Dear Margot,
Yes, idea bouncing is one of my favorite sports.
Well done straw bale walls hold up to damp air (but not liquid water) fairly well in all but the most torrid of hot humid climates (possible in a greenhouse, even in Alberta) They " breathe ", allowing humidity to pass through them, thus gradually equalizing the greenhouse and home humidity. Putting concrete plaster coverings (non breathing) over them is generally a recipe for disaster, but natural earthen/lime type plasters are hydroscopic - allowing water vapour to pass through and greatly reducing condensation problems.
While many straw bale houses have never had any problems at all - I have seen a few with issues. The problem I have seen most often in some of my friend's buildings relates to the fact that the plaster covering over the straw bales must be perfectly sealed. A small thin/missed area of plaster, or more commonly, a slight settling of the foundation or the wall getting bumped by a car and UGH! you get a microscopic crack that allows liquid water (as opposed to water vapour in the air) in. Wet straw composts, leaving a wall made up of 2 plaster sheets, a big air space and some lovely black compost at the bottom of the wall. Sometimes builders will dip the straw bales in a tank of clay slurry before stacking them on the wall - to make sure as many cracks and crevices as possible are coated with clay. Of course, a good hat ( roof) and good boots (foundation) are vital.
Another version of building with clay and straw is the technique often called light clay straw. This process thoroughly coats each blade of straw with clay. Well coated straw essentially doesn't rot, mold or burn well - and is helped in its insulation value via the air pocket inside the centre of each stem of straw.
The bales of straw are opened up and loosely piled on a tarp. Then a fairly thin clay slurry is poured over the straw and mixed with pitchforks - like tossing a salad with salad dressing. The coated straw is then packed into 2 parallel wooden forms, usually about 2 feet high and 1 foot apart. This is sort of like poured adobe or poured concrete walls. The coated straw can be tramped down by walking on it or by hand, using firming tools to get into the corners. After that layer of wall is dry, the forms are moved up and another layer of wall is created over the first. If the straw is well tamped down, the wall surface is fairly smooth and attractive. Varying the proportions of straw to clay produces different effects. It can be covered with a lime plaster, etc if desired.
Straw bale, light clay straw and adobe are examples of a continuum of wall building techniques that use clay and straw. Straw bale has the most straw (insulation) and the least clay (thermal mass) - adobe the most clay and the least straw. Light clay straw is more or less in the middle. It still has pretty good insulation properties - approx 30 R value for a foot thick wall, but of course strict R values are hard to apply/compare when using natural building materials. Effective R value is maybe a more useful term , but a certain amount of estimation has to be used, as natural materials and exactly how they are applied vary so much. I feel there is a point of diminishing returns anyway when it comes to insulation - somewhere in the range of 30 for the walls is ok for me, esp when the walls are monolithic in size. The thermal mass of the wall ( and surrounding floors,etc) plays a big part in this equation. Another reason I have chosen light clay straw walls is the high amount of thermal mass they possess.
The greenhouse I am designing will allow the winter sun to shine in and onto the wall (made out light clay straw 1 foot thick) separating the greenhouse form the house, allowing direct thermal heating of the wall. There will also be a row of 55 gallon black water drums in front of this wall because nothing beats good old water for storing heat. There will also be 2 foot high raised growing beds filled with dirt, a concrete patio stone walk way and, up in the rafters, more water tanks for pre-heating domestic hot water - all will help store heat.
I plan on covering the surface of the northern wall of the greenhouse with a layer of lime plaster (likely mixed with a little ground charcoal to darken it for heat absorption) to increase its moisture resistance. I am comfortable that the light clay straw wall and lime plaster combo will not mold and cause air quality problems, even in a greenhouse. In fact, the clay in the wall acts as an air-purifier. I have read of this type of wall holding up well even in saunas.
You might suggest a rammed earth, light clay straw, poured adobe or adobe brick wall covered with lime plaster to your clients for that northern greenhouse wall and use straw bale for the other walls of the dwelling area. I find that the general public now `gets`straw bale construction, but it takes a bit of effort to explain these other, less familiar options to them. They seem less romantic somehow. To fire the imagination, think about using other natural materials instead of straw for the mix. I have seen video of dried cowpatties being used in Africa and have heard of chopped cat tail stems or fluff,dried willow tops,shredded paper,saw dust,etc being tried.
I am tentatively planing to use durisol (recycled wood and concrete mix) blocks for the below grade construction of the house and greenhouse, including the cold cellar. These are insulated forms that are put in place and secured with rebar, then regular concrete is poured inside them. This still uses some concrete, which I would prefer to avoid, but it does minimize it. They are made in Canada - check out their website and let me know what you think. Another possible options are using earth bags or rubble trench foundations.
http://www.durisolbuild.com/default.html For insulating the floor of the greenhouse, I am leaning toward having a dirt floor covered in the pathways with simple concrete patio stones to keep things cleaner. The growing beds on either side of the central path will have a dirt floor going down the natural earth. The insulation is in the foundation and knee wall AND (here`s the trick) extending out under ground at a slant from the foundation like a skirt. The idea is to pump heat into the dirt under the greenhouse and trap it there with the foundation and skirt insulation. A very large fly-wheel effect that should provide residual heat from the summer warmed earth through the winter. Pumping the heat into the underlying dirt can be passive or active, with fans and airtubes in the ground. The permaculture consultant that I hired, Gregoire Lamoureux suggested this system. Have you heard of itÉ I still need to do more research on it.
Anyway, enough for now. I will check out the Solviva info - thanks for the lead. It helps me to write some of this stuff down - I have to semi-organize my thoughts. The idea ball is in your court. Good luck and hope to hear from you soon - although in the summer I only go on the computer on rainy days like today.
Mary
