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Repairing Clay Plaster (with toilet paper?…)

Questions concerning the durability of clay plaster – especially as an exterior plaster, and even more especially in cold and wet northern climates – get raised any time we suggest using clay plaster to a client. We recently had the experience of returning to the first building we clay plastered, back in 1994. What we saw and learned greatly increases our confidence in the use of clay plasters!

What do we mean by “durable?”
When we talk about durability, what do we really mean? Let’s say we’re comparing two kinds of exterior siding: clay plaster and vinyl siding. Intuitively, we’d probably say that the vinyl siding is more durable. But scratch the surface a bit… no material is indestructible, so what we really mean is “how long before it needs fixing or replacing.” Vinyl siding can last quite a long time before it wears out or breaks. But it does wear out and break, and when it does what can be done? Typically, nothing. It gets removed, taken to landfill, and replaced with new material.

The clay plaster may be more susceptible to wear (especially if it’s placed too close to the ground, as we’ll soon see!). But when it is damaged, it can be easily repaired at almost no cost and made as good as new, with no landfilling and no need for replacement.

Using Clay Properly
The first step in making clay plaster durable is to plan properly. The worst section of damage on this 12 year old home was next to the utility door on the north side. The building is way too close to grade… we recommend 8-12 inches minimum, but didn’t do that here. It was also unprotected by a roof overhang… despite the whole building have wide overhangs, this northern corner protrudes out to be almost in-line with the roof. Two strikes! And yet, here in the worst possible scenario – with rain hitting it, snow piling against it and no sun striking it to help dry it out – the plaster was still intact and still protecting the bales, it just didn’t look pretty anymore. Other places on this building saw some cracking, a result of not using enough fiber in the mix. Our clay plasters have for years now featured high quantities of fiber and we’ve avoided these kinds of cracks.

Getting the repairs going
We addressed the two areas that had seen a fair bit of erosion with new clay plaster. But clay plaster mix is terrible for filling cracks… the large aggregate and high fiber content that make for great plaster also makes for a mix that does not want to be pushed into long, narrow cracks.

Even though we opened up all the cracks with a pallet knife, the openings were nowhere near the size needed to push in an actual plaster mix. In fact, a mix with almost any aggregate (sand) in it does a lousy job. Even if it fills the crack adequately, there is always sandy mix left on the surface of the plaster calling attention to the repair forever after. And if we used straight clay, the shrinkage would be extreme and there would be micro-cracking along the crack.

Toilet paper to the rescue!
As we contemplated how to make a mix that would adhere to the existing clay, but would have such a fine aggregate that it could be wiped off the surface, we started to think about cellulose… little paper fibers that would be very fine but still add a lot of strength to the repair mix.

Earthen plaster repairs

Toilet paper provided the cellulose fiber we needed, and mixed in the blender with clay (and a bit of talc) created a smooth mix!

We came up with a highly scientific formula: 6 arm-spans of toilet paper (two-ply) to 2 cups of clay, with a bit of talc powder and water to the desired consistency. What we got was a sticky mix that was easy to work into cracks, that bonded well with the existing clay, didn’t shrink at all and was very easy to work with!

We were able to fill all the cracks to their full depth using a putty knife and pulling back and forth across the crack until it wouldn’t accept any more material. Then one pass with the putty knife left the surface scraped back cleanly to the original plaster.

Low impact repairs… like, really low impact!
The materials we needed to do all the repair work were right on site. The clay that had been leftover from the initial plastering in 2004 was left in a small mound near the house. Slowly, that mound became a “garden” of sorts. We were able to shovel clay from the back side of the pile and leave the garden undisturbed. Some natural pigment, some sand (and some TP in the cracks)… that’s all that was required.


I don’t think we could even calculate a carbon footprint or embodied energy for these repairs!

Mixing and applying a new clay paint
The largest area of the house had a red clay paint applied 12 years ago. There were enough cracks and repairs on this section that we decided to re-coat it with a coarse clay paint. We mixed 20 parts of the site clay with 10 parts of fine sand and 3 parts of pigment, and applied this runny mix using a sponge float.

A wetter mix with only 3 parts of fine sand was brushed onto the narrower bands of colour at the top of the wall. It was easy to cut a smooth line with this paint, making for crisp lines between the colour bands.

Fast work, faster next time
There were enough areas that needed attention on this house that we decided to completely re-paint the whole building. From first arrival at the site to colour matching the mixes to application and final clean-up, we spent a total of 3 days for 2 people (about 42 hours) on these repairs.
When this plaster needs work again in the future, there will be a paint mix in all three colours ready to be re-hydrated and applied. And since the colours match, spot repairs can be done instead of a whole new coat. If we’d been smart enough to do this the first time around, we could have cut the time for the job in half! We don’t expect cracks to re-open again, as no new cracks opened up on the building after the first couple of years.

A final layer of protection
One of the reasons we feel this clay plaster held up so well – despite being a less than ideal mix placed too close to the ground – was the inclusion of a top-coat of Primasil, a silicate paint primer from PermaTint.

Though it isn’t intended to be used as a “clear coat” finish, we have applied it this way on several buildings and it has done a great job of protecting the plaster from water damage while remaining highly permeable. In the future we will experiment with adding PrimaSil to our finish plasters and clay paints instead of water and see if building the silicate right into the material has a positive effect.

An endlessly repairable finish
The beauty of clay plaster is its ability to be maintained and repaired indefinitely. We had no waste from these repairs other than some sand and clay on the ground, and we had no expense other than a bit of pigment and a roll of toilet paper. And the pigment will be suitable for about a century’s worth of repairs of this extent! Now the plaster is once again gorgeous to look at and ready to handle another decade or two of keeping out the elements… Try doing that with vinyl (or anything else!).

Why We Love Earthen Floors

Take one step – especially with bare feet – on an earthen floor and chances are you will be sold on the idea. You will want an earthen floor of your own. And not only will you be making happy feet when you choose an earthen floor, you’ll be making one of the most radical-yet-simple sustainable building choices… one that could dramatically reduce the environmental impacts of the built environment in a meaningful way.

earthen floor workshop and how-to

Clay, sand, fiber… that’s it!

A true game-changer
With the construction industry touting just about every option as being “eco-friendly” these days, it can be hard to know what choices really do make a difference. Earthen floors are a truly eco-friendly option. Using just four basic, natural, chemical-free and abundant materials that are minimally processed on site, an earthen floor creates a durable, healthy finished floor with the lowest possible environmental impacts. Mix the right proportions of clay, sand, natural fibers and drying oils and you’ll have a floor that is as beautiful as it is planet-friendly. The embodied energy of a 3/4″ thick earthen floor is 0.16 MJ/square foot, a tiny fraction compared to 3 MJ/square foot for hardwood, linoleum and concrete flooring of the same thickness, and 10-25 MJ/square foot for tile.

Really, a dirt floor?
It is often difficult for anybody in the “developed” world to consider an earthen floor as part of a clean, modern home. But earthen floors can be the visual showpiece of a home. A well-made earthen floor is a thing of beauty, bringing a texture and visual impact that cannot be replicated with any other material. Natural clay colours or natural pigments offer a wide palette, and a variety of fiber options can be used to great effect. And then there are the oil finishes which can add a rich lustre and additional colour options.

Are hearten floors durable?
Earthen floors are not a common option, and therefore most people do not have experience with seeing an earthen floor wear over time. In fact, these floors have very similar wear characteristics as most other natural floor materials like wood, bamboo and linoleum. All of these floor types can have a long lifespan under typical use conditions, although all are susceptible to scratching and gouging if mistreated, and all will require occasional refinishing to protect and enhance the surface of the material. Earthen floors are no different, and are quite easy to repair and refinish should some damage occur. I witnessed the earthen floor at Arts Centre Hastings spend a night under water after a large cooler full of melted ice broke, and yet after mopping up the spill the floor was not affected at all!

Place them wisely in the building
Though durable, it is wise to place them appropriately. Entryways, especially those that will see a lot of salt from snowy boots, can stress an earthen floor. Areas which will see a lot of dragging of chairs and furniture may not be appropriate. But if the use of the floor is for interior foot traffic, they hold up very well.

How does it work?
The clay/sand/fiber mix of an earthen floor may not seem like an ideal combination in a heavy-wearing scenario like a floor. These elements combine to make a substrate that can be easily packed and levelled. A typical earthen floor mix is 1 part of clay, 4 parts of sand, and 1 part of finely chopped fiber. As clays and clay soils can have different properties, it is always good to experiment with new materials before pouring an entire floor. Once this mix has been poured and troweled level, it is allowed to dry. Then the real magic occurs: several coats (anywhere from 2-6) of natural oil finish is applied to the floor. The oil penetrates into the clay/sand mixture and hardens around it, creating a tight and water-resistant finish that is very durable. The process is similar to natural linoleum, where linseed oil is mixed with sawdust. As with linoleum, the result is surprisingly solid.

Where can an earthen floor be used?
Earthen floors can be laid over many typical floor bases, including concrete slabs and plywood sub-floors. As

earthen floor clay floor how-to

A living room with a wood stove is a great place for an earthen floor

long as the floor base is stable and doesn’t have excessive flex or deflection, then an earthen floor can be laid. Typical thickness for a finished earthen floor is 3/4″, though it is possible to make them thicker. The floors can be laid over hydronic heating tubes, or used under wood stoves or other sources of heat. Simple substrate preparations are used if the base is either very smooth and shiny or if it is water absorbent.

It’s easy to learn to make an earthen floor
The steps involved in mixing, laying and finishing an earthen floor are very straightforward. If you think an earthen floor might be in your future, you can check out our upcoming earthen floor workshop, where you’ll get a chance to mix, pour, level and finish a complete earthen floor.

All About Natural Paint

There is no easier or better place to shift away from toxic petrochemicals and move to using natural, non-toxic options than with the paint we put on our walls.

Anybody Can (and Should) Do This
We hear from many people who wish they could build a home with natural materials, but because they live in an existing home they seem to feel there is no way for them to use natural materials. But using natural paints is something that anybody can do, at any time, in any home, and on any wall surface. And the benefits are profound. In terms of your family’s health, it can be better to have a non-natural home painted with natural finishes than to have a natural home painted with toxic petrochemicals. Natural paints are also better for the planet.

Why Not Just Use No-VOC Paint?
By all appearances, the paint industry seems to be getting “greener.” So why not just choose a good no-VOC paint and use that? Turns out, there are quite a few reasons. Firstly, paints labelled as “Low-VOC” or “No-VOC” are far from being non-toxic. Secondly, the petrochemical paint industry has a huge environmental and carbon footprint.

The Dirty Secret About No-VOC Paintdisturbing paint facts
The impetus to reduce the quantities of volatile organic compounds (VOCs) from paints actually had nothing to do with human health concerns. VOC reductions were imposed on the paint industry because they contributed to smog, and only those VOCs that directly contribute to low-level ozone production are covered by these regulations. The US Environmental Protection Agency (EPA), the body that first imposed VOC restrictions, has this to say after testing a range of paints that qualify as low-VOC and finding surprisingly high concentrations of VOCs:

“EPA Reference Method 24 is probably not an adequate method for measuring the VOC content of low-VOC latex paints. …Current bulk analysis and emission test results showed that the VOC contents of low-VOC latex paints are well within the uncertainty range of Method 24, and the method is apparently not precise enough to accurately define the VOC content of those paints.” –Inside IAQ EPA/600/N-98/003

What If It’s Labelled as “Green”
There are some labelling programs that do ensure acrylic (commonly called “latex”) paints are less harmful to occupants. However, the most common labels do not. GreenGuard and Ecologo are the labels most commonly seen in paint stores. They are administered by Underwriters’ Laboratories (UL). Here’s what that standard has to say about its commitment to human health:

“1.14 While this practice lists specific chemicals and associated maximum allowable concentrations, as required by criteria indoor air procedures and specifications, it does not assess the human risk involved with use of the materials either as an installer and/or as an end user.” –UL 2821

green seal logoIf you want to trust a label, find paints certified by GreenSeal GS-11. This is the only standard I can find that actually excludes a wide range of toxic chemicals and has a direct concern for human health.

And Even If It’s Got a Good Green Label…

Despite the fact that they are called “water-based,” all acrylic paints are made from petrochemicals. Coatings consumption worldwide reached 80 billion pounds and $120 billion in value in 2013, according to “Global Paint & Coatings, 2013-2018,” by polymer and chemical market researchers Kusumgar, Nerlfi & Growney. That means that our use of petrochemical paint carries with it the same environmental impacts as any use of crude oil. Don’t like offshore drilling, oil sands, pipelines, greenhouse gas emissions, oil spills, etc? Every time we use acrylic paint, we contribute to all those impacts.

From raw material harvesting through production and end-of-life waste, the 80

From raw material harvesting through production and end-of-life waste, the 80 billion pounds of paints produced annually have a massive impact on the environment.

The embodied energy and embodied carbon emissions of acrylic paint are also very high. Using data from the Inventory of Carbon and Energy V2.0, the paint needed to coat the interior of a typical 2,000 square foot home (primer and two coats of finish) would use about 7,300 megajoules (MJ) of energy to produce, and emit 303 kg of carbon dioxides (or equivalents). That’s the energy in 1.5 barrels of crude oil or 61 gallons of gasoline required to paint every home, and somewhere in the neighbourhood of the same weight in CO2 emissions as the combined weights of the home’s inhabitants!

Now the Good News!
Don’t want to inhale toxic chemicals or contribute to oil spills and climate change? The good news is that there are plenty of accessible, affordable and practical paint options available that are non-toxic and low-impact. Most of the paint manufacturers listed here provide full disclosure of their ingredient lists, meaning that there are no hidden toxins. All have been recommended by people with chemical sensitivities.

Natural paints come in a number of different categories, based on the type of binder they use, and each type of paint has a range of different surfaces it may be used on:

Natural Oil Paints

  • Drying oils (linseed, sunflower, tung, etc) polymerize when exposed to air
  • Some natural oil paints are emulsified with water
  • Indoor & outdoor use
  • Used on almost any substrate

Although many people will have an initial negative reaction to the idea of “oil paints,” these bad associations are from very toxic petrochemical oil paints. Natural oil paints are a whole different breed. The emulsified oil paints are the most straightforward natural paints to use, and give results that are consistent with modern petrochemical paints. Washable, durable and tinted to any available colours, these paints can be used to replace conventional acrylic and alkyd paints with no change to expectations about application, coverage and durability. All the brands we’ve used are non-toxic and fully bio-degradable. Most can be obtained in just about any imaginable tint.

Auro Wall Paint, available in Canada from Tockay
Allback Linseed Oil Paint available in Canada from Living Rooms
AFM Safecoat Naturals available in Canada from Living Rooms
Kreidezeit Wall Paint, available in Canada from Tockay

Lime Paints

  • Calcium carbonate binder, often with additional natural binders
  • Indoor use (outdoor use for lime washes)
  • Most wall substrates, surface prep may be req’d

Lime paints have been used for thousands of years, and the modern versions are excellent products that can be used on most wall surfaces. Naturally anti-septic, these paints come in a variety of textures from quite smooth to quite grainy. They add a depth and beauty that is hard to explain but is immediately obvious upon seeing them. They are durable and do not wash away with water. They are an excellent choice for any wall that receives light to heavy contact, and are available in a wide range of colours.

Kreidezeit Lime Paint, available in Canada from Tockay
Auro Lime Paint, available in Canada from Tockay

Clay Paints

Non-toxic paints

Kreidezeit clay paint can be brushed or rolled onto wall surfaces primed with a casein primer

  • Natural clay binder, often with additional natural binders
  • Indoor use only
  • Most wall substrates, surface prep may be req’d

Clay paints are the champions of low-impact and low-toxicity. The fact that they are gorgeous to look at is an additional bonus! A variety of grain sizes and tints are available. They are durable (no dusting, will not brush off the wall) but are not washable. They can handle some direct wetting, but will wash off with scrubbing or constant abrasion. Good for use on any wall that does not receive direct wetting or a lot of touching/contact.

Kreidezeit Clay Paint, available in Canada from Tockay

Casein Paints

  • Milk or vegetable casein binder, often with additional natural binders
  • Indoor use only
  • Most wall substrates, wood

Casein paints can be made from vegetable or milk casein. Similar to the clay paints, they are capable of dealing with some wetting and abrasion, but shouldn’t be used in places where this will happen consistently. A wide variety of tints are available. They can be used on walls, and also on raw wood.

Homestead House Milk Paint, available from Homestead House
Kreidezeit Vegetable Casein Paint, available from Tockay

Mineral Paints

Non-toxic paint

Eco-House silicate dispersion paint can be used on interior and exterior mineral surfaces

  • Potassium or sodium silicate (“waterglass”)
  • Indoor & outdoor use
  • Mineral substrates only (plaster, brick, concrete, etc)

Silicate dispersion paints are unique in that they don’t coat a surface, they mineralize onto the mineral surface and become an integral part of the surface. This makes them extremely durable. We use them a lot as a finish for exterior plasters, where they have the Goretex-like effect of protecting walls from bulk water penetration, but maintain the permeability of the plaster. They can be used indoors or outdoors on any surface that is mineral-based, including clay & lime plasters, concrete, brick, stucco and stone. They come in a wide range of colours, and colour matching is available.

Eco-House Silicate Dispersion Paint, available in Canada from Perma-Tint

Non-toxic Clean-up
One of the unsung benefits of using any of these paints is that they are all biodegradable. Even the “cleanest” conventional paints have a petrochemical base that ends up in waterways or in soil during cleanup, with an aggregate of thousands of gallons entering the ecosystem annually. Natural paints clean easily and the wash water can safely go into septic systems or onto the ground.

So Many Viable Options
All of the paints listed here are products that we have used with excellent results. Each type of paint has specific uses and surfaces, meaning there is no surface in or on a home that cannot be treated with a natural paint. Costs tend to be slightly higher than mid-range conventional paints, and in line with higher-end conventional options. None of these paints are unaffordable, and the slight extra cost is a small price to pay to be surrounded by non-toxic surfaces that are not off-gassing into your home, and did not have a deep impact on the environment. A worthy investment for any home!

Want to Try These Paints?
Endeavour’s Eco-Paints workshop is a day long opportunity to learn all about natural paints, and to actually use all of the paints mentioned above.

The Art and Science of Natural Plaster DVD Now Available

The Art and Science of Natural Plaster is a 140-minute DVD created to help homeowners figure out how to use natural plasters on their own projects, created and narrated by Chris Magwood of Endeavour Centre. The
DVD is now available for purchase through PlasterScience.com in hard copy or online streaming formats.

Produced by Bart Glumineau, a graduate of Endeavour’s Sustainable New Construction program in 2013, and co-founder of PossibleMedia.org, creating original video content to share the stories of individuals and groups of people who are actively engaged in creating a better, more sustainable future.

The DVD covers base coat and finish coat mixes and applications that are suitable for a wide range of sustainable and conventional wall surfaces, from cob and straw bale to drywall. Chapters include:

  • Introduction to Plaster
  • Types of Plaster
  • Substrates and Substrate Preparation
  • Mixing
  • Tools and Trowels
  • Body Coat Application
  • Finish Coat Application
  • Paints, Washes and Sealants
  • Repair and Maintenance of Plasters

Much of the hands-on footage for the DVD was filmed at Endeavour’s 2014 project, a straw bale office building for the local teachers’ union.

We are excited to have some of our teaching and methodology presented in an accessible video format, and hope that the DVD inspires more people to take up natural plastering on new builds and renovation projects!

Thatch roof update

In 2009, we undertook the first permitted thatch roof in Ontario as part of the Camp Kawartha Environment Centre on the nature preserve grounds at Trent University. This roof sits atop a timber framed entryway for the building and greets all those arriving at this busy public building. The roof is nearing the end of its fifth winter, and a hike along the nature trails near the building gave us a chance to inspect the thatch under a blanket of snow and find that it is still water-tight and holding up very well.

Thatching is a roof system that has ancient origins and is still widely used in a modern context… just not in North America. Only a small handful of buildings on this continent have thatched roofs, and the skill set is extremely limited. This despite the fact that the material for thatching roofs is a widely available invasive reed known as phragmites (or elephant grass). An abundant supply of these reeds grows along many highway medians and ditches.

Thatched roofs exist in a wide range of climates world-wide, with the northern European roofs in countries like Denmark and Germany most closely representing Canadian conditions. In these places, thatching typically lasts 40-70 years, an impressive improvement over the commonly used 25 year asphalt shingles.

The actual process of thatching a roof is a bit more labour intensive than conventional shingling, though experienced crews in Europe move a rate that is not far off conventional practice here. For our project, the manual harvesting and preparation of the reeds was the most labour-intensive aspect. This would be quite easy to mechanize (as has been done elsewhere in the world), which would make thatching a much more viable proposition in this part of the world.

Given that the material for thatching grows annually, for free, along our highways, and that the environmental impact and working lifespan of this type of roof are far better than conventional options, it would be great to see more thatching happening in this part of the world.

Building bottle wall features

Many natural buildings feature bottles incorporated into walls. Bottle walls add colourful light and whimsy to a wall and open up all kinds of great design possibilities.

Here’s a little “how-to” guide to building your own bottle wall. It’s quite an easy process, and is applicable to interior walls and renovations as new buildings. Your own creativity is the only limit when it comes to using bottles in your building!

Rocket mass heaters with Andrew Brunning

I have lived almost my entire life in homes that have been heated with wood in one way or another. From a giant wood furnace in the basement of an old Ontario farmhouse to an elegant little pellet stove in a city home in Peterborough, I have enjoyed the process of burning wood to keep warm.

Rocket stove revolution

With this kind of background, it’s no wonder that I have followed closely the development of “rocket stoves” over the past decade. From their beginnings as a means to provide efficient cooking heat from minimal fuel in developing countries, the promise of rocket stoves has been intriguing for any wood burning enthusiast. However, the open “J-tube” style of most rocket stoves meant that the feed tube for the fire was open inside the home with all the attendant dangers. In addition, the wood used in J-tube stoves is small dimension, which is perfect for cooking where fuel is scarce but as a home heating device means constant attention and stoking is required. For these reasons, I have been hesitant to recommend rocket stoves as a home heating system, except for the strong-hearted devotees of the idea.

Rocket mass heaters – suitable for indoors!

However, the development of “rocket mass heaters” brings the rocket stove idea to the point where it is a feasible home heating device. This style of rocket stove blends the safety and efficiency of the masonry heater with the do-it-yourself approach of the rocket stove. I was privileged to be able to take a workshop on building rocket mass heaters with Andrew Brunning of Rocket Mass Heaters.

The design of the rocket mass heater, or batch box rocket stove, was developed by Peter van den Berg, and its genesis is explained in this article in Permies. The heater combines the simple construction and burn characteristics of a rocket stove with a full masonry burn box, as with a masonry heater or typical wood stove, which can have a closed door with or without glazing. One fill of the burn box equals several hours of burn time and many more hours of heat from the mass built around the stove.

How to build a rocket mass heater

The workshop with Andrew allowed the participants to help build the rocket mass heater, as well as the large mass bench that would be the recipient of the heat generated. The photo gallery below gives a good overview of the process:

Rocket mass heater workshop coming to Endeavour in 2015

I look forward to building one of these rocket mass heaters for myself. And Endeavour looks forward to bringing Andrew to the school in 2015 for a hands-on workshop!

How to build with earthbag

Earthbag building is one of Endeavour’s favourite building techniques. We’ve used it for foundations on many projects, and have built an entire buried root cellar with this material.

We’ve put together our experience with earth bag in a photo series. We hope it inspires you to consider this choice for your next building project!

Building with Hempcrete or Hemp-Lime

A group of lucky participants was treated to an excellent weekend workshop on building with hempcrete (or hemp-lime), led by UK architect and hempcrete pioneer, Tom Woolley. Tom is the author of Hemp Lime Construction and Low Impact Building, and has been involved in many hempcrete and sustainable building projects throughout the UK.

The weekend began with a classroom session, during which Tom covered the materials and techniques for successful hemp-lime building, and showing the group photos and details of a variety of building projects, including his charming hemp-lime cottage.

We then moved on to making some sample mixes to demonstrate the combination of materials. We were working with two different mix types, and made a sample of a third type of mix. For all three mixes, the weight ratios of materials were the same:

  • 1 kilogram of chopped hemp hurd (also known as shiv)
  • 1.5 kilogram of powdered binder (natural hydraulic lime or hydrated lime and metakaolin)
  • approximately 1.5 kilograms of water

The chopped hemp hurd or shiv needs to be fairly course (particle sizes ranging from 1/4 to 1 inch) and be relatively dust- and fiber-free. We were able to source Canadian-grown and processed hemp hurd from Plains Hemp in Manitoba.

Most UK-based hempcrete builders work with a natural hydraulic lime (NHL) as the basis for their binder. There is no North American source for NHL, so it tends to be expensive to import from Europe. We used an NHL 3.5 from St. Astier as one of our mix options. For a more locally-sourced version, we used a typical North American hydrated lime and a fired kaolin clay (called metakaolin) called Metapor. The NHL is a lime that chemically sets (hardens) through a reaction with the water content of the mix. North American hydrated lime does not set hydraulically (with water), but when mixed with a pozzolan like Metapor the two materials together have a hydraulic set.

The dry ingredients (hemp hurd and lime) are mixed together so that the powdered lime is covering all of the hemp, and then the water is introduced. Having done some work with hempcrete at Endeavour, we were surprised at how little water Tom uses in his mix. The final mix is just moist enough to lightly hold together when squeezed in a hand.

With some small scale mixes placed into test cones, we then moved on to installing hempcrete in some larger wall panels. These panels were built by Sarah Seitz for her PhD research work at Queen’s University, where she will perform tests to help determine the thermal insulation properties of hempcrete.

The panels simulate a typical double-stud construction, with a 2×4 frame on the “exterior” side of the panel and a 2×3 frame on the “interior” side.

As one group mixed batches of hempcrete in the mortar mixer, the others placed it into the forms and lightly tamped it into place. As the forms fill up, they are moved up the wall. The hempcrete retains its shape after less than 20 minutes in the forms. The filling and tamping continues right to the top of the wall. Once everybody was settled into their roles, it took less than 1.5 hours to fill a whole wall form.

In the end, we placed 40.25 cubic feet of hempcrete into the two walls. We used seven 40-lb bags of hemp hurd and seven bags of powdered ingredients to reach that quantity. With the small amount of water used in the mix, we’re anticipating a drying time for the 14-inch thick walls of about 2 weeks. This is much shorter than for the wetter mixes we have made in the past.

Cost-wise, we used $63 of hemp, $42 of hydrated lime and $21 of metakaolin, for a total material cost of $3.13 per cubic foot of insulation.

We will share Sarah’s thermal testing results when she has completed them. We are expecting to find their performance to be around R-2 to 2.5 per inch, meaning that our 14-inch wall would surpass current code requirements for thermal insulation.

This was a fun and informative workshop, and we’d like to thank Tom Woolley for sharing his deep knowledge of this subject with us!

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