Which will keep a house the coolest? Straw bale, cob or rammed earth?

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?

Hey there Markus,

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!

I will second everything that Cody wrote!

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.

Having lived in the tropics for decades including offgrid, no electricity as this might be, I would never build with high mass materials. They retain air temp during day then emit all night,  you dont need direct sun for high thermal mass to retain its 33C for many months. Best for living house I ever experienced was old railway line pole, slightly elevated home, no walls bar the bedroom but flyscreen around all sides with covered verandah all round house perimeter. This allows no insects but catching all breezes. Using mist water spray on perimeter and you have cooling of the house. If you have small solar, ceiling fans do all thats reqd.
Have a look at Papua New Guinea native homes, elevated low mass structures and they have huge amounts of clay and mud to use right at their door if desired.
Having lived 50C Deserts and Tropical my half century, those two while appearing same issues work differently in real life living.
Best and easiest for me was just living no electricity or refrigeration under a tin roof no walls at all for years in the hot Tropics, too many insects, snakes etc though if living with loved ones, besides dogs.

I'm about to embark on an adobe brick project in my backyard in the greater Phoenix area and I've been researching this very topic. I've yet to reach any firm conclusions. I can't find much literature on adobe or rammed earth in these extreme highs, so I'm making it up. It's true that here, high mass may only get you a big oven. The HoHokam lived here in mud homes for hundreds of years, but their villages weren't surrounded by 100 square miles of concrete and asphalt, and it cooled off at night. Here, we sometimes hit the 115F mark (41C), and overnight lows in this huge metropolis won't dip below 90F (32C) at that time. Thermal mass will continue to radiate heat all night when the outside median temp is over 100F. My plan is to build a double wall and vent the area between to the outside. My vents will be in the wall (perhaps the roof too) and easily accessible so I can open the vents in summer and close in winter. Proper venting should create a draft between the walls and prevent heat from building up and radiating to the inside. At least it sounds good in theory. Plus, my south and west walls will be very well shaded. I don't want to insulate. I want the raw look of adobe block, but if it proves too hot then I will contemplate rigid polystyrene on the outside,followed by mud plaster. I will also use a swamp cooler --- low in power usage and very effective here because the humidity usually sits around 5 to 10 percent most the year, excepting the monsoon season, and a swamp cooler injects humidity inside, which is good in such a climate as this. If you use adobe, cob or rammed earth, you should be prepared to insulate the exterior should your situation require it. Also, some folks are using rigid insulation inside of cob or rammed earth walls, out of sight. Should you build in such heat, you should document the results and share, because there's not many talking about it. Most mud research and literature comes from people working in the New Mexico highlands and Colorado--- quite different from the extreme heat of the Sonoran Desert, the city, and like environments.

Wilmer Smith wrote:Thermal mass will continue to radiate heat all night when the outside median temp is over 100F. ... Should you build in such heat, you should document the results and share, because there's not many talking about it. Most mud research and literature comes from people working in the New Mexico highlands and Colorado--- quite different from the extreme heat of the Sonoran Desert, the city, and like environments.

Yes, exactly this!!!

I live in the Tucson area and from what I've read it seems strawbale is actually the best for insulation. Rammed earth, etc. creates thermal mass that will just keep your house hot all night long in the summer. I'm not sure non-Arizonans understand what we mean when we say "hot".

Unfortunately I don't have much info to add here, I just wanted to chime in and say I'm very interested in this topic and hope more hot-climate experts can share what they've learned!

Thanks.

Mass walls ( adobe and cob ) have traditionally worked well in the Southwest because the temperatures in desert regions typically swing above and below our comfort zone, so a well designed mass wall will even out the swings with its stored heat and coolth. When the daily temperatures don't average out to one we like, whether too hot or too cold, then a mass wall is no longer your friend.  Since most climates have seasons where the average temperature is above or below what's comfortable, you are better off insulating your walls than depending on the thermal flywheel for comfort.

It sounds like lots of insulation, with mass on the inside of the insulation envelope, is best for the temperature swings of desert areas. Am I reading the situation well?

The constant heat of oversized cities in the hottest zones is another thing.

brett johns wrote:Having lived in the tropics for decades including offgrid, no electricity as this might be, I would never build with high mass materials. They retain air temp during day then emit all night,  you dont need direct sun for high thermal mass to retain its 33C for many months. Best for living house I ever experienced was old railway line pole, slightly elevated home, no walls bar the bedroom but flyscreen around all sides with covered verandah all round house perimeter.

Hi Brett,

Thank you for your insights, here are the weather conditions at my location.  





I have been thinking hard and long on the question of what to build.  These are the ideas I have considered:

• Traditional cement construction (do not like the aesthetics, environmental impact or fact it must have an AC to be comfortable)

• Straw Bale (concerned about mold and/or insect infestation)

• Cobb, Adobe, Rammed Earth and/or Wattle and daub (these seemed to be the best options... until that is I read your post)

I had not considered that Adobe works well in the Southwest primarily because the temperature drops in the evening.  Where I am located the temperature is much less varied.

If I understand you correctly Adobe would not create a comfortable indoor temperature in the tropics, even with 16" walls?

Your advise then would be to build an elevated pad, with very few internal walls screened off from the bugs.  A few ceiling fans to create a constant breeze.

If you happen to have any examples of what you referred to in your post I would love to see them.

Thank you

Your climate makes all the difference, careful about importing "permaculture solutions" designed for a different climate.

You are in the hot, wet/dry tropics, so these solutions developed for desert and temperate regions are not appropriate for you.

Thermal mass is not your friend at all in the tropics. The average yearly temperature is warmer than is comfortable. So thermal mass evening out day/night temperature swings just makes it too hot 24/7. A light, elevated house as Brett suggests above, ready to capture all available breeze, with wide verandas all around and hopefully high shade above like palms is probably your ticket. With kick-ass netting of course. And ceiling fans for when the breeze dies down.

I found an interesting diagram of a typical Malay house, for instance.

An idea I played with since the mid ‘90’s, more for city/suburbia, and was able to test in college, green energies, was “second layering”. A second roof or wall, atop or in front of any thing getting sun. A roof 2-3” over a peak roof, by placing say 2x3s over existing plywood, that has been radiant painted, and adding another layer of plywood or metal roof, vented at the bottom and top, and peek vented. Similar method on sun faced walls. Passive cooling shading, with no radiant pass through. It made a huge difference.
I have seen drawings of this hinted, online, every once in a moon. I lived in north Mojave for a while, where many mobile homes would put a giant carport over a double wide for shading. I was a carpenter on the north shore of Chicago, when thought of, and wondered how much energy this would have saved, cities wide. Most, just shrugged, at the idea, “too expensive, for the second layer.” There are always ways to learn how to take a good idea and make it less expensive. That’s why sites like this exist.
I have been learning since then about more off-grid living(thatch and growing roofs) and all kinds of better ideas across the board. But still think this concept would really help more city/suburbia areas, that don’t do much of anything, but A/C and cutting down their “dirty trees”.

Not the most economic solution for some, but did get me an accommodation and top of class in college, in my 40’s. Hope this may help, inspire a better concept.

Jerry McIntire wrote:It sounds like lots of insulation, with mass on the inside of the insulation envelope, is best for the temperature swings of desert areas. Am I reading the situation well?

The constant heat of oversized cities in the hottest zones is another thing.

Maybe using just thermal mass in the walls would work well in situations with large diurnal temperature swings. I would figure out how wide the wall needs to be for the temperature to take 12 hours to move through and put a large roof overhang to protect from direct summer sun. This way the day heat is radiating into the house at night and cool night temps during the day.

Lived there for 6 months through the furnace season.....or rather the crucible season. I understand.