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 SURFACEIf 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 WALLStraw 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 FINISHESIn 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|
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.