Plaster earthbag walls with a three-coat earth plaster inside and a lime or cement-lime plaster outside, applied directly to the bags with no mesh in most cases. In regions with little rain, an earth plaster with a high proportion of cement or lime stabiliser can work outside.
Part of Earthbag Building
The plaster system is the combination of coats and binders that protects the bags from sunlight and rain and binds them into one wall.
The Bag Surface Does Half The Work Of Keying The Plaster
Earthbag walls are rougher than straw bales. The hollows between courses act as keys, so plaster grips without mesh. The decision is therefore about binder and weather, not attachment.
| Face | Plaster | Why |
|---|---|---|
| Interior | Earth plaster, three coats | Same mix as straw bale interior plaster |
| Exterior, wet climate | Lime or cement-lime | Sheds rain, lets the wall dry |
| Exterior, little rain | Earth with a high proportion of cement or lime stabiliser | Can work where rain is minimal |
For the interior earth mix, the parent article points to the straw bale interior plaster: about 1 part clay-rich subsoil to 2-3 parts sharp sand, plus chopped straw for reinforcement; see Earth Plaster (Interior). For lime, see Lime Putty and Plaster and Rendering. For stabilisers see Stabilizers.
What You Need
- Clay-rich subsoil, sharp sand and chopped straw for earth plaster.
- Lime (matured putty) and sharp sand for exterior plaster.
- Trowels, a hawk, water and a sprayer.
- A finished wall that has settled and dried.
Method: Coat By Coat
Step 1: Test the mix on a sample section before committing.
Step 2: Dampen the wall so it does not suck water from the plaster too fast.
Step 3: Apply the first coat pressed firmly into the hollows between courses and scored for the next coat.
Step 4: Let each coat cure before the next. Keep lime coats damp, since lime hardens by absorbing carbon dioxide from the air rather than by drying.
Step 5: Apply two more coats to reach the finish, each slightly finer than the last.
Common Mistakes
- Cement plaster on an earth-filled wall in a wet climate. It can trap moisture in the earth. The parent article specifies lime or cement-lime outside.
- Fast-drying plaster in sun and wind. It cracks and does not cure.
- Plastering a damp wall. See Moisture Management.
Safety
Lime and cement burn
Both are alkaline. Quicklime slaking is exothermic enough to boil the water in the pit and spatter, so slake with goggles, gloves, boots and full cover, from upwind. Mature putty and cement mixes still burn skin and eyes. If either gets in an eye, hold the lids open and flush with clean water for at least 30 minutes and seek care. Do not neutralise with vinegar: heat from neutralisation adds injury, as Chemical Burns explains. Wear a dust mask when handling dry lime or cement.
What’s Next
- Learn next: Waterproofing Methods — keeping the plaster dry underneath
- Combine with: Lime Plaster (Exterior) — lime in more depth