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 rebarWhy? 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 stuccoWhy? 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 baseWhy? 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|
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