There is a lot of talk about how all of these styles of home have a thermal mass that will keep a man warm in the winter and not waste heat, but which of these building styles is best for someone who lives in a very hot climate and wants to keep their home nice and cool?
I see no one's jumped in on your thread so allow me!
I hate to be the 'it depends' guy, but... =)
Any of those systems provide enough thermal mass to effectively cool a house in summer and keep it warm in winter. Integrating that wall style with passive solar design, appropriate tech heating/cooling systems and other elements of house design can create a home that is comfortable year round.
Which is most appropriate for your specific area depends on some other factors. How humid is the region? How wet are the soils? How much rainfall? Water plays a big part in the long term health and efficiency of walls. Also what's the elevation? How much wind reaches the site? Is it from a consistent direction, or is it more seasonal?
Rammed earth is traditionally seen in more arid circumstances, though there are notable exceptions to that rule. They're great for absorbing the heat throughout the day, and releasing it throughout cold desert nights.
Cob and strawbale are more universal in their use, though it's my understanding that straw bale is a little more common in temperate situations (where straw is more abundant/readily available).
There are a number of other strategies that are very helpful to keeping a home cool in the hotter parts of the world:
- Move heat sources outside. An outdoor/detached kitchen, shower, and laundry facilities help to prevent you from heating up the house you're trying to cool.
- Build to where the wind blows. Catching a cool breeze can make a huge difference in a home's comfort, and building with the direction of the prevailing wind in mind can mean a one-time action with benefits for years.
- Get low. This one is very dependent on your soils, but as we see in Paul's WOFATI building style, sinking a home a few feet into the earth can greatly contribute to the coolness and temperature regulation of a structure.
- Grow your house. Integrating plant systems, especially when paired with your ventilation system, can cool a structure significantly, while providing clean refreshing air at the same time.
- Cool in, hot out. Speaking of ventilation, being intentional about how air moves through the structure can help keep temps low. Venting hot air from the top of a structure, and using that movement to pull in cool air from near or even under the ground (using the Bernoulli effect) can continually cool your house, using the energy of the heat that you're expelling.
- Shade. Shading your roof and walls, either with plants or nonliving shade materials, can reduce the amount of heat energy entering a structure.
I know this didn't directly answer your question, but I hope it was helpful!
"The highest function of ecology is the understanding of consequences."
Also, the different wall systems can be used in combination inside the same structure. Straw bale walls have more insulation value than cob walls. Cob walls have more thermal mass than straw bale walls. It can be an effective strategy to use the cob walls where one wants to gain solar heat (or sink heat in the shade) and use straw bale walls where one wants to keep a room well insulated from outdoor temperatures.
For the region in which I live that means an efficient structure would have cob walls on the south (and perhaps part of the east and west walls) and straw bale on the north, east and west walls. Of course, it goes without saying that roof overhang and glazing location/amount should be designed to make the most from the angles of the sun throughout the seasons. And the flooring system is also critical.
For hot regions it is also a nice touch to make ceiling height as tall as is practical. This will allow for the room temperature to stratify.
In hot climates most would say that full earthen construction is the best for maintaining a cool inside environment--I would side with what Justin said and use both of best worlds and create a well insulated house with lots of interior mass to help regulate the temperature. Cob Cottage Company in Oregon has started demonstrating how cob and strawbale function wonderfully when hand in hand. Currently I am building a balecob house-- 14" of strawbale with 8-10" of cob on the interior, my blog is in my signature if you are interested in photos.