Straw and earth walls can last for centuries. Some straw bale homes in Nebraska have stood for about a hundred years, and earthen buildings around the world many times older. What they all share is protection from one thing: water.
Natural builders sum it up in a phrase: every natural building needs good boots and a good hat. The boots are a foundation that lifts the walls above wet ground and splashing rain. The hat is a roof that keeps rain off their face. Get those two right, add walls that can breathe, and the rest is forgiving.
- 1Name the four ways water gets into a wall, and how to stop each one.
- 2Compare foundation types for natural buildings.
- 3Explain the code's key moisture rules for straw bale walls.
- 4Design a roof and site that keep walls dry.
- 5Keep a natural building dry for the long run.
Four ways water gets in
Straw can stay sound for generations as long as it can dry out. Trouble starts only when it stays wet for a long time. That happens in four ways, and each has a simple answer.
Rain
Falls on the top of the wall and runs down its face. A wide roof overhang keeps most of it off.
Splash and wind-driven rain
Bounces up from the ground and blows sideways in storms. A raised foundation and tough plaster on the weather side handle it.
Wicking
Water climbs through concrete, soil and straw like a paper towel in a puddle. A capillary break stops it.
Vapor
Warm, moist indoor air can condense inside a cold wall. Breathable plasters let it dry out instead of getting trapped.
Good boots: the foundation
A foundation does three jobs: it carries the heavy walls, it reaches below the frost line so freezing ground can't heave it, and it lifts the walls up out of the wet. Natural walls are heavy and thick, so the foundation must be as wide as the wall.
Concrete stem wall
A concrete footing and short wall, the most familiar option for building departments and engineers. Uses more concrete, but approvals are simplest.
Rubble trench
A trench below frost depth filled with drained gravel, topped by a stone or concrete stem wall. Drains water away and uses far less concrete. Often needs an engineer's sign-off.
Stone stem wall
Dry-laid or mortared stone from the land, on a footing or rubble trench. The most lasting "boots" of all, and the most work.
Earthbag or gravel bag
Bags filled with gravel or soil, stacked and tamped. Cheap and owner-buildable, but usually needs special engineering approval for a home.
The rules for keeping bales dry at the base
The straw bale appendix in the building code sets clear minimums. They're good practice for any natural wall:
- Bales at least 8 inches above the ground. Measured from the outside grade to the bottom of the bales.
- Plaster stops short of the ground. Exterior plaster must end at least 6 inches above soil, or 3 inches above paving.
- A capillary break below. A moisture barrier goes on top of the concrete, and the bales sit at least ¾ inch above it on a "toe-up" of wood sill plates with gravel or insulation between.
- Rot-proof sills. The sill plates under the bales must be naturally durable or preservative-treated wood.
Rubble trench and stem wall
Trench measured along the outside walls. Stem wall height includes about 4 inches below grade. An 80 lb bag of concrete mix makes about 0.6 cubic feet. Planning numbers only; your engineer sets the real sizes.
A good hat: the roof
The roof is your walls' first defense. The code doesn't set a minimum overhang for bale walls, but experienced natural builders recommend generous ones, commonly 24 inches or more, and deeper on the stormy side or on tall walls.
- Wide overhangs on every side, deepest toward storms
- Gutters and a drip edge so water leaves cleanly
- Downspouts that carry water well away, or into a rain tank
- A simple roof shape, with few valleys and corners to leak
- A porch on the weather side doubles as wall protection
The overhang that keeps rain off also shades the south windows in summer. Size it with the overhang calculator in Lesson 2, then check that it's at least as deep as your builder recommends for rain. If the two disagree, a porch or a separate awning over the windows can satisfy both.
Deep eaves, a sheltering porch roof and thick plastered walls are what make natural homes look so settled and kind. The "good hat" is one of the most beautiful parts of the house.
Walls that breathe
Straw and earth stay healthy because moisture can move through them and dry out. Seal them in plastic and that moisture gets trapped. That's why the code is so specific about finishes.
| Do | Don't |
|---|---|
| Plaster directly onto the bales with lime or clay plaster | Put plastic sheeting, foil or vinyl between the bales and plaster |
| Use breathable finishes: lime wash, clay paint, silicate paints | Coat the walls with ordinary latex or oil paint, which can trap moisture |
| Check bales with a moisture meter before plastering: under 20% | Plaster over bales that got rained on and haven't dried |
| Flash windows carefully, with sloped sills that throw water off | Leave flat sills where water can sit against the straw |
| Protect shower walls with a proper waterproof backing | Put a shower directly against a bale wall without protection |
The code forbids Class I and II vapor retarders (like plastic sheeting) on straw bale walls and requires finishes of at least 3 perms. In cold climates it asks for a slightly more vapor-resistant finish on the inside. Lesson 5 covers plasters in depth.
The site, and the years ahead
- Ground slopes away: a 6 in drop in the first 10 ft is the standard rule
- A gravel strip at the base instead of soil or mulch
- No sprinklers or thirsty plantings against the walls
- Downspouts emptying at least several feet away
- Snow cleared away from the base of the walls
- Walk around after a big storm and look for wet spots
- Patch any plaster cracks before winter
- Refresh lime wash on the weather side every few years
- Clean gutters and check flashing at windows and roof
- Fix plumbing leaks immediately
Build a model foundation and roof
Run three simple experiments that show why natural walls need boots, a hat and room to breathe. Then walk your own house in the rain.
You'll need
- Paper towels and a shallow tray of water
- A handful of gravel and a small piece of plastic
- Two of your test blocks from Lesson 1 (or sugar cubes)
- Small stones and a piece of cardboard for a "roof"
- A spray bottle or watering can
- A string, a line level and a tape measure
Do it together
- Wicking test. Stand a folded paper towel in the water tray and watch the water climb. Then stand another on a pile of gravel in the water. How high does each one get wet?
- Boots and hat test. Set one block on the bare tray and one on a little stone "foundation" with a cardboard roof that overhangs it. Spray both like a storm for one minute.
- Compare. Which block got wetter, and where? Let them dry and look for damage.
- Breathing test. Wrap a damp paper towel in plastic and leave another open to the air. Which one dries by tomorrow?
- Walk your house in the rain. Where does water drip, splash and pool? Mark it on a sketch.
- Measure the grade. Stretch a level string 10 ft out from a wall and measure how far the ground drops. Is it at least 6 inches?
Little builders
- Be the storm with the spray bottle
- Watch and describe the water climbing the paper towel
- Find the puddles around the house in the rain
Older builders
- Measure how high the water wicked in each test
- Run the foundation calculator for your Lesson 2 plan
- Calculate the grade slope as a percent
Add a foundation and roof to the plan and model from Lesson 2, with overhangs and clearances marked.
Built to last
A natural building with good boots and a good hat can outlive everyone who built it. That's a remarkable thing to give your family: a home that, cared for simply, could still be standing for your great-grandchildren.
Next, we raise the walls themselves: straw bale and light straw-clay, step by step.
Your Lesson 3 worksheet
Work through these together. Your answers save on this device as you type, and print cleanly for your records.
Progress record
What this lesson practices:
- ScienceWeathering and erosion by water; testing materials and capillary action (NGSS 4-ESS2-1, 2-PS1-2).
- EngineeringComparing solutions and testing which best meets the criteria (NGSS 3-5-ETS1-3, MS-ETS1-3).
- MathVolume, measurement and slope as a percent (Common Core 5.MD.C.5, 7.RP.A.3).
- Life skillsHome maintenance and protecting a building over time.
Standards mapping is a draft. If you use charter or ESA funds, confirm it with your education specialist.