Okay, so here is a possibly very dumb question, but it's eating me (and I can get straw bales cheap and put them in place myself, whereas I need to rent a guy with a big machine for the earthworks), so if anyone can help clarify this, I would be grateful...
Since the R value of straw bales is at least three times the R value of dry earth, if I understood Paul's wofati article right, could I replace the dry earth between the two layers of poly with 1/3 that thickness of straw bales?
Would the 8 inches of (wet) earth on top of the second poly be heavy enough to keep the whole thing down?
It's not all about r-value. Thermal mass is also very important and in an earth-integrated design like a wofati, it is KEY. Dale is right, the straw would rot. As it does so it would shrink and collapse. It would also feel less solid over time as you walk on it. And it would only insulate the indoor climate of the house. The thermal mass ENSURES temperature consistency by acting as a thermal battery. The thermal mass itself moderates the temperature by either radiating or absorbing heat depending on the time of the year. Straw bales don't do that, they only insulate.
posted 4 years ago
Okay, i understand the argument of the thermal mass: it will lose the stored heat a lot quicker than earth, so you won't get thermal inertia to carry you through the winter. That in itself is enough reason to abandon the straw.
But aside from that the straw shouldn't rot if it is between two layers of poly and is supposed to be dry from the start, no? if it doesn't get wet how can it rot? or is everyone counting on the poly being pierced?
Location: Provo, UT
posted 4 years ago
You're right, it'll only rot if there's water. But I'm also worried about the degradation over time by people standing on the surface above the house. With each step there would be flex and that would wear the straw down, too. But rot is still a concern because you can't always control the factors, especially if there's capillary action from below. When earth get's wet it isn't AS big a deal so it's more forgiving.
Straw handles water and vapor very well in all three modes, as well as wood or even better. When sandwiched as core with COB skins it performs even better. It is more than thermal mass people do not understand. It is Enthapy and the first law of thermal dynamics that creates what I refer to as " Hygroscopic thermal mass" that is more than a "thermal battery" that gets better in colder or hotter climates or really wet ones due to latent(hidden) heat of evaporation and phase changes that produce energy. The system has to be understood and designed right. I have threads named "Breathable Walls" and "2016 Healthy Buildings" that explains in depth and provides supporting data proven for centuries since the late 19 century in the USA. No need to re-post it here. Poly will prevent this system from performing correctly and cause the straw to rot.
I'd need a sketch of the design and the climate zone to give any more advice.