For a non-insulated earthen floor, we could proceed to pour our adobe right on top of the gravel. Keep in mind that the earth below frost level maintains a fairly constant temperature of 52-58° F (11-14° C). This can make for a comfortable floor in the hotter summer months, but in the colder winter months, can result in a floor that is uncomfortably cold. For this reason, it's to our best advantage to insulate against this cold seeping through the floor.
For a low-tech insulated floor, we mix a high ratio of straw to clay adobe. In order to prevent the adobe from penetrating too deeply into our gravel base, we first spread a two-inch (5 cm) layer of loose, clean, dry straw (Fig. 16.4).
In general, a mix that contains 25 to 35 percent clay-rich soil, and 65 to 75 percent sandy/gravelly soil, like reject sand or road base, is combined with as much long straw as the mix will accommodate and still feel fairly stiff, but not dry. This is often an occasion for a mud-mixing party, with much stomping and thrashing about (Fig. 16.5).
This straw-rich base layer goes on about four to six inches (10-15 cm) thick, depending on your stamina.
Screed or trowel this layer as level as possible. One way to help maintain a level surface is to use stringlines or partition the floor into sections with boards that delineate the height of your pour (Fig. 16.6).
Leave the surface textured to provide a keyin for the next layer. Though it seems obvious, it bears reminding to work your way towards a door, rather than trapping yourself up against a wall, with no way out except across the floor you have worked so hard to level.
16.5: Mixing a straw-rich adobe insulative layer can be likened to shampooing a big shaggy dog.
16.6: When one section is poured and leveled, the boards are moved and the adobe poured into the new section. Continue this method until the pour is complete.
16.4: The loose straw soaks up any extra moisture from the adobe and provides that much more insulation.
16.7: Finish floor troweling tip: A padded kneeling board protects both the floor and your knees. Supporting your weight on a wood-float frees your other hand to trowel with just the right amount of pressure.
This high-straw pour will take days to dry in hot, dry weather and longer in a more humid climate. It could take weeks in rainy or cold weather. In this case, to inhibit mold add a cup of Borax to every wheelbarrow load (see more on this natural mold inhibitor in Chapter 15, under "Additives"). We use a cement mixer for all our earthen plasters, cob mixes, and floor pours. Cement mixers are cheaper and easier to come by than mortar mixers. A cement mixer can also handle coarser material, like gravel and long straw.
Once the straw/clay base is dry, it should sound hollow when you knock on it. If it doesn't, it may not be dry all the way through, or the ratio of mud to straw could have been a little off. A hollow sound indicates lots of air spaces, meaning an insulative success. It's imperative that this insulative layer is completely dry before continuing on to a finish. Complete drying allows time for any shrinkage that might occur. Cracks may occur during this drying process, but the next layer will fill those cracks in, and the small cracks provide a key-in for the final coat. If the insulative layer is not dry before you apply the final coat, cracks can occur that travel up into and through the finish coat. So let it dry completely to save you from extra work and unnecessary frustration.
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