The New Strawbale Home: Good Shoes, Good Hat

Good Shoes

Long-lasting foundations must be strong enough to support the weight of the building, make it impervious to moisture, and typically extend as deep into the ground as the winter frost depth. If your building site is on a slope, care must be taken to insure that rain and ground moisture have a convenient drainage path around the house. Raising straw bales 6 to 10 inches above grade and installing a moisture barrier (or “damp-proof course”) between the stem wall and first course of bales is common practice to prevent moisture wicking up from the ground into the straw.

A “toe-up” is commonly two parallel, rot-resistant 2x4s bolted to the slab, with the space between them filled with pea gravel or pumice and sprinkled with boric acid and diatomaceous earth to discourage pest entry.

Besides plain old concrete, effective foundations can be created with mortared stone, a grade beam over a rubble trench, pumicecrete, concrete piers with wooden beams and insulated concrete forms (ICFs). It's best to build the foundation wide enough to support the 1 to 2 inches of heavy plaster on both sides of the bale wall. It's also wise to support straw bale walls a couple of inches above the final floor level, with a “toe up” to prevent an indoor plumbing mishap from soaking the bottom course of bales. In most climates it's cost effective to insulate the foundation and slab from the ground around it—in fact, it's often required by energy codes. While some builders in cold climates have experimented with using bales to insulate below a monolithic slab, this is not recommended. Monitoring has revealed that bales in contact with the earth will, over time, absorb moisture and eventually decompose.

A Good Hat

A roof design that incorporates wide eaves (two to three feet, if possible) is also highly recommended. Not only will it shed rain and snow far from bale walls, but it will also protect earthen plasters from erosion and cement stucco from becoming water saturated. Additionally, wide overhangs, portals and porches offer the cheapest living/storage space possible and are useful in any climate. Flat roofs and parapet walls, common in the Southwest, are not recommended. Unless their detailing and maintenance is impeccable, they will eventually leak, and cause problems no matter what your wall system.

There is a big difference between vertical and horizontal moisture intrusion. Even a driving rain hitting the sides of an unplastered straw bale wall will not penetrate very far, and the wall will quickly dry out. But water seeping into bales from above will soak down into the middle of the bales, where wall thickness makes it more difficult for moisture to escape.

Proper design and detailing at doors and windows is also critical, as water will find its way into cracks between dissimilar materials. Commonly, windows are set close to the outside of an opening, leaving a wide shelf or window seat on the inside, and the minimum surface needing protection from the weather outside. Where windowsills and ledges are exposed, protect them with a moisture barrier such as tar paper. (This is one of the few places appropriate for sheet-type moisture barriers.) Install drip edges at the top and bottom of openings to shed rain and snow away from bale walls.

Gutters and wide overhangs shed rain and snow away from bale walls.