A straw bale wall sits on a stem wall at least 30 cm above finished grade, built on a drained rubble trench foundation, with a moisture barrier between the masonry and the first course of bales. Bales must never touch the ground, because ground moisture wicking into them causes rot within weeks.
Part of Straw Bale Construction
The foundation system of a straw bale building lifts the bales out of splash and rising damp and spreads the wall’s weight into the ground.
The Foundation Is The Boots In “Good Boots And A Good Hat”
Straw rots when it stays wet, and bales sitting on or near the ground stay wet. The foundation therefore does two jobs: it carries the wall, and it lifts the bales above splash height and cuts off rising moisture. The parent article names the design rule “good boots and a good hat”: an elevated stem wall, moisture barrier and drainage away from the building, plus a generous roof overhang.
Three parts make up the system:
- The trench and its drain: Rubble Trench Foundations.
- The stem wall that raises the bales: Stem Wall Height and Materials.
- The barrier on top of the stem wall: salvaged plastic sheeting, tar paper or even birch bark, placed before the first bales, since otherwise moisture wicks from the masonry into the bales.
For other foundation types see Foundation Types and Stone Foundation.
What You Need
- A trench below frost depth, drained to daylight if possible.
- Stem wall material: dry-stacked stone with earthen mortar, urbanite, adobe bricks or gravel bags.
- A moisture barrier for the top of the stem wall.
- Ground shaped to slope away from the building.
Method: Trench, Stem, Barrier
Step 1: Dig the trench below frost depth; the parent article gives a range of 30-150 cm by region.
Step 2: Fill with compacted gravel or rubble and lay the drain.
Step 3: Build the stem wall to at least 30 cm above finished grade (45-60 cm in wet climates or areas with heavy snow).
Step 4: Lay the moisture barrier across the full top of the stem wall.
Step 5: Check the top is level before the first bales go on.
Common Mistakes
- A stem wall too low. Bales touching ground rot within one season.
- No barrier. Moisture wicks up out of the masonry.
- Foundation with no outlet for the drain.
Safety
Trench collapse
Frost depth in cold regions can put the trench bottom deeper than about 1.2 m. Slope the sides or shore them, keep spoil back from the edge, and never dig alone. Full precautions are in Rubble Trench Foundations.
What’s Next
- Learn next: Wall Systems — the walls on top
- Combine with: Roof Overhang and Rain Protection — the hat