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In conventional building practice, moisture is prevented from migrating into the wall cavity through the use of continuous plastic vapor barriers. This addresses the very real concern of air leakage in stud-framed homes, but it is unnecessary and a structural compromise with bale walls.

Attaching a vapor barrier to a straw wall offers many complications. The barrier can only be attached to the top and bottom of the wall, making it hard to maintain a taut surface unless wooden attachment points are added to the wall. A vapor barrier prevents the plaster coating from attaching itself directly to the straw. This not only makes plastering much more difficult — and will likely require more metal reinforcement for the plaster — but eliminates the substantial structural benefits of bonding the plaster to the straw. Remember, bonded together, the two materials create a stressed skin panel far stronger than the sum of their independent elements. A straw wall without a vapor barrier is less time-consuming to build and eliminates the use of a manufactured product with a high embodied energy. Finally, straw, metal stucco mesh, and plastering tools all increase the risk of introducing punctures to the vapor barrier, rendering it less effective.