surrounded by showy yellow blooms of skunk cabbage, listening to the long complex song of the winter wren, like tiny raindrops on a still lake. Rain chains hanging from the corners of the living roof of the Sanctuary building echoed the wren’s song. As my ear was drawn to the patterns in the song, so my eye was drawn to the pattern in this chain of tiny bowls linking roof to earth, and the perpetual motion machine of the water trickling down it. Inside the building, my hand reached for rough earth plastered walls and the uneven smoothness of roundwood posts. My feet pressed against the cool softness of an oiled earthen floor. My eye traced the linear path of beams to their meeting with curved walls, and subconsciously I found pattern in thousands of details of this building, each of which represented a choice made by a builder, and a statement about beauty.

Traditionally beauty is in the realm of art, not science. Lately, however, the two may be intersecting. Biophilia is a word used by evolutionary biologist E.O. Wilson to describe an evolved human affinity for nature. In 1961, as a young naturalist, Wilson first visited the rainforests of Surinam. As he became lost in the richness of the natural world here, he realized there was some larger idea that was eluding him. More than two decades later he would write “the image of the land I kept for many years symbolized the tangle of dreams and boyhood adventures from which I had originally departed, the home country of all naturalists, and the quiet refuge from which personal beliefs might be redeemed in a permanent and more nearly perfect form.” A poetic description of the naturalist’s experience, but Wilson came to believe that his own intense connections to nature are merely expressions of a broader human connection to the natural world, which has a genetic basis.

According to Wilson’s theory, being in contact with nature makes humans happier and healthier; and since it’s in our genes it is cross-cultural – everyone, rich or poor, rural or urban, will find solace in nature. One of the implications of this is that we may find something beautiful, or comfortable, because it emulates patterns found in nature. From there it’s only a short leap to design buildings that emulate natural patterns and processes. Termed biophilic design, it’s a leap that a number of architects and psychologists have already begun to make.

I’m reminded of a scene from the movie Black Robe, in which a Jesuit priest, lost in the Canadian forest, looks up into the trees and sees for a moment the columns in a cathedral. It’s a cinematic moment that has stuck with me when most other details of the movie have dropped away. Mathematician Nikos Salingaros suggests that some of the greatest religious architecture relies on natural patterns and symmetries to connect humans to the divine. According to Salingaros, “we engage emotionally with the built environment through architectural forms and surfaces. We experience our surroundings no differently than we experience natural environments, other living creatures, and other human beings.”

Biophilic design is not new; arguably it is best exemplified in traditional, or vernacular, architecture and often finds itself in opposition to modern, minimalist architecture. Consider some of the shared aspects of the forest and the sanctuary building. My senses were engaged. The earthy smell of moss in the forest, the faint odours of earth plasters and natural oils; songs of varied thrush and winter wren, the bell-like sound of the rain chains; the bark of a cedar tree and the feel of earth plaster that my hand instinctively reaches out to touch. We tend to think of what we see, but we engage the world with all of our senses.

Visually, I am drawn to ordered, but complex patterns. The parallel rafters as they meet the rounded wall, the rain chains. Or moss, like snowflakes never twice the same. This repetition of similar patterns has been called visual rhyming. Rhyming patterns occur on different scales, in the building or in the forest. Sometimes the pattern is only a texture when perceived from afar, but greater and greater detail is revealed as one moves closer. Michael N. Corbett describes this as an attraction to neither rigid uniformity nor wide variation, but rather small variations and imperfections in a general pattern. We are attracted to that which is made by hand, rather than by machine. At some very base level, it seems, we are all luddites.

This doesn’t imply that we all need to live in a cob house. Natural materials and design aspects can be incorporated into any building, and always have been. Many conventions in architecture and interior design probably derive from the natural world. I live in a 100-year-old building with wood floors and beautiful mouldings, and (recently added) earth plasters. I would say that many of the deliberate, and unconscious details of this building do reflect biophilic design. My addition of earth plaster only enhances this. The trouble with biophilic design is that so far we don’t really understand what it is, or what rules it follows. Even so, it’s a valuable concept to keep in mind while designing or choosing materials.

A good book of articles about biophilic design is edited by Stephen Kellert.

I haven’t seen this film yet, but I will when I have the chance.

Biophilic Design: The Architecture of Life (Trailer) from Tamarack Media on Vimeo.

Harold Orr’s Superinsulated Retrofits

Recently I had the privilege of interviewing Harold Orr, who was the project leader on the Saskatchewan Conservation House in the late 1970′s. He was involved in the invention of the residential HRV, and blower door tests, and his work influenced the Passive House and Net Zero movements. Now in his eighties, his brain contains a library of information on energy efficient building, and he talked to me for two hours straight. Orr’s main passion for the past several decades has been superinsulated retrofits of existing buildings, and he says the need for deep energy retrofits was obvious to him from early on.

The Saskatchewan Conservation House, now seen as a milestone in energy efficient building, was finished in 1977. “We recognized that as a first step,” Orr tells me, “the next step was to see if we can do this on a larger scale.” The province of Saskatchewan organized a competition, in which builders submitted proposals for a showcase of energy efficient homes – the challenge was to design and build homes that use only 25% of the heating of a conventional house. Orr was involved, along with Rob Dumont, in evaluating the proposals, “but we realized even as we did this that the number of houses that we build every year in a city is a small percentage of the houses that are already in a city.” Only in cities with a major building boom can you achieve a significant energy reduction, Orr explains, “so we were concerned about how we might do this on a conventional house.”

Orr and Dumont started looking for a house to retrofit and study the results, and by the end of 1981 they had found one in Saskatoon. This was the same year that the Superinsulated Retrofit Book, by Marshall and Argue, was published, describing double wall retrofits. The house Orr and Dumont had found was a 1968 bungalow with 2×4 stud walls and 2.95 air changes per hour (slightly better than the average house of that era).

“We decided to do a major energy retrofit on the house, and we wanted to bring it up very close to the level of the Saskatchewan Conservation House,” Orr says. The whole process of this renovation is described in a report that Orr wrote with Robert Dumont. They performed blower door tests at each stage of the renovation to see how air-tightness of the house was affected. They took off the stucco and wrapped the walls with polyethylene, which was sealed down to the foundation and up to the top plate of the house, and not surprisingly the house was considerably more air-tight. Next they hung a second 2×4 wall off the exterior of the house, with an eight inch gap between the old and new wall. By the time they had insulated the cavity and the new wall, the combined insulation (including the existing insulation in the old wall) was about R50.

“That did the walls quite well, but we wondered what on earth to do about the roof,” Orr says. “Because one of the major problems in housing is the leakage between the house and the attic space.” Because of wood shrinkage there is nearly always a gap where the drywall meets the top plate, which Orr estimates is commonly 1/16 of an inch. Drywall is also not normally air tight at the floorline – so in most older houses air can travel behind the drywall, from the living space into the attic.

“So we thought why don’t we cut the tail end of the rafters off so it’s nice and smooth at the edge of the wall,” Orr recounts, “and we’ll put a piece of plywood over the raw edges that we’ve cut off, and then we can carry the vapour barrier that we’ve already put on the outside of the wall right over the roof and down the other side.” This is what is now known as the chainsaw retrofit – a time lapse of a later chainsaw retrofit was filmed by Orr’s son Robert.

“So anyway we’ve got the vapour barrier on the roof and we’ve got it tight,” says Orr. “Now we put 2×8′s, one at the edge of the roof, one at the peak of the roof and one half way in between. On top of this we put new rafters down the roof. In the 2×8 we put R28 and in the 2×4 rafters we put R12 which gives us R40 on the roof. Plus the insulation we already had in the attic which is likely around R20. Now the we’ve got R60 in the roof. We’ve got the outside walls of the house and the roof done, and we’ve got the house very very tight.” In fact Orr says that the 1981 retrofit was almost identical in its performance to the Saskatchewan Conservation House.

Orr has worked on a number of retrofits since, most recently a four-suite apartment in Regina. This renovation of basement, walls and roof had a cost of about 11$ per square foot for materials (including metal roofing), and about the same again for labour. Because the retrofit turned it from an undesirable to a desirable place to live, with commensurate increase in rent that could be charged, it has an eight year payback time – making it a phenomenal investment.

So the economics of the double wall, or superinsulated, retrofit are not bleak, though it’s a large investment, and finding the right contractor to do it is going to be important. But how does it compare to just tacking some foam to the outside of the house and re-siding it? According to Orr there is no comparison.

“I took four walls and assessed them,” Orr says. “One I put 2 inches of styrofoam on, at R5 per inch that would be R10. When you put 2 inches on you really have to strap it, because you cannot put siding on over 2 inches of styrofoam. And unfortunately 1×3 strapping is the same price as 2×4′s. So if you’re putting strapping on, why not use 2×4′s?” And why not center them away from the wall, for a double wall retrofit? Since foam insulation is so much more expensive than batt insulation, says Orr, “I can put in R60 for the same price as R10. Now you’ve got to persuade me that R10 is better than R60.” That’s just materials, labour will change that somewhat, but the point is made.

Orr has more to say, however, adding that “when you put styrofoam on the outside of a house you’re not making the house any tighter, all you’re doing is reducing the heat loss through the walls. If you take a look at a pie chart in terms of where the heat goes in a house, you’ll find that roughly 10% of your heat loss goes through the outside walls.”  About 30 to 40 % of your total heat loss is due to air leakage, another 10% for the ceiling, 10% for the windows and doors, and about 30% for the basement. “You have to tackle the big hunks,” says Orr, “and the big hunks are air leakage and uninsulated basement.”

Air leakage in a typical house, from Keeping the Heat In

“I think the problem is that people don’t properly analyze where the heat is going. Get the book called keeping the heat in, it’s a publication of Natural Resources Canada [available as a free download], and anybody doing any work of this type should get this book and study it. If you look at where the heat goes the big chunk is air leakage, and usually putting styrofoam on the outside isn’t going to affect anything.”

I close the interview by asking why, so many years after retirement, he’s still doing this kind of work.

“It’s a passion with me,” he says. “I enjoy it. And I’m enough of a scotsman that it bothers me to see people wasting their money. I go by houses every day and I see them putting on an inch and a half of styrofoam, and lord help me – why don’t you do something for the same price and do it better?”

More information


History of the Chainsaw Retrofit