Straw Bale Construction and Water

The greatest enemy of straw bale construction is water. People often question whether moisture is a problem and seem to equate moisture with water. There is a difference. In the right climate, moisture can enter a wall and exit a wall in a short enough period of time so not to cause damage. For example, it is typical for a home to drive moisture through the walls to the exterior during the cold winter months when interior heating both creates and moves moisture towards the cold exterior. As long as this moisture is allowed to escape to the outside, there will not be a problem. If, by the use of housewraps, the moisture is held in the walls, the damage can be extensive.

The idea of eliminating housewraps, like Tyvek® and Typar®, may seem strange to builders coming from the conventional building world. For years, we have been told to wrap the house up tight and eliminate the movement of moisture through the walls. Housewraps have become increasingly “intelligent” and can even adjust their transparence rates relative to the levels of humidity. That said, they are not used in straw bale construction other than to protect the bottom courses from rain splash. Other than this one use, they are eliminated entirely from the building process as they have been shown to cause more damage than they prevent.

Many builders have worked hard to master the art of flashing openings and will be set back by the loss of housewraps. The vapor barrier is an essential part of flashing openings in conventional construction as the head and legs of the flashings are tucked under the barrier to create a path for water to move away from the opening. In straw bale construction, openings present the biggest risk to water infiltration. Proper flashing is essential. More than ever, the builder has to think like the enemy: think like water. How does water flow down a surface? How does water wick into dry materials. Where would you go if you were water? The answer to these questions has helped design proper flashings around openings and has highlighted the need for protection both above and below the bales. If a roof leaks, where is the water going to go? Straight down into the house? Maybe, but it is more likely to sneak down the rafters to the plate and then drip down into the wall cavity. Protecting the bales from this type of slow leak is very important. Proper protection along the box beam will minimize the risks and impacts of water leaks by directing moisture towards the plaster where it will show up as wet marks long before damage to the bales has occurred.

Many builders can tell you horror stories about broken pipes and blown washing machine hoses. The damage created by these incidents can be huge, especially if the owners are out of town when they happen. Fixing rotten drywall and swollen sub floors is a pain, but it is not catastrophic. Imagine the repairs if the leak flooded into a wall made of straw bales. The bales could wick all of the water right off the floor and up into the walls. It is possible that the walls would not be able to sufficiently dry out and would therefore be ruined and need replacing. That is a catastrophic repair and it is exactly why we do not place water pipes in the bale walls and why the bales never sit directly on the ground. Plumbing is either relegated to interior walls or is protected by continuous sleeves or faux wall construction. For additional protection from water damage, we provide a, minimum, 3″ “toe-up for the bales to sit on. The toe-up provides a break in the sub straight so that no water can possibly wick into the bales. Therefore, the walls stay dry and the damage is only reflected in the flooring. Considering that most bale homes have integral color, polished concrete slab floors, the damage created with a major leak or broken hose is often insignificant.

If you think like water and plan for the worst, your bale home will last forever. It is well worth the added insurance to over flash an opening or over protect the top and bottom of bale walls. Keep in mind that moisture, and small amounts of water, will find its way into the wall no matter what type of construction you use. If you believe this is not true, you are fooling yourself. Over time, Mother Nature is always stronger than what we build. What makes a well built house last forever is its ability to shed the water and moisture that sneaks in. If you think like water and build with the intention of keeping water out of the home, you will be successful.

ABOUT THE AUTHOR

Andrew Morrison has a passion for straw bale construction that is matched only by his desire to teach his knowledge to others. He has a wealth of experience in designing and building both conventional and straw bale homes. After years of building, he has moved his practice entirely to consulting and teaching. He shares his knowledge with thousands of people via his DVD series and this website and teaches roughly six-eight hands on workshops each year. For more on his workshops, please visit www.strawbale.com/store/category/workshops.  Andrew received a BA degree from Hampshire College in 1995 for Glacial Geology. He also has a degree in construction technology.

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