14 January 2011

Yes you can! Build with Strawbale in Wet Climates

I often hear people lament: "I would love to build with strawbale, but you can only do that in the Southwest."  If that were true, I wouldn't have a job!!  All new buildings I design here on the East Coast U.S. are strawbale, and usually have multiple natural building features...cob, living roofs, cordwood, natural plasters, etc.  There are specific details that need a shift from the perceived norm of strawbale construction.  But it is definitely possible to build durable strawbale structure in any U.S. climate. 

The bottom line is that IF YOU CAN BUILD WITH WOOD, YOU CAN BUILD WITH STRAW!

This strawbale home is nestled in the snowy woods of New Hampshire.
Water is a problem when it accumulates to higher than 20%.  At this moisture content, two things happen in organic materials.  First, any dormant mold spores are activated to bloom.  Second, the microbes that cause organic materials to biodegrade become active.  This means any wood or straw will begin to rot.  The particular issue with strawbale walls is that the rotting generally begins deep inside the wall, so by the time you know there is a problem (by observation at the surface of the wall), your wall is well on its way to compost.  The key is differentiating between liquid water (rain, water in pipes, etc.) and air-borne vapor (humidity).  You want to keep liquid water completely out of the wall.  However, air-borne vapor is only a problem if it is allowed to condense, and thus become liquid, inside the wall.

I have taken clues from how we build durably with wood to inform how to build with straw in a wet and humid climate.  We learn from long-lasting wood construction that there are a few basic rules to follow...protect the base, provide a good roof, keep liquid water out, but let walls breathe.  The question then is how do we translate that to strawbale construction?  Simply follow these four easy rules to durable strawbale in any climate.


1. PROTECT THE WALL BASE
Strawbale walls should always be lifted up off the ground.  Water generally can enter the base of the wall in two ways: rising moisture from the ground and splashing rain off the roof.  If your climate gets a lot of rain, you want to lift your bales 18" to 24" above the final ground height to prevent splashing rainwater from consistently wetting the same spot on your strawbale wall.  You also want to create a deep roof overhang that extends away from the house, so that any rainwater falls well away from the house.  I typically use a 2-foot eave.  The exception is sometimes on the South side of a building, when a roof eave needs to be smaller to allow for winter solar gain.  Then I adjust the South overhang only to the sun angle.

lift straw off the ground & provide a deep overhangs


2. PROVIDE REDUNDANT MOISTURE PROTECT AT ANY HORIZONTAL STRAWBALE SURFACE
If there is an unfortunate leak at a window or your roof, you want to know you have a problem right away so you can fix it before any damage is done.  This is true for any type of construction!  The difference between straw and wood construction is that strawbales can absorb an enormous amount of water and never give you any sign that there is any problem.  So I recommend installing a waterproofing diversion at the top of any horizontal strawbale surface.  This allows any leak to be diverted to the side of the wall, where it will stain the plaster and indicate to you that there is an issue that needs to be addressed.  I use two layer of roofing felt on top of all window sills and at the tops of all strawbale walls, as shown below.

waterproofing at sills and tops of walls


3. AVOID CONDENSATION POINTS INSIDE THE WALL
Straw is warm to the touch at ambient temperatures.  Air-borne vapor does not condense on a strawbale.  However, a material that is cold to the touch at ambient temperatures, such as metal, can cause humidity to condense on its surface.  Similar to water condensing on the outside of a glass of iced water when the air around is warm and humid.  This same phenomenon will occur inside your wall if you use an ambiently cold material to pin your bales together.  Air-borne vapor will condense on the cold surface, turn to liquid, and collect over time to create a wet spot inside your wall.  So, instead of using metal rebar to pin bales, I recommend bamboo or wood.  Bamboo has the added benefit in our region of being invasive, so people are generally more than happy to have you remove it for free.  Similarly, I avoid the common detail of using pea gravel in the base of a strawbale wall.  The stone is cold, so condensation can occur on their surface, creating a wet spot at the bottom of the bales.  (Plus now you have a spot in your wall with no insulation!)

use bamboo or wood pinning (avoid rebar in the walls)


4. USE BREATHABLE FINISHES
In addition to avoiding condensation points, as described above, you want to create surface finishes that allow any vapor to travel freely through the wall.  If they travel most of the way and then cannot escape through your finish, you could end up with moisture build-up inside your strawbale wall.  This means plasters and paints should have excellent breathability if you are building in a humid climate.  I use wood siding or lime plasters on the outside since they both shed rain water so well.  And I use natural clay plasters, usually dug right from the site, to finish the straw on the inside.  I also avoid cement-based plasters, since they are brittle and not very breathable, so liquid water penetrates any inevitable cracks, but cannot readily get back out (ie, moisture build-up).  Paints and sealers should also be breathable, so I avoid acrylics, in favor of clay, lime, or casein based paints, or simply a burnished or clear linseed oil finish to prevent dusting.

use natural, breathable plasters & finishes

Resources
I think these two books are the best resources to learn about how to build with strawbales in wet climates.  All of the authors live in snowy, wet, humid climates.

         
click the book covers above for more info or to purchase


For more information on this topic, see also my online article: Five Tips for Keeping Strawbale Walls Dry in a Wet Climate.

42 comments:

  1. Strawbale homes can be so beautiful. My husband is going to enjoy reading this blog when he gets some time. Strawbale is one option we've talked about for our next property, there's so many good things about building with it. We have a different climate and are in a different hemisphere!

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  2. If your climate gets a lot of rain, you want to lift your bales 18" to 24" above the final ground height to prevent splashing rainwater from consistently wetting the same spot on your strawbale wall.

    What is the preferred material to lift bales , more concrete ; wood ; something else ?

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    1. I have at least a dozen different details for this condition to get the bales up off the ground. And it totally depends on several factors:
      1) the contours of the building site
      2) the foundation
      3) the materials we have in abundance
      I try to avoid more concrete, since that is a huge greenhouse gas contributor.

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  3. Very interesting Sigi and thank you for sharing. My wife and I are living in Taiwan, with typhoon seasons during the summer: violent winds and heavy rains; and otherwise a high humidity environment. We are still pondering on whether we could build our dream house with such weather. Reading you is one step toward a "yes" answer. One can spot in the countryside some old cob houses that are still standing. But I worry about typhoons and strawbale (it would be rice straw by the way here)!

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    1. Well, I would say build the foundation & the roof however you normally would to protect from typhoons. Maybe extend the roof overhangs an extra bit and lift the bales up a few feet off the ground. And make sure the ground around the building slopes away from the house really well. But I am giving you theoretical advice since I have never designed for a typhoon climate. We do get hurricanes here (occasionally) and I can say that strawbale survives that just fine.

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  4. Sigi, do you have any advice, for avoiding problems, with code enforcement and permitting? I am trying, to gather materials, to build, an earthbag home. And, all the planning department tells me is to follow normal building codes???? Well, normal building codes don't apply, to a structure, which isn't wood/metal. At least, I don't think they do. Any advice you could impart is highly appreciated. Thank You, Susan

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    1. Hi Susan,
      So my first question is where are you building? The biggest issue with any thermal mass building material...cob, earthbag, rammed earth, adobe...is that it will not meet code requirements for insulation if you live anywhere cold. So where are you building?

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  5. I just wondering how about the fireproof ?

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    1. Fire requires both fuel and oxygen in order to burn. A strawbale wall is full of fuel, but there is not enough oxygen for the densely packed straw to burn. In fire tests, strawbale walls outperform conventional wall systems. In a two hour fire test, the wall had a 2,000F furnace on one side, and not only did the straw never flame or fail, you could still touch the outer wall, as the temperature had only gone up 10 degrees.

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  6. Beautiful designs! I learned about strawbale houses when I took a permaculture design course with Sowing Solutions at Sirius EcoVillage in Western MA, and I would LOVE to build one someday!

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  7. You got a really useful blog I have been here reading for about an hour. I am a newbie and your success is very much an inspiration for me.

    Cheap Shoreline Restoration in FL & Rip Rap Installation in FL

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  8. I am planning to move to Costa Rica sometime this year. I have started to research all types of homes including cob, earth ship, straw bale and shipping containers. In your opinion can all of these be successfully built in that climate? The do have quite a rainy season and it a bit humid.

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    1. You are basically a humid climate, but reasonably consistent in terms of temperature. If that is the case, and you are not needing to augment with heat for comfort during colder seasons, then your best bet would like be a clay structure. That includes adobe brick, rammed earth, or cob (which is adobe but sculpted in place). You for sure would want to lift the walls up off the ground (likely with stone or something similar). And you would want fairly deep roof overhangs to keep rain off the walls during the rainy season, and sun off the walls when it is hot. The clay walls will then moderate temperature and humidity over time. Does that make sense and is that helpful?

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  9. Thanks, that does help a lot. I do plan on having a rain catchment system, but one last question. Why do you need to shade the walls? Is it to avoid the radiant heat or does it actually damage the structure somehow?

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    1. If the mass material is in the sun, then it will heat up and not act as passive cooling. Sun will not hurt the material, you just will have more of a challenge controlling the inside temperature if you are in a hot climate. 100% shade means the only energy input is from the air temperature (not additionally from direct sun's heat). Hope that helps.

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  11. Hi Sigi, thanks for all your valuable information. I am building a cob home in southeast AZ. The exterior walls have had the 3rd coat of lime plaster, and now getting ready for the lime wash and paint. I have a recipe that includes boiled linseed oil in the lime paint mixture. I've read that linseed oil prevents breathability of the cob. I also read that some people use it in their final coating. Can you please clear this up? Thank you

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    1. Linseed oil is a breathable finish. It can reduce the total breathability of cob if you put many many (we're talking 5 or more) coats of it on. You are just adding a little bit (probably a 1/4 per gallon or something), so this has no impact on breathability. My only recommendation would be to do some test areas with the recipe and process for whatever limewash or natural paint you use before you commit to the whole house.

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  12. Hi Sigi,
    We are about to build a SB home in tropical north Queensland Australia where it is hot and we receive a LOT of rain. My wife was concerned about the humidity so thank you for your article. You mentioned that you clad on the outside? This is what we want to do as well. Q. Do you still render underneath before cladding?
    Thanks,
    Ryan and Amy

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    1. Hi, and how fun! Congrats on your SB home. For cladding, first I "sew" vertical supports to the bales (sewn with baling twine). Then I plaster between the verticals, so that all the straw is coated. Then you can attach your cladding to the verticals. Hope that helps. And best of luck with your home build!!

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  13. Based on your answer to the person wishing to build in Costa Rica I'm wondering if Cob might not be a better choice for building in Hawaii than SB? The climate does change seasonally *somewhat* but it is more a matter of needing Cooling rather than heat. If one were to build with a full surrounding porch / veranda (deep roof overhangs so that the walls are in total perpetual shade) it sounds as if Cob would provide the better cooling option, since it could then provide some degree of cooling without any energy input whatsoever?

    How does an exterior Cob wall hold up to blowing RAIN?

    Given that Cob is also organic material, would the same concerns apply, in terms of avoiding concrete based stucco, in order to avoid decomposition, in a very humid / wet climate?
    Thanks in advance for your answer & also, thanks for this lovely & very Inspiring blog! :)
    ~Alyce J

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    1. If you will use energy to change the inside temperature...for heating or air conditioning...then you want insulating walls. That would be strawbale. If you are primarily trying to keep cool, then thick thermal mass walls, shaded from direct sunshine, are the ideal. In tropical climates, ventilation is also key to staying comfortable. Walls and roofs were often built as a double system...one roof, then vented air space, then another roof to shade the under roof. Same for walls. This may also be an appropriate approach in your climate.

      Also note, cob is not organic and does not decompose. Cob is made with clay, so it is mineral based (inorganic)...like stone.

      Hope this helps you.
      ps. it may help to find a local architect or builder that specializes in natural or indigenous structures to help you think through what is most appropriate for where you are.

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  14. Hello - I have read rodents can be a problem. How do you plan for them?

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    1. I'm not sure where you heard that, but it's not the case. Rodents can be a problem in a barn stacked with straw, because there is food (grain seeds) and warmth (straw) and...most importantly to your question...there are gaps between the bales for rodents to run around freely. A strawbale wall is built solid and tight, with no gaps. Plus there is over an inch thick of plaster on either side of the bales, completely encasing the straw. Any rodent would literally need to dig through the hard plaster, and then dig a channel into the solid straw. This is unlikely, and would leave behind a huge pile of debris (so you'd know exactly where they were). My experience in 20 years is zero rodents in plastered strawbale walls.

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  15. Hi Sigi, we have the house shell built with cladding on outer walls, then construction ply on the inner side ,then an air gap between the studs, then the strawbales, that we are building. The strawbales rest against the studs but not in between them. We have been advised to use a clay slip on the side between the straw and cladding, but was wondering if this is absolutely necessary or can we just use clay plaster on the other internal side.The shell has been up for a few years now and is always totally dry on the inside. Is the reason for clay slip on the cladding side, for fire or rodent protection? thanks in advance Mhorag

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    1. I would strongly recommend at least a slip coat of plaster on the backside of the strawbales. It won't really provide rodent protection...they will happily live in the stud cavity and eat any remaining seeds in your bales, whether you have a little bit of plaster or not. It does improve fire resistance in the wall cavity, though, since the clay won't surface-burn, but the straw will if exposed to the oxygen in the stud cavity. But also, if the strawbales are only plastered on one side, and that plaster is heavy (which thick clay plaster is), then your wall will really want to tip toward the inside. You can also pin the wall from above (go through a plank or brace of some kind and then through the top couple bales).

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  16. So is it cool to build on a raised concrete base? or will the temperature difference eventually lead to moisture seeping in?

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    1. Also if you live in a wet climate can you not just wrap the bales in a water repelling plastic or material before covering?

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    2. you can have a concrete foundation system, but if you live somewhere that you need insulation, then you need to insulate the concrete appropriately. Otherwise you have a cold thermal bridge in your wall system. For your second question, no, you cannot wrap organic materials in a vapor barrier...that guarantees rotting. True for wood as well. Buildings need to breathe.

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    3. Damn damn, I've got my heart set on polished concrete floors but the east coast of Australia spans the full spectrum of cold to warm and dry to humid. Are there other natural alternatives that hold up to hay similarly regarding practicality and aesthetic? Really appreciate your insight

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    4. I'm not understanding your question...there isn't a relationship between the floor you choose and the walls you choose...

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  17. Hey sigi, i have a plant to build my own home from straw, but we just have straw from the rice field, is it possible to use this kind of straw and the place where i want to build is place with 95 humidity and is it possible to build this kind of building there.can you give me some advice, by the way i live in indonesia. Thanks

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  18. Hey sigi, i have a plant to build my own home from straw, but we just have straw from the rice field, is it possible to use this kind of straw and the place where i want to build is place with 95 humidity and is it possible to build this kind of building there.can you give me some advice, by the way i live in indonesia. Thanks

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    1. yes, absolutely, you can use rice straw and you can also build with straw in a humid climate. Here's an article with tips: http://www.buildnaturally.com/EDucate/Articles/DryStrawbale.htm

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  19. Hey Sigi:

    I would love to hear your thoughts on passive cooling and ventilation in a humid climate. I'm further south than you, and we have outdoor humidity readings over 80% (!) for about 3/4 of the year. Opening windows at night lets in that moist air, and during the summer it's too hot to keep the windows open during the day.... which puts us in a bit of a pickle for trying to keep houses cool.

    - ziggy

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  20. Hey sigi, thanks for the good info, I'm considering building with straw bales is western Washington state USA. We get about 56 in(a bit over 1.3 meters) of rain each year, mostly in winter. It doesn't snow much, the average winter low is a little bit above freezing, so cold and wet. Would this be a suitable climate? I talked to a local building department official and he said that he approved 2 straw bale houses, both of which rotted. Perhaps the builders used cold materials like metal or stone? Also I am still unclear about how to do the foundation. One source recomends a gravel trench covered in earth bags to prevent condensation. I was hoping to avoid using plastic if at all possible, could you list some alternative methods? Thanks!

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    1. Any biodegradable material will last if it stays dry. This is true for straw, just as it is for wood. So the bottom line is that there is a way to build durably with straw anywhere that it is also feasible to build durably with wood.

      Most people can visualize the wet they can see...rain, melting snow, leaky pipes etc. What often gets forgotten is condensation. That means, humidity in the air will condense into liquid water when it comes into contact with something cold. Think glass of ice water on a humid summer day with water on the outside of the glass. When this occurs inside a wall, you end up with a build-up of moisture over time, and eventually its enough to start mold growth and decomposition.

      So the simple solution is to avoid any cold condensation points inside your strawbale walls. That means no metal rebar to pin, no gravel base at the bottom (which, by the way, also gives you a huge thermal bridge with no insulation), etc. Earthbags will not prevent condensation and they will not give you any insulation...so that to me is a tricky solution to get right and make sure that your building envelope is completely insulated. You also want breathable finishes on both sides...so no concrete stucco.

      If it would help for me to talk directly with your building official, I would be happy to do so. That way they can have a clear understanding of what to approve and what will fail.

      As for what type of foundation...you could just do something pretty straightforward. A rubble trench footer into the ground to frost depth, with a stone knee wall or concrete grade beam on top, and then an insulated framed floor at a height above grade that makes sense for your site and your snowfall? In this scenario, you can simply add blocking between your floor framing under the inside face of the straw, and start your strawbale walls on the framed floor.

      Hope that helps.

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  21. Hi Sigi. I know this is an old post, but if you're still reading the comments... You say you avoid a gravel base at the bottom of the wall. What do you use instead? Or do you just build the straw right onto the floor? Thanks.

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    1. I insulate the base.
      This article may help: http://buildnaturally.blogspot.com/2013/12/three-strawbale-no-nos.html

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