If you don’t want to use straw, you could use sod:
https://nebraskastudies.org/en/1850-1874/the-challenges-of-the-plains/living-in-a-sod-house/
Let me give another example. Would dense Styrofoam work better or worse than less dense. Would there be an optimum and how could I mathematically arrive at that.
How contained would a moist straw wall have to be to exhibit spontaneous combustion? (presuming it’s between something like two brick walls tat prevent evaporation or significant loss of heat)
I’ve never heard of it happening.
Just compare R-values/inch thickness.
A quick Google search shows it’s pretty rare for wet straw to spontaneously combust (unlike hay).
Thank goodness for modern insulation. 
I would think that methane would have to be present for that to happen. The bacteria that causes the heat starts to die off below 160 degrees. I know it has happened, and I have always wondered about the exact circumstances that allowed it to happen.
I don’t think regular ol’ Styrofoam (expanded polystyrene or EPS) is used much in construction nowadays. Extruded polystyrene (XPS) has a number of advantages over it.
I am not doing anything with insulation an hour from now I won’t even be thinking about it. I am just curious about the science behind it.
Google thermal conductivity.
Stooks. Stooks of hay or straw.
As seen on the Saskatchewan Coat of Arms:
You have to lower the pressure a lot before it matters. At 1/1000 of an atmosphere, the mean free path of nitrogen at 22 C is only 0.07 mm–still way too small to make a difference in most circumstances. The thermal conductivity of air is thus almost constant until you get to very low pressures, like a millionth of an atmosphere.
This depends on the size of the voids. If you’re making a thermos type bottle, you need quite a good vacuum. But if you have some kind of powder insulation with really small voids, atmospheric pressure is good enough. For any insulation there’s a level of vacuum that enters this Knudsen regime, and plenty of insulations are practical to do this with.
There’s a kind of prefabbed insulation that comes in panels with aluminized plastic sheet covering, filled with fine insulation and evacuated low enough to take advantage of this effect. The stuff has an uncannily low thermal effusivity – hold it in your hand, and it feels warm almost instantly.
Anorher form of construction from olden times is rammed earth. Great for thermal mass, not as good for insulation.
Rammed earth is an ancient construction technique that’s been revived in recent years for its sustainability and aesthetics. It’s a load-bearing material that can be used to build walls for houses, museums, and cemeteries. Rammed earth provides:
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Thermal mass: Rammed earth provides excellent thermal mass.
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Durability: Rammed earth is very strong in compression and can be used for multistory loadbearing construction.
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Low maintenance: Rammed earth is low maintenance.
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Fire proofing: Rammed earth is fire proof.
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Pest deterrence: Rammed earth deters pests.
Rammed earth works best in humid environments with moderate temperatures. In cold climates, rammed earth walls require additional insulation, and in areas with high rainfall, the walls need extra protection from the rain.
I have always been fascinated with ram earth construction but never could figure out how they kept it from absorbing water or being washed away gradually. Does the compacting mitigate a lot of the moisture problems? Does it require clay soil or other specific types of soil?
There’s usually a roof.
Another ancient construction related to insulation. Ice storage in the midleast.
A Yakhchal, a pyramidal structure.
Impressive