So You Want to Live in an Earthen Home – Part 2: Choosing a Building Method

In Part 1 of the series we discussed the ins and outs of choosing the right land – building codes, soil conditions, water access, neighbours, proximity to markets, etc. As you will see, the qualities of the land and the unique sites found on it will shed light on the type of building method you’ll want to employ. Choosing the ideal building method hinges on four main criteria: 1) Climate, 2) Availability of materials, 3) Embodied energy, and 4) Size, design, and layout of the home. The last thing you want to do is impose a building method on a site to which it doesn’t match.

Climate: The Insulation and Thermal Mass Variables        

We’ll start with climate, which should factor heavily into your decision. For areas with cold winters you will want to choose a method that keeps heat in. Straw bale, including the unusually large ‘big bales‘, and double wall stack wall construction (a.k.a cordwood), which Saskatchewan’s Cliff Shockey has shown to be well suited to harsh prairie winters, are well suited for cold climates. As much as it hurts to hear it, a cob house just doesn’t make a lot of sense in the middle of winter in provinces like Alberta and Saskatchewan, or for that matter most of Canada (the very West Coast being the arguable exception).

Fitting somewhere in-between are light-clay straw and Hemp-Crete, which blend good insulation with thermal performance (more on this later). Mild climates pose fewer problems when choosing a building method and, really, the sky’s the limit when it comes to choosing which method to use. Earth bag, cob, rammed earth, adobe, and papercrete may be successfully used, though it’s important to consider the precipitation patterns and solar potential when deciding between methods. High levels of precipitation can be overcome in most places as long as you have a good hat (roof) and boots (foundation) for your home. The key point is to research each method thoroughly before committing to a full project. It’ll save you headaches and heartache.

Now it’s important to remember that insulation and thermal mass are nearly inversely proportional, meaning that the more insulative a material the less thermal mass it contains, and vice versa. While some people will be drawn to using R-values to evaluate one method against another, it should be kept in mind that R-values are static ratings. They fail to capture the thermal mass performance, which can be as important a factor as insulation. While R-Values measure thermal resistance (the resistance of a material to resist movement of heat through its structure), thermal mass describes how much “inertia” the material provides against temperature fluctuations. It’s essentially the ability of the material to absorb and radiate warmth and coolth. This is precisely why a material like light-clay straw may be found in more temperate climates despite only having an R-Value of approximately 1.5 per inch. It blends thermal mass and insulation beautifully.

Capturing the Sun: Using Site to Your Advantage

Those wishing to incorporate passive solar design, use a masonry heater (or rocket mass heater) or both, will want thermal mass in their home. And I can’t emphasize enough – of you have access to a good solar resource, use it! Strategically placing windows (glazing) and using thermal mass to store heat when the sun is shining (or when the fire is going) and having that heat release during the night when temperatures are at their lowest, is just good, smart, design. It’ll cut the amount of wood you’ll need to chop and haul, the amount of gas propane you’ll have to burn, and you just can’t beat the comfort of passive heat. It’s important to evaluate building methods by considering not only  it’s R-Value, but it’s thermal mass performance as well.

Availability of Material

The availability of materials is also a crucial factor to consider, as this can either work for you or against you. As Rob Roy say’s in his book Mortgage Free, you’re best to tailor floor plans to structural considerations and available materials, not the other way around. In other words, if your land is covered in trees and doesn’t have a speck of clay, you’d be best served building a log or cordwood home instead of a cob home. Having to transport large quantities of material to your site will likely be laborious, costly, and it will increase the environmental impact of your home, which leads into the next point – embodied energy.

looking for clay

This is the sum of energy inputs from cradle (e.g. the ground) to grave (e.g. landfill). This includes the fuel energy and electricity to extract, process, manufacture and transport a material; the energy used to maintain the material over time; and the eventual disposal of the material. For example, the embodied energy of earth – the basis of cob, adobe, rammed earth, etc. – is extremely low, while concrete (cement) is nearly off the charts in comparison. It is also worth considering what will come of your home after you are gone. Will you be leaving an unsightly, even toxic, legacy or will it simply “melt” back into the landscape?

Design

Lastly, don’t forget that when it comes to choosing a building method, size does matter. This became apparent while working on a 2000 square foot cob house in New Zealand a number of years ago. The project had been 8 years in the works and it had yet to be finished. There’s a reason that most cob homes are only a few hundred square feet! Though the finishing work on cob is minimal relative to straw bale construction, the building process moves at a glacial pace in comparison. If you have a big family, have a home-based business, need more roof area for rainwater capture, or just want extra space, you’ll want to keep this in mind when choosing a building method. And remember, you can always add on later. My advice is: keep it simple and keep it small.

The desired design and layout also factors into this decision. If you want round, curvaceous walls, rammed earth or straw bale might not be the best choice. Cob will have you singing. Some techniques are better suited to multi-story construction than others. This is an important consideration because a second story can be added without added roofing or foundation expenses – two of the most expensive elements of any build. Two story designs are also more energy efficient.

Don’t Be Overwhelmed: Where You Live Will Naturally Provide Many Answers

As much as there is to consider, this process becomes much more limited by where you end up living. If you have access to straw but few or no trees, then you might rule out cordwood construction and you might not heat with wood. If you live in a mild area with lots of clay and plenty of sunshine, a cob house with good passive solar design may be all you need. Of course, you may decide that it’s a cob house or no house, so this would obviously inform your land search. Pay attention to these details and your project will be more fun, affordable and attainable.

Happy choosing,

Ashley

Next month – Part 3: Siting your house


How We Can Help You:

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