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

Comments

  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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  12. Hello Sigi,
    1. Thank you for existing and being a super helpful resource.
    2. My partner and I are planning a rubble trench, earthbag stem wall (with gravel inside), strawbale home with clay plaster. My question about this post is: How do I make a barrier between the earthbags, toe up and strawbales? Should I put in a vapour barrier between the earthbags and the toe up OR do some clay-straw mixture? If clay-straw: what would that look like?

    Thank you!

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    1. Hi there,
      I'm not an earthbag proponent, so I'm reticent to weigh in on this detailing. Sorry...

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  13. Thank you so much for explaining so sensibly about the SB building. I just ordered "Serious Straw Bale Building" you suggested, FYI the other link doesn't seem to work, but will hunt it up when I finish the first one.

    I've been researching different building options even though I get a lot of "you can't do that here {I'm in Southern NH}!" I was excited about the earthship idea because of the rain harvesting and green house filtration and being off the grid, I hope I can do the same with a straw bales because I think I might manage SB over packing tires since I'll have to do most of it on my own and I;m in my 50s. I look forward to researching deeper.

    Thank you again.

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    1. Thanks for the heads up on the broken link. It's possible that book is out of print. I'll have to check on that.

      As for building in NH...I would stick to super insulating walls (like strawbales). I designed a home near Keene and they were super comfy all winter with very little energy use. The thing with earthships is that they were a design solution to a very particular problem, and when you transport that solution to a different place you can get unforseen negative results. There are eartships in cold climates, for example, that are uninhabitable due to mold (from condensation). And definitely definitely please do not bury tires around your home. They leach carcinogens into your soil. Best of luck with your home! :)

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    2. Hi Sigi, I'm in MetroWest Boston, so somewhat in the same area as Keene. Do you have more info on the house in Keene? I'd love to visit, or ask them about how they went about getting your design built...

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    3. I definitely have more info on it. As for a visit, I can ask them if they are open to it...but I try to respect the privacy of my clients so I don't just share their contact info without permission.

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    4. Here's a start: http://www.buildnaturally.com/PRojects/HIJY/PR-HIJY.htm

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  14. I really enjoy your posts I wondered do you have a preferable heating system for SBH; I'm in NH. I've seen mason heaters in large expensive SBH and adobe fireplaces out west in small homes. I haven't seen rocket mass heaters used in SBH. Do you have a preference? I'm thinking 1-2000 square foot simple SBH. I've read Serious Straw Bale as you suggested and definitely want to go natural/recycle for my building.

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    1. I love masonry heaters. My favorite is Temp Cast (made in Canada), because you purchase just the firebox kit and can face it with any mass that makes you happy (like cob or stone). They are more efficient than a rocket mass heater and can heat up to 1750 SF if the home is well insulated (like strawbales are) and if the heater is centrally located. I also like hot water radiant floors, and had one client that used an exterior wood boiler to heat the water for their floor. That works really well.

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  15. Thank you for the suggestion, I'd hadn't heard of them before. Unfortunately $4000-$5000 for the just the kit - quote from Temp Casts website -is far out of my budget, though I would definitely got one if it was. However I may look into getting monitor heater which worked well for a relative . Thank you again.

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