Building a Light Straw-Clay House

Building a Light Straw-Clay House

Building a Light Straw-Clay House 150 150 Strawbale Studio

Building a Light Clay Straw House     by Jacob Schmidt


This is an introduction to light clay straw construction (a.k.a. “straw-clay”). In the summer of 2010, I started construction on my first house (20 ft. x 12 ft.). It was stud framed with light clay straw infill. I had helped with other natural building projects and had some conventional experience. This article is the product of all of my mistakes (and few successes) so that you, my fellow builder, do not have to walk the road that I have. Fortunately for y’all I built a chicken coop in 2011, and that helped me to learn more about building correctly with light clay straw. (One mistake being I should have done the coop first, but at least my chickens are happy.)

Straw-clay construction is not suitable for all buildings, but it is an easy building method, eco-friendly, and cheap. The straw provides insulation, and the clay slip gives some thermal mass. Before building, I suggest reading general construction, green building, framing, and foundation books. Clay-straw is more appropriate (in my opinion) for smaller buildings (>1000 sq. ft.) because of labor and time issues. In Germany, this type of construction is known as leichtlehm which translate to “light loam”. Light clay  straw is akin to wattle and duab which has been used in Europe since the 12th century. After the second World War, straw-clay was used between structural members (studs), whereas traditionally the building was timber framed.

Building with Light Clay Straw: Overview

The process of building the walls is simple but pretty labor intensive. After deciding the building basics (dimensions, orientation, layout), you can decide how thick the walls should be. Most sources recommend 12 inches thick, which results in an r-value of 19. Plaster adds very slightly to the r-value, and the clay/straw ratio affects the r-value (more straw nets a higher r-value, and more clay means greater thermal mass). Walls can be as thin as 4 inches.

Consider what type of foundation (see below) and the type of framing you will build. The options for framing are timber framing, post and beam, and stud framing. To finish construction by the winter, you should start as early as possible. Dig your trench and put in your foundation, and frame and roof your building by the early summer. Finish your walls by mid-summer to allow for a long drying time. Lastly, apply the plaster and allow to dry (~1 week) by the first frost.

A straw-clay chicken coop

Foundation for a Light Clay Straw House

A good foundation is essential for the longevity of your building. The top of the foundation needs to be at least 8 inches above grade, and should really be 12 inches high (a “footer”, get it?). You will need to have the final grade slope away from the wall, and if you’re in areas of heavier rainfall, you will need to drain around the perimeter. Good site location can save many headaches and backaches. I suggest a rubble trench foundation which is simple and eco-friendly. The idea is this: you dig a trench as thick as your grade beam is wide and 6 inches below the frost line. Then place a 4 inch perforated drain tile (pipe) in the trench and backfill with 3/4″ clean gravel. The tile needs to drain to daylight, so the tile should be at a slope towards the outlet(s) which are solid drain tile. Bring the gravel to near grade and add a ground beam (earthbags, rocks, bricks, concrete, urbanite, or concrete blocks).

Other alternatives are to put in a concrete foundation and stem wall or a slab on grade, or build on piers. (There are many books available on these techniques). You will need to place a moisture barrier (a.k.a., a capillary break) on the top of the footer/ground beam to prevent moisture from rising into the wall. A last consideration is that the foundation should be an inch thicker on the inside and outside of your walls so your plaster has a lip to rest on.

Framing Your Building

Framing the building is something I just have ideas about. Work with folks around you who know framing, or get books on the subject, or both. If you are stud framing, your studs will be flush with the wall when complete on both the interior and exterior. If you want to build a 12 inch wall, you can use 2x4s that are tacked together with 2×4 joiners (perpendicular), spaced every two feet (vertical). You could also use 2x6s (which should be joined as well) or one 2×12. Put studs at recommended spacing for roof loads. If you double up boards, then you can put twice as much space between studs.

Post and beam or timber framing having framing members set into the wall, and are ideal since temporary boards (for building the light clay straw walls) need something to attach to. However, they do not have to be completely flush with the walls like stud framing. Another consideration is if you’d like the finished walls to be flush with the frame and then plastered over, or if the plastered walls will be flush with an exposed frame.

When roofing your building, you will need to have at least 18 inches of overhang (or more in high rainfall areas, 24″).

Materials for Making Light Clay Straw

The tools and experience necessary to build light clay straw walls are minimal. After deciding your wall thickness, length, and height, you can determine how much straw and clay you will need. My calculation is that a two string bale can do 15 cubic feet. You can use barley, wheat, rice or oat straw, but do not use hay. Wheat is recommended for its higher insulative qualities. The amount of clay needed depends on the percentage of clay in your subsoil. The minimum clay ratio is 50%. This can be determined by a simple jar test (this method is described in other books). If your clay content is 75%, a cubic yard can do about 100 square feet of wall (12 inches thick).

Also, it’s best to frame and roof your building so that you can store your bales under cover. You should never use moldy or wet straw.

Making clay slip is fairly easy. You can sift (through 1/4″ hardware cloth) dry clay into a 5 gallon bucket, and once it is half full, add water until the bucket is three quarters full. Mix the clay with a paint paddle, a stick, or your hands (or for larger quantities, a cement mixer). Mix until you reach the right consistency (it should resemble heavy cream) or until you can no longer see your finger prints. Add water slowly, as it is easier to add more water than to take it away. Another method is to fill a barrel (55 gallon) three quarters full of screened clay and put at least two inches of water over the top. Let it sit for a couple of weeks and it will be a lot easier to work with. Take material from the barrel and mix in smaller quantities as described above.

You will need to put vertical bracing in your walls to support the straw and prevent settling. You can use sticks, bamboo, or any small diameter object that can hold the weight. The stabilizers should be every two feet and attached to the structural members. The bars are easiest to add as the walls are being built so as to not hinder wall building.

Building Your Walls

Now you are ready to build your light clay straw walls. First attach a 3/4″ piece of plywood or OSB to the frame on both sides. the boards should be two feet high and long enough to span the distance between framing members. Break up half a straw bale on a 8’x12′ foot tarp (or plywood) and pour half of a 5 gallon bucket of slip onto the straw. Mix with a pitchfork and add the rest of slip as needed to coat all the straw evenly. Some folks suggest mixing the slip and straw and allowing it to rest for a day or two so that the mix is easier to tamp, and so the straw absorbs more slip.

Fill up the space between the plywood forms, and then detach the forms and move them up to the empty space above finished wall sections. The walls do not need to rest as they are being built up. To start, simply put mixture in-between the plywood and compact it. You will not need to use all of your strength, but the mix does need to be compacted. Some people use small 2x4s to push the mixture down. When you reach the last section of wall under the roof, simply place the outside board all the way to the top and the front one half the distance up. You then stuff and tamp and move the inside piece all the way to the top and allow the wall to cure for an hour before removing the boards.

Drying Time For Light Clay Straw

Once the wall has dried for a week, you may need to add more mixture in gaps that appear. You will need to mist down the surrounding area with water (but don’t saturate it) for better cohesion. The rule of thumb is that for every inch of thickness the wall will need one week to dry, so a 12″ thick wall means 12 weeks of drying. Occasional rain hitting the walls is okay, but if it is too wet, more drying time is needed. The walls should not freeze before drying, either. Depending on your straw, you will most likely get a lot of missed seeds sprouting. When they have shriveled and died, your walls may be dry enough. If you have a bale probe, and when the moisture content of the walls reads below 18%, you can begin to plaster. If you are not going to be able to plaster your walls before frost, you need to protect them by hanging old billboard vinyl, or tarps, preferably with airspace between the walls and its protection.

Plastering Straw-Clay Walls

Before you start to plaster your walls, you need to go around and trim your walls to make a good smooth plane for the plaster to adhere to. Also, it’s not actually necessary to plaster. You can attach wood siding, roofing metal, or whatever you want to your framing members in order to protect your walls. This will decrease the longevity (in my opinion) of your building, but may save you many headaches if you do not enjoy plastering. The problem with siding is that your walls need to breathe in order to stay dry. Another problem is that metal sidewalls could have problems with condensation. Plaster can breathe (slowly), whereas wood and metal can not. If you are going to use siding, it would be good to add at least one coat of plaster to seal and insulate the building more.

A weed whacker is nice for trimming loose straw, especially around windows and doors, and also chopping up straw for plaster. Before plastering the interior, and perhaps the exterior, it is good to go around the building and hang nailers (for future shelves) onto framing members. Usually, 1 inch wide boards are good, but for heavier shelves or tables it might be best to use 2 inch wide boards. If you don’t want shelves hanging from the wall, then skip this step, but if you ever might, this is the best opportunity to hang them. Once the plaster is up it is not impossible, but you will have to remove all the plaster and find the structural members to attach boards to. Also, use screws to make shelves after plastering to avoid cracking the plaster.

I think that The Natural Plaster Book is a good reference, or consult with folks in your area who have plastered in the past. The internet has many plaster recipes as well. With light clay straw, it is only necessary to do two coats of plaster. I suggest three on the outside for areas of slanty rains or if the walls are under 12 inches thick. If the plaster is going to go over posts or timber or studs, you will need to ‘span the gap.’ you can get small nails and do pepper nailing (which is driving nails leaving a half inch of the head out and no more than 3 inches in between each nail in a zig zag pattern). Stapling burlap over the frames can work, too. Lastly, make a wheat paste (a.k.a. adhesion coat) to paint on over the frame. This is done by mixing 1 cup white (not whole wheat) flour and 1 cup water, and heating and stirring until it reaches a boil. If lumpy, screen out the lumps. I suggest using earthen plasters on the inside, and lime plasters on the exterior. If you live in areas with little driving rain, or simply don’t want lime plaster, then you can use earthen plaster. Or you could do lime on both sides if inclined. When you are doing plastering and it is green hard (1-2 weeks), you can whitewash your plaster to further protect it. One to three washes should do it. Also note that plaster needs to be green hard before freezing season.

Random Closing Thoughts

A few random closing thoughts. Build your structure so that it is passive solar. The benefits of orientating your building to the sun are amazing. Light clay straw can also be used to insulate between rafters. The mix should have an inch or two of airspace between the metal (or other roofing material) to avoid condensation. It is not necessary to use as much slip here, but use enough to deter mice from eating your straw. Light clay straw can also be used to insulate your floor between the floor joists, and it can be used in internal walls of a building. Lastly, if you decide to build (exterior) walls that are 8 inches or less, you will need to use a very wet (clay) mixture, which in turn makes the mixture heavier and more likely to sag, so horizontal bracing is more important. This gives the walls more strength (and thermal mass), which is necessary to help with plastering and unintentional wall damage while drying.

This article is just an attempt to get more folks to build with light clay straw. There may be mistakes or differences of opinions, but this is the best knowledge that I can currently offer (without a lot more words and drawings and parentheses). So start small and do experiments on what works for your climate.