- Part 4

Rammed Earth Construction is the ultimate in building with local materials. It is build from earth that is dug up from the building site. The technique consists of building a form, similar to a form used to pour concrete, and then filling the form with a mixture of sand, clay and gravel with portland cement as a binder and then compressing it together with a tamper.

Rammed earth construction has been in use since the neolithic ages and there are archeological sites in China from 5000 BCE where rammed earth was used for walls and foundations. In the past binders such as blood or lime were used.

Rammed earth construction is particularly good in passive solar design, as it has a large amount of thermal mass, which will even out the temperature fluctuation during the day. It also has the advantages of being almost soundproof and fireproof. In some sites, other materials, such as glass or shells are added to the mixture to give it additional texture. It can also be coloured by adding pigments to the mixture. Variations can be created by using different mixture in the various layers. Since the rammed earth is used as the finshed wall, it is very non-toxic, as the materials are soil and cement, so it is a good construction method for those with environmental allergies.

The technique for construction is quite simple. Forms are put in place in the shape of the desired wall and then a damp mixture of sand, clay, gravel and portland cement is placed in the forms four to ten inches deep. The mixture is tamped down with either a mechanical pogo stick tamper, a pneumatic tamper or by hand with a plate tamper until it has been compressed to about half it’s original thickness. Another layer is then put in and the process is repeated until the top of the form is reached. The form is then removed and move up so the process can continue until the desired wall height is reached.

An interesting variation on rammed earth was developed in British Columbia, called SIREwall, which stands for Stabalized, Insulated Rammed Earth wall. In their process, a piece of foam insulation of the desired thickness is placed vertically in the center of the form and the rammed earth is placed on either side around reinforcing rebar and then tamped down using custom selected mechanical tampers. This creates a wall that has an insulated core, but has the thermal mass exposed on the interior and a durable rammed earth exterior protecting the insulation. This results in a wall between 14 and 21 inches thick. The SIREwall process has a protocol that controls the soil consistency, the method of mixing and curing to produce a wall with predictable, cost-effective results.

Properly done, a rammed earth construction will create a building that is weather resistant, soundproof, insect and rodent proof, inexpensive to heat and cool, very comfortable to live in and will probably last for centuries.

I consider Structural Insulated Panels (SIP) to be one of the best options for an environmentally friendly home. This is because they form a wall that has little or no air infiltration, use less lumber than conventional framing, and in some cases the foam can be made from renewable resources. There have even been SIPs made using straw as the insulating medium. Also, since they are built in a factory, there is little jobsite scrap and what scrap that is produced at the factory can be easily recycled. The SIPs can be manufactured in a variety of thicknesses, with 6 inch being the most common. I would recommend going with 8″ for a R32 wall.

SIPs usually made of two layers of Oriented Strand Board (OSB) separated by a layer of foam insulation. The foam can be Expanded Polystyrene (EPS), which is a foam made of small beads fused together, Extruded Polystyrene (XPS), commonly a blue or pink solid foam board, or Polyurethane foam, a solid white or beige foam. The SIPs are made to order in a factory and then shipped to the building site. The sections can be anywhere from 4 feet to 24 feet in width. While the wider sections provide a better insulate wall (since there are no joints), they require the use of a crane and a larger building crew for installation. The SIP sections are joined together during installation using a spline, often made of lumber and then sealed with low expanding foam, specialized mastic and/or SIP tape. SIPs can be used to build not only the walls of a building, but can also be used to build the roof of the building, however, extreme care must be taken to make sure the joints are completely sealed, since if air can pass through them, it will carry moisture that can condense and produce an environment conducive to the growth of mould. This has been the cause of structural failures in the past, particularly in very cold climates.

A Do It Yourselfer can build with SIPs, but you would have to make sure to get smaller panels that can be handled by one or two people. The cost will be higher than building with standard framing, but will result in a better sealed home. If you are contracting the work out, or doing your own general contracting, SIPs would be a good way to go since the labour costs would be less than for framing and insulating.

Some disadvantages of SIPs are that you have to be quite exact in your measurements when building the foundation, as the SIPs are cut to fit the plan, and field modifications require the use of specialized equipment. The other issue is that SIPs don’t have a lot of thermal mass, but this can be offset by designing extra mass into the interior of the house.