Why Building Paper Matters in Straw Bale Construction and Natural Plaster
You have probably seen building paper stapled to wood before a plaster crew gets to work. It is the black material that covers wood posts and beams and often outlines the edges of bale walls. Many people install it without knowing why they use it or how much it helps. A single layer can make a significant difference in the durability of natural plaster and the long-term health of a wall. I learned this years ago when I repaired a section of plaster that had bonded tightly to a wood post. The repair was simple, but the lesson was clear. If you give plaster a way to move, you avoid problems later.
Why You Use Building Paper
You use building paper for three reasons. Each one supports the goals of straw bale construction and natural plaster.
Moisture protection
Plaster and stucco hold moisture during application and after rain. Wood swells when it absorbs water. Building paper protects the wood from that moisture and acts as a simple drainage plane. It lets water that reaches the surface move out instead of getting trapped. This matters for wood posts, beams, and any non-straw-framed walls that receive plaster.
Bond separation
Wood expands and contracts through the seasons. Plaster does not move the same way. If plaster bonds tightly to wood, the movement shows up as cracks. The paper creates a slip layer. The plaster remains intact while the wood behind it moves. This separation is small but important.
Durability of the finish
Wood draws moisture out of plaster quickly. If plaster loses water too fast, the base coat weakens and cracks. Building paper slows that exchange, allowing the plaster to cure at a steady rate. You end up with a stronger and more even finish.
Where You Apply Building Paper
Use building paper when plaster comes into contact with wood or any surface that is not straw and porous.
Common locations include:
Window and door bucks
Posts, beams, and plates
Larsen trusses or nailers
Plywood or OSB trim strips
Any transition between materials that move at different rates
On a full bale wall, you plaster directly on the straw. The paper is only for the solid transitions where plaster needs support to move independently from the structure.
How to Install Building Paper
Good installation supports good plaster. Follow these basics.
Use Grade D or similar building paper
Keep the surface smooth and fasten the paper tightly (a hammer tacker or staple gun works great)
Start low, overlap upward, and wrap corners
Keep overlaps small and consistent
Trim the edges so the scratch coat has a clean surface to meet
One layer works well in most straw bale applications. In conventional stucco work, you often see two layers. The outer layer separates from the inner layer, which creates a small drainage space. This helps when you plaster over a framed wall instead of a bale wall.
Liquid Applied Membranes
People often ask about liquid membranes. They work for weather protection, but they bond tightly to the plaster. You lose the slip layer, and cracks may appear at the wood interface. Some builders use small patches of liquid membrane (as do I), but building paper remains the safer choice for most transitions.
If you want hands-on experience applying plaster and prepping walls with confidence, join us at a workshop. You will learn these details in real conditions and bring the skills home to your own project. Please let me know if you'd like help selecting a workshop that suits your plans.
