Hello. I am new to the site, so my apologies if this question has been answered in another thread, but I have been planning to construct a small, hut-like earthship. And I was wondering if the benefit of having a traditional Earthsip roof--cement and rebarb, etc.--is really all that significant. I understand that cement is naturally high in thermal mass, but is it possible to achieve a similar result with a wood frame and some dirt or other material for the roof?
A major feature of concrete roof construction is that it is rotproof, so any wood used in a structural roof would have to be protected from moisture better than concrete. As long as you have a good waterproof membrane on top, and keep the wood exposed to the interior air circulation so it can't build up moisture, and you size the timbers appropriately, it should work fine.
Concrete has significantly more thermal mass than wood, but it also conducts heat significantly faster, and as a roof would need major insulation to be viable. What is your climate? Hot, cold, dry, wet, and what size and speed of temperature changes do you get?
See the threads on wofati construction here for detailed information about structural wood roofs underground.
Thanks for the information! The Wofati houses are an interesting alternative. However, do you know how feasible it would be to construct one in very rough, rocky terrain--like the deserts of Colorado? Also, you're saying if I do choose to use wood for the roofing material, ensure it shares the same air as the inside of the house? And this will prevent molding and rotting of the wood? And one further question: for passive solar homes, insulation should be placed only (including around windows and door frames) on the outside of whatever material is high in thermal mass, correct? So, for instance, if the walls of the structure are earth-rammed tries, I should apply only a water membrane and plaster coating to the inside, but wrap the outside with insulation. Or does this depend entirely on my climate conditions? If it is a hotter climate, will I need less insulation? Now I am confusing myself. Please, clarify.
The local climate absolutely influences the optimum selection of materials and methods. The main development of earthships appears to me to be in dry temperate areas, where moderating temperature swings is the most important factor, followed by insulation to keep heat in. In northern areas which are just plain cold for months on end, moderating from cold to colder is fairly useless, and you need serious insulation to impede constant heat loss. In Florida, insulation can be fairly irrelevant if you have sufficient thermal mass coupled to the interior and feeding from underground.
To determine if a wofati will work well in a given climate, I would first ask what the average ground temperature is 5' or so below the surface. If it is cold, you would need to insulate the wofati's earth mass from below as well as from above, or it will never reach a comfortable equilibrium. Similarly, if the climate is frequently damp, it may not be practical to get an envelope of dry soil which would have any useful insulation value.
The wood, unless naturally rot-resistant, will need to be located so that it can lose water to be not too humid, and not touching cold damp surfaces which would concentrate moisture. In some climates this would be easy, in others it would be extremely difficult.
Passive solar in any climate would have what insulation there is on the outside of the thermal mass so it can moderate the interior temperature.
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