02 December 2013

Three Strawbale No-No's

Strawbale construction has been enjoying an international revival over the past few decades.  That revival began in the Southwest U.S., a very dry climate with minimal rainfall and very low humidity.  Why is that relevant?  Because many of the methods of construction developed in the 1980's in that arid climate are still presented as "best practices" details...however, what works in a dry climate does not necessarily work well in a wet climate, where details need to address condensation, rainfall, snow melt, and more.

I would like to reset the standard on how strawbale buildings are constructed appropriately in wet/humid climates.  First, let's dispel the myth that it is not wise to build with straw in a wet climate.  You can read more details HERE, but the bottom line is that anywhere that you can build with wood, you can build durably with straw, because straw and wood are biologically similar.  Second, let's talk specifics about three common details seen in books and on the internet.  These three common strawbale practices may work fine in super arid climates but are NOT appropriate for wet climates.  (However, you can transpose wet/humid climate details to any dry climate.)

#1 Do not pin strawbales with internal rebar

Why?  Rebar is steel reinforcing used to strengthen concrete.  The metal is an ambiently cold material, that is, it remains cold to the touch unless there is a heat source.  When humidity is present, moisture in the air condenses into a liquid along the cold surface of the metal.  The result: a constant drip of water down the metal whenever the air is humid.  That water will eventually build up above 18%, at which point, decomposition of the straw will begin.
Additinally, steel requires a lot of energy to manufacture and is way way way stronger than is needed to hold fuzzy strawbales together.

What to do instead?  Pin your strawbales together with any ambiently warm material, such as bamboo stakes or wood dowels.  Both are strong enough for the job of holding bales together and since both have the same ambient temperature as the straw, you will not create a condensation point inside the wall of straw.

inserting bamboo pins into a strawbale wall


#2 no cement stucco

Why?  Cement is a non-breathable finish material, which means that it blocks air-borne moisture (humidity) from transferring through it.  Additionally, cement is a brittle material, which means that it develops mini cracks with any movement of the building (and all buildings have movement).  What happens is that the cracks allow moisture into the wall and the lack of breathability prevents that moisture from drying back out.  The result is moisture build-up over time.  And again, if/when that moisture gets to 18%, any biodegradable material will begin to decompose.

What to do instead?  Use breathable plasters & finishes, such as clay-based plasters, lime-based plasters, and natural (non-polymer) paints, that allow humidity to transfer freely through the wall.  I use lime plasters outside to save on maintenance and clay plasters inside because they are so beautiful and wonderful to work with.  For paints, I use natural, mostly home-made paints (you can read more on how to make natural paints HERE).

finishing touches on a clay plaster


#3 no gravel base

Why?  Most stone, like metal, is ambiently cold....that is, cold to the touch at room temperature or below.  So, like the example of metal above, the stone creates an artificial condensation point inside your wall.  Consistent condensation means build-up of moisture over time, and...you get the picture by now...eventually you get to that 18%, and decomposition begins.
Plus, you have just built this super-insulated strawbale wall and then eliminated insulation along the entire base...so huge thermal break in your wall system.

What to do instead?  Your base should contain only insulating and ambiently warm materials, like the straw.  There is a stone that will work for this application...pumice...which is a volcanic rock that has many air pockets, making it an insulating material.  But otherwise, you can use any insulation material, such as light-clay-straw, vermiculite or perlite, wood chips, or even reused packing peanuts.

wall base detail under a strawbale wall, with rigid foam insulation


So let's shift these three common details and build durably for wet climates!!

RESOURCES
Here are my 2 favorite books on how to build appropriately with strawbales in wet climates.  All of the authors live & build in snowy, wet, humid climates.

         
click the book covers above for more info or to purchase

20 comments:

  1. Hi
    I like the way you start and then conclude your thoughts. Thanks for this nice information regarding regarding I really appreciate your work, keep it up. Santa Barbara Green building

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    1. thanks for your kind words. I'm glad the information is useful.

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  2. Hi, I live in Taiwan and want to build a straw bale house in the future. Thank you for sharing such important information online. I follow your blog and FB pages. You are an innovator and change the world by spreading your knowledge. Thank you so much !

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    1. so glad you find the information to be useful!

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  3. Thank you for all this good information. Wish we were closer and could attend your workshops. We live in North Idaho in a wet area with good winter snowfall. We have plans to build a straw bale insulated post and beam house. In our plan now, the south facing side is all windows for passive solar, the north and east sides have a 10' porch that will help protect the house from snowfall. The west side has no porch or windows. We were thinking about using corrugated metal flashing on the bottom of the exterior wall to protect from snow. But, will this also cause condensation? Do you see this as a problem?

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    1. Should be ok as long as there is an air gap (sometimes called a rain screen or weep) between the metal and the straw. The idea is that any condensation runs down the metal and out the bottom, instead or going into the straw. Also, be sure to put at least one healthy coat of clay plaster over the straw.

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  4. Would Straw Bale be appropriate for building in Hawaii where there is extremely high rain fall & also very frequent (small) earthquakes? Besides the 3 modifications suggested above, are there any changes you'd suggest for a HOT & Humid climate with lots of rainfall?

    Also, how does Straw Bale perform in a Hurricane? Are you aware of any actual examples of Straw Bale surviving a Hurricane of significant strength (or a tornado, which are even more destructive)?

    Can you suggest any way to combine the benefits & aesthetics of Straw Bale insulation & construction techniques with the proven strength & hurricane resistance of Concrete Block construction, without creating the issue of an internal condensation surface (which would lead eventually to decomposition of your straw bales)?

    If we eliminate the rebar entirely, it seems it would be necessary to use post & beam techniques rather than having a load bearing straw bale wall? (Please correct me if that assumption is in error.)

    Thanks in advance for your time & suggestions!
    Sincerely, Alyce J

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    1. Part of your question I answered on your other comment regarding cob. :)

      For hurricanes and strawbale, there have been independent tests done on strawbale wall systems, including a hurricane test. A plastered strawbale wall passes the hurricane test with minimal deflection. Definitely do not use rebar with a strawbale wall, but you can use bamboo stakes to pin or you can use external cabling to compress walls. As long as the metal is not inside the straw. Post-and-beam is always an option too...

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  5. Hi, I live in Hungary, where strawbale walls are often suggested to be covered with chicken-wire first, and then to be covered with clay plaster. Does it cause condensation? I had a feeling it contradicts with your thoughts on not putting metal into a strawbale wall.
    My family plans to build an earth-sheltered home with brick walls + strawbale insulation and clay plaster on the outside, and I'm trying to gather information for that.

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    1. The "no metal" rule applies to metal inside the wall...specifically, between the strings of the straw. So technically, no, the chicken wire will not cause a condensation problem inside the wall. However, it is also not providing any function and it can get in the way of getting a good coat of plaster worked into the straw. So I would eliminate the chicken wire entirely. (I never ever use it. Ever.) Also, for clay on the exterior, you will either need to protect it with very deep overhangs or you will need to repair it annually. If you do not want to do that, then I would recommend 3 coats of lime plaster instead of clay plaster on the exterior.
      Best of luck to you

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    2. Thank you so much for your answer and advice!

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  8. Hi Sigi! My name is Diego, from Brasil. =)
    I have a question to you, if I do a cob walls with a gravel base i will have the same trouble in the case of strawbale?
    Vey nice to meet u.

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    1. Hi Diego, I'm not sure I understand your question...but the short answer is that cob walls have slightly different rules than strawbale walls but you want to protect the base and the top of the walls from weather (especially rain)...always

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  10. Sorry!I want to know about the moisture in the walls made of cob, if a make a house with a stone base with 1 or 2 feet above the floor and make the cob walls i will have the same trouble in the case of strawbale? The moisture will comes from below and reach the straw in the cob mass or in this case this does`t happen?
    Sorry my bad english. ;)

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    1. I'm still not sure I understand fully. But you don't need to worry about the straw in cob, as long as you build a base that gets the cob off the ground. This ensures that the cob will dry out, and thus protect any straw in the clay mixture.

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  11. Yes, you told what I needed to know, I'm gonna be sure to make a base that gets the cob off the ground. Thanks again!

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