Slip straw is an easy and energy efficient way to build lightweight homes or walls. It uses materials that are almost always locally available and also offers quality insulative properties. To build with slip straw you will need an abundant supply of straw, clay and some sort of wood to make forms.
The straw you use should be from some sort of grain such as rice, wheat, barley, etc. Hay, which comes from different types of grasses, isn’t suitable since it is usually much thinner than the thicker stalks of grain-based straw.
The first step in slip straw construction is to build a timber frame for your home. Since slip straw walls are so light weight, they are only usable for infill walls and can´t bear the weight of a roof. Once your timber frame is finished, you need to set up a form (plywood works great) that is around 4 to 6 inches thick.
When the form for your wall is ready, mix up a heavy clay slurry by mixing 1 part clay (high clay content is best) to 2 parts water. A paint mixer attached to an electric drill is a quick and efficient way to mix up the clay and water to make your slurry. You are aiming for a consistency of melted ice cream.
Take your straw and dip it into the clay slurry making sure that all the individual fibers get coated. Then, put the clay soaked straw (this is the slip straw) into your wall form and use a stick to compact it as tightly as you can. Continue adding more slip straw and compacting until your wall is completely “filled up.” Let your slip straw dry for several days before removing the form.
The dried clay acts as a glue that holds the individual fibers of the wall together. Your wall will be very lightweight yet strong as well. The spaces between the straw fibers are great insulation as well. When you have finished building your in-fill walls with slip straw you can plaster over them with either an earthen plaster or a lime stucco.
Earthen Floors and Earthen Plasters
Whichever type of natural home you decide to build, earthen floors and earthen plasters are a great option to add character and warmth to your home. It is said that earthen plasters turn a mud house into an earthen home.
One of the most beautiful types of natural plasters is “tadelakt”, an ancient Moroccan form of lime based plaster. Tadelakt can be used as a plaster over earthen homes, straw bales homes, or earthbag homes. To build with tadelakt, you need to first make a lime based plaster by mixing 2 parts lime putty (lime powder mixed with water to the consistency of a thick milk shake and let to sit and mature for several weeks) with 1 part very find sand material. Marble dust (used in Venetian plasters) is a great option, adds strength to the plaster, and can be found at most pool supply stores.
The trick to tadelakt plasters is in the process of polishing. It is a very time consuming process as you can only do a small section of each wall at a time. Place a little bit of the lime based plaster on your wall and smooth it so that is flat. Once the plaster starts to set, take a small polished rock with a smooth side and begin to polish your wall repeatedly. The goal is to try and force out of all the water in your lime based plaster. You will need to polish a small section of wall for 15 to 30 minutes with your smooth rock until the plaster is smooth and dry.
Once you´ve finished plastering your wall, you can seal the wall with a natural soap such as olive oil soap and you will have a bright, smooth plaster that will make you feel as if you´re living in a polished piece of pottery. When done correctly, tadelakt plasters are waterproof and can even be used for earthen sinks or bathtubs.
A home with slip straw walls and tadelakt plasters wouldn’t be complete without an earthen floor. Earthen floors offer a huge source of earthen mass that works great to capture the heat of the sun. When combined with passive solar design, an earthen floor can capture enough heat to keep homes warm even during the coldest winters. Slip straw walls add extra insulation to keep that heat in the house.
To build an earthen floor, follow the simple constructions below:
- Fill the perimeter foundation of the living space with 1-inch washed and cleaned gravel. Be sure not to use surfacing gravel, which is commonly used on roads.
- Pour a 1-inch-thick layer of sand over the gravel.
- Cover the sand with a layer of the 30-ml plastic sheets.
- Pour another 1-inch-thick layer of sand over the plastic. The sand on both the bottom and the top will protect the plastic sheeting from being torn by the gravel.
- Pour about 2 inches of road base on top of the sand. Spray it lightly with water, tamping down the road base by hand until it is tightly compacted. Add the road base in this manner until you have about a 5-inch layer of road base.
- Mix together six parts sifted sand, two parts sifted clay and one part finely cut straw in a wheelbarrow. The amounts of each material vary depending on the size of the room.
- Spread a 2-3 inch thick layer of the adobe onto the floor. Let dry for at least a week. It will crack.
- Once dried, lay another 1 inch thick layer of adobe onto the floor using the trowel to create a smooth finish leaving it to dry for another week. It should crack significantly less.
- The final layer should be no more than half an inch thick and applied with a trowel to give a fine finish. Let dry completely.
- Once dried, apply linseed oil to the entire floor with a rag. Make sure to put down as much oil as possible without creating puddles. Let dry, which can take a couple of days.
- Mix three parts linseed oil with one part oil thinner, and apply the mixture to the entire floor. Let dry completely.
- Mix two parts linseed oil with two parts oil thinner, and apply the mixture over the second layer. Let dry completely.
- Mix one part linseed oil with three parts oil thinner and apply the mixture over the third coat. Let dry completely. It is important to make sure that between coats, the oil is completely absorbed into the floor.
- To seal the floor, one can add melted beeswax as well.
Modern-day homes most likely have VOC-emitting paint that is applied to drywall that covers a stick frame built from pressure treated 2x4s and formaldehyde-laden plywood. Floors are covered either with polyurethane carpets that are sprayed with cancer causing fire retardants or laminate flooring made from MDF, which is also a major source of formaldehyde that off gasses into the home. This construction process essentially ensures that you will be living in an environment where the indoor air quality is overloaded with volatile organic compounds (VOCs) that have the potential to negatively affect your health.
On the other hand, a home made from clay and straw, with beautiful hand-polished walls with lime and sand, and a floor made from the earth beneath your feet will not only be beautiful and aesthetically pleasing, but will also offer a healthy, natural environment.