A primer on Stuccoing Straw Bale Walls
by Herbert Nordmeyer
ASTM standards and building codes are enacted for good reasons. Know and understand them before varying from them. If you don't have the time to study the appropriate documents and understand them, hire an architect or an engineer to do your design work or at least to review it. Remember, know the consequences before you make decisions rather than finding out after it's too late. Be different, but build safe housing.
Always think about resale value. Your house will be put on the market either by you or your heirs. While rounded and bumpy walls (beer-belly walls) may be intriguing and adequate to you, there are people who would consider such a wall unacceptable. Sales price is determined in part by the number of people who would be willing to purchase the house. Be different, but build salable housing.
Why stucco a straw house? We can spend hours going over various reasons, but the final decision is usually based on two factors:
Straw, wood, metal, concrete, and other building materials move. They react (move) differently to changing temperature and changing moisture conditions. If the house is not built to take these movements into account, cracks will occur. Often the stucco skin cracks.
Build your straw walls so they are straight and flat. If they were not, do the best you can to correct the problem. A chain saw is good for rough trimming and may be used to trim down a two inch high spot on a wall. Use a string line to judge straightness. Stuff loose straw into holes. Fill depressions with a mixture of loose straw and stucco. Then use a weed eater to remove stems and tufts of straw.
Install window and door frames if they are not already installed.
Corners, door frames, and window frames can be laid up square, or rounded. Decide before the first bale is laid what you want. That decision will impact how you build your window and door frames. If corners are to be rounded, they should be rounded as the bales are laid up. Measure roundness with a template, not by eye.
Stucco will not bond to wood or metal on a permanent basis and can transfer moisture and hydrated lime to wood, thus causing it to rot. Face wood and metal with felt paper and staple it into place. For wood, use a staple gun. For metal use "Robert" pins.
Stucco and plaster are weak in tension and flexure. Metal lath is added to increase the tensile and flexural strength. People build without using metal lathe, but consider the impact on the resale value before you make that decision. Reinforce around each door and window with 2.5 pounds per square cubic yard galvanized expanded metal lathe. The sheets are 24 inches wide. Either use them at that width. or trim them to 12 inches wide. Staple the lath to wood members every 6 inches.
Add the chicken wire. "Chicken wire" comes in many forms. An 18 gauge, 1.5" mesh wire stucco lath is available. This wire is self-furred so it will stand out from plywood or gyp sheathing. That will not help on a straw wall. But if you have a portion of the house that needs sheathing, it is worth considering. Regular 1" chicken wire is commonly used. Try to find and use 18 gauge rather than 22 gauge. If you use 22 gauge, be sure to spray the stucco into the straw so the straw will be carrying some of the weight of the stucco.
A wall can be chicken wired by either wrapping the wire around the wall (a 2 or 3 person job) or by stapling the wire to the top of the wall and stepping on the wire to tighten it and stapling the bottom (1 person job). Staple to all window and door frames. If the wire sags after the wire is stapled, use "Robert" pins, or tie through the bales to insure that the chicken wire is against the bales. Small sags can be removed by grabbing the wire with a pair of pliers and twisting.
To prevent structural cracking, install expansion joints every 20 feet or less. In most cases you will satisfy the 20 foot requirement by installing expansion joints above doors, and above and below windows. Anchor the lath under the expansion joint to the wall and cut it before installing the expansion joints. If the lath is continuous under the expansion joint, the expansion joint will serve no useful purpose.
Casing bead should be applied at the top and bottom of each wall, around widows and door frames. Anchor it firmly. A layer of stucco eight feet high, one inch thick, and one foot wide will weigh over 100 pounds. How many 3/8 inch staples are needed to hold this load? Corner beading can be applied to give crisp corners. The inside gets the same treatment.
For exterior stucco start with a mix of:
Vary the sand and water content to get a mix that will stick to the trowel, come off the trowel as needed, and stick to the wall
Every Portland cement, every hydrated lime, and every sand behaves differently. You will have to adjust the formula. "Mortar fats" are designed to replace lime in mixes. They work, but until you are experienced with stucco, do not use them. Slight miscalculations in measuring them can result in major problems. Adding liquid detergent to a stucco makes it light, fluffy, and easy to apply. It also slows the set time and substantially reduces the ultimate strength. After it is dry, it will powder if you rub your hand across it.
Use a hydrated lime plaster or gypsum plaster for interior use. This will give you a softer, more acoustically absorbent finish while stucco is hard and causes echoes. Gypsum plasters can be purchased pre-mixed and will set in less than an hour. For a hydrated lime plaster start with a mix of:
Without Portland, the stucco will stay workable in a humid house for a day or two. With half a bag of Portland, it will set in about two hours. A high lime plaster is a wonderful mix to learn with. You can work it until you get tired of working it. Leaves, rocks, and other things can be embedded in lime plasters. Lime plasters can be applied over a stucco scratch coat. Gypsum plaster cannot.
Fiberglass fibers protect against plastic shrinkage cracks for the first 24 hours. After that, the high alkalinity in many Portlands will cause the fibers to start breaking down. Polypropylene fibers protect against plastic shrinkage cracks and reinforce the stucco for the life of the structure. Chopped horse hair is traditional and works well if you can find a horse. I have never tested human hair or dog hair as a reinforcement.
The first coat is called the scratch coat. It is used to tie the substrate (straw) to the rest of the stucco. If troweled on, it may not bond to the straw. After 15 minutes or so, a small amount of moisture is absorbed by the straw and the surface of the straw gets slick, that is, the mechanical bond is reduced. The stucco may break loose and fall.
Spraying with a gunite rig may result in a buildup of .75" to 2" on the first coat. Bonding may be poor due to the stiffness of the mix. Spraying with a drywall rig gives more control. Stucco needs to be thinner than when it is troweled on. Thinning stucco will reduce the strength of the stucco if it is not formulated specifically for thinning but it allows greater penetration into the bales and thus a better bond. A modified hopper gun will spray stucco, but it needs to be filled every 60 second. A word of caution about spraying: A local contractor purchased a drywall rig so he could stucco one whole house per day. He accomplished his goal on the first house, but spent the next seven days cleaning up the mess. Spread plastic or newspaper over windows and doors. Protect everything you don't wish to have stuccoed. After the spraying is complete, remove the masking tape as soon as possible so it won't harden and cause additional problems. After the scratch coat is applied you may think that someone forgot to flatten the wall. It will look uneven. The brown coat, or second coat, is used to level the wall. I prefer to apply this coat by hand since a heavy coat is needed in many areas to level out the wall. A gunite rig with an excellent operator also works out well.
The third coat is the finish coat. Stucco can be applied smooth, with minor texture, with strong texture, or with various designs. It can be applied to look like brick or like stone. It can be tinted or applied gray or white. It can be painted or it can be left natural, Stay away from the "adobe" look that is formed by lightly coating the wall so the shape of the bales can be seen. This may create conduits to allow water to flow into your bales.
Ferrous sulfate and ferric nitrate will stain stucco towards reddish or golden-brown. The color will vary with the absorption of the wall, leaving a blotchy, or mottled look. Do not expect this type of staining to look like a paint job.
Stucco will transmit water. Codes require a water-resistant membrane behind the stucco. You cannot do this with straw. Design and flash to keep water off the walls. Insure that the stucco is at least 0.75" thick and is put on in several coats. Do not leave any places in the wall or around windows that will catch water.
Stucco shrinks when it freezes. Moisture expands when it freezes. This is a case of an immovable object (stucco) meeting an irresistible force (ice). Keep water out of stucco. Stucco cannot correct foundation problems. Stucco cannot support a roof. Do not use it for that purpose on a Nebraska-style wall.
Herb is a chemist involved in the development of stuccoes, mortars, and wall systems. He describes his work as being on the cutting edge of stone age technology. When not in his lab, he is out spraying scratch coat stucco on straw bale walls or helping write ASTM standards. At the September meeting Herb gave a presentation/demonstration on making a flagstone -like finish with stucco. He is expected to publish a book on straw bale construction in 1998.