What is the difference between cob and straw bale building?
We assume you want a comparison of building method, insulation, and function/wear. There are several aspects that need to be addressed to answer this question. Basically, a home can be built with either cob or straw bales, or both. Their function is different, though. Straw bales are insulators, holding in the temperature and keeping the interior of your home warm or cool, depending on time of year. Cob functions differently, not as an insulator but as thermal mass, in that it absorbs any radiant heat or cooling and then slowly releases it as it cools off.
When you are building with straw bales, you are essentially using the bales as your framework, especially when building a load-bearing, Nebraska-style structure. Straw bale building is done with straw bales (not hay bales), which then need to be covered – usually with earth plaster, lime plaster or cement stucco**. A covering is just that – a cover material (generally, layers totaling about 1″ or so thick) that protects the bales.
Cob is a lot like earth plaster, although it is much more specific. You want the right amount of clay-soil (binder), sand (aggregate), water, and straw or horse manure (tensile strength). Cow manure also makes a nice additive binder to clay-soil, creating a nice texture that functions differently from horse manure, more as a binder but not a strengthener. You want your mix to hold together, for the individual clay platelets to cover each of the tiny bits of sand. Clay binds it, sand (or other aggregate) keeps it from cracking, and water makes it all possible. Straw/manure/fiber makes it stronger. You want to get your cob mix right so that it doesn’t crack too much when it dries out. If you rely on water to bind your materials without getting the mix right, you might be disappointed with the results. Also, clay and water is really pleasant to work with, but too much clay and not enough aggregate will definitely crack too much and fall apart. You have to have a lot of aggregate – a lot more than clay, generally 2 or 3 parts aggregate to 1 part clay-soil. All of this specificity is important when building a cob oven or something structural.
When building with cob (or cobb, as some refer to it), you are making the balls or loaves into the actual building material, essentially sculpting your house with no framework; the cob is the framework. Cob can also be used within framework, such as within stud walls. One application might be to use cob in a south-facing wall for heat absorption when the sun is lower in the autumn, winter and spring, allowing for slow release of heat on cool nights. Also, cob can capture the cool of a summer night and slowly release that cool into your home during the day. With these bitter cold Missouri nights, a really good application is to use cob on the interior of your straw bale house – in the form of a bench or trombe wall – capturing and absorbing the sunlight pouring in through your south-facing windows. An earthen floor will also capture solar gain. Everything works best if your foundation is well insulated, as well as good ceiling insulation and weatherization techniques.
Straw bale building really performs optimally. We personally love the marriage of straw bale and cob architecture. You can have the insulation you need for colder winters by building with straw bales, and cob can be used on your home’s interior for absorption/thermal mass, to round out straw bale walls and/or to add sculptures and depth. How you use these materials just depends on the climate where you live and the dreams you want to manifest. Missouri is the perfect place for a marriage made in heaven, a love between two “naturals” – clay and straw. 🙂
**About Cement Stucco: We aren’t really recommending cement stucco for covering bales anymore, now that we have more experience and knowledge about the function of earth plaster vs. cement stucco. Here in moisture-rich Missouri USA, large roof overhangs or a covered wrap-around porch adequately protect straw bale walls from water. Further, if earthen walls become moistened/wet, the hygroscopic properties of the clay/earth kicks in. Clay gets wet and tiny clay platelets expand; as clay dries out, platelets shrink. If your bale wall takes on some moisture (using precautions mentioned), the clay will literally suck/absorb any moisture back out of the bales. This is the “breathing” process often referred to – straw bale walls need to be able to “breathe”so that moisture doesn’t get trapped in the straw, causing rot. This is why you do not paint with synthetics on your interior walls (see Natural Paints and Finishes).This is also why we would not now recommend using cement stucco as a cover material for straw bales (or cob or anything natural, for that matter). Cement will absorb water, but it does not release water very quickly or in the same way that earth plaster does. We have cement stucco covering our house – so far, so good, but we live in a protected woodland, and our house has large overhangs; due to Missouri’s moisture, we would recommend having an earthen exterior. Even though clay/earth (not topsoil) is well suited to absorbing and releasing water, you still need to have good roof overhangs (approximately 2 feet). As a final cover, lime plaster (with or without colorant) will give a lot of added protection to your walls.