Put a flat moisture barrier of plastic sheeting, tar paper or birch bark between the top of the masonry stem wall and the first course of bales, so water cannot wick up from the masonry into the straw. Use it only as that horizontal break: a straw bale wall has to dry, so a vapour-tight layer wrapped around the wall or placed where it blocks drying traps moisture inside the bale and causes the hidden rot it was meant to prevent.
Part of Straw Bale Construction
A wall moisture barrier is a thin water-resistant layer that interrupts the path of liquid water from masonry into a bale wall.
A Barrier Stops Liquid Water Rising, But Must Not Stop The Wall Drying
Masonry stem walls draw ground water upward by capillary action, the same wicking that moves water through fine soil. Bales laid straight on that masonry soak it up. A horizontal sheet between the two breaks the path: water cannot cross a layer it cannot wet through. That is the correct use.
The second half of the problem is the opposite. Any wall gets some water from driven rain, leaks and vapour from inside the house, and a bale wall survives only because that water can leave again, through the breathable plaster described in Lime Plaster (Exterior) and Earth Plaster (Interior). The same principle is why cement plaster is ruled out on bales. A layer that seals the wall faces, or one that turns up and covers the bale sides, shuts that exit. Water that gets in behind it stays, the straw rises above the 20% limit, and the wall rots from inside while looking sound.
So the rule is by position, not by material: block liquid water from below, keep every other path open for drying. No vapour-permeability ratings are given here, because none can be sourced; judge a material by whether it is a sheet that water cannot wet through, and keep it out of the wall faces.
What You Need
- Barrier material: salvaged plastic sheeting, tar paper, or birch bark, as the parent article lists.
- A level, clean stem-wall top; see Stem Wall Height and Materials.
- Enough width and length to cover the full top of the stem wall without gaps.
Method: Lay It Flat, Overlap It, Stop It At The Edge
Step 1: Clear the stem-wall top of sharp stone points and wet mud that would puncture or float the sheet.
Step 2: Lay the barrier across the full top of the stem wall, in the width of the bale.
Step 3: Overlap joins generously, with the upper sheet lapping over the lower in the direction water would run, so it cannot enter a seam.
Step 4: Do not turn it up the bale faces or wrap the wall. Cut it flush, so plaster can run down onto the stem wall and the straw is free to dry.
Step 5: Lay the first course at once, and cover the barrier edge with plaster as the work reaches the base.
Step 6: Check later. Take bale moisture readings low in the wall after the first wet season, as in Bale Moisture Monitoring; a reading rising toward the 20% limit at the base means water is entering despite the break.
Diagnosing A Barrier That Is Doing Harm
| Sign | Likely cause |
|---|---|
| Damp or dark straw just above the barrier | Water entering from the face, trapped by the sheet |
| Musty smell at the wall base only | Barrier turned up or plaster sealed at the base |
| Moist base only after rain, dry otherwise | Splash or leak; check overhang and drainage |
Common Mistakes
- Plastic sheeting around the whole wall. It traps moisture in the bale.
- No barrier on a porous stem wall. Moisture wicks straight into the first course.
- Barrier edges buried in plaster at a different height. Water collects behind them.
- Skipping the check. The failure is hidden.
Safety
Wet bales rot unseen
Do not cover a suspect wall with plastic to “keep the damp out” or plaster over bales at 20% moisture or more. Opening a mouldy base releases spores: wear a dust mask. Salvaged tar paper and bitumen are flammable; keep flames away.
What’s Next
- Learn next: Roof Overhang and Rain Protection
- Combine with: Foundations and Stem Walls