Exterior plaster on a straw bale wall is lime, mixed as 1 part slaked lime putty to 2.5-3 parts sharp sand, because it sheds rain yet lets moisture in the wall escape, whereas cement traps moisture and causes hidden rot. Lime cures by absorbing carbon dioxide from the air, not by drying, so keep each coat damp for several days, and treat wet lime as a corrosive that can burn skin and permanently damage eyes.
Part of Straw Bale Construction
Lime plaster is a slaked-lime and sand render that protects the outside of a bale wall while letting it breathe.
Lime Sheds Rain And Lets The Wall Dry
The exterior plaster has two competing jobs: to keep rain out and to let moisture in the wall out. Lime does both. It is weather-resistant yet breathable, and it hardens by carbonation, absorbing carbon dioxide from the air. Cement plaster is waterproof in the wrong way: it traps moisture inside the wall. The parent article says not to use cement plaster on straw bales.
Lime putty improves with age. Historical builders aged it for months or years; see Lime Putty for preparation and Plaster and Rendering for use. If you must make lime from limestone, see Limestone Burning.
What You Need
- Matured lime putty, mixed 1 part to 2.5-3 parts sharp sand.
- Sharp sand that is clean and angular; see Lime Mortar.
- Goggles or a face shield, gloves, long sleeves, boots, and a source of clean water for flushing, within a few steps of where you work.
- A sprayer to mist the coats while they cure.
- A dry, trimmed wall under a finished roof.
Method: Three Damp Coats
Step 1: Mix 1 part putty to 2.5-3 parts sharp sand.
Step 2: Dampen the wall so it does not drink the water from the plaster.
Step 3: Apply the coats in the same three-coat system as for earth plaster; see Application Technique.
Step 4: Cure each coat slowly. Keep it damp by misting for several days. Do not let direct sun, wind or frost dry or freeze it, because rapid drying stops the carbonation and the coat stays weak.
Step 5: Let the full thickness harden before painting or limewashing; see Plaster and Lime Wash.
Common Mistakes
- Cement-based plaster outside. It causes hidden rot.
- Fresh, unmatured putty. It is gritty and poor-working.
- Fast drying. The coat dusts and cracks.
- Plastering with a leaking roof above.
Safety
Lime burns skin, blinds eyes and boils in the pit
Wet lime plaster and limewash have a pH of about 12-13. That is strongly alkaline: it dissolves skin fats and tissue, an alkali burn can be painless at first while it goes deep, and a splash in the eye can cause permanent damage. Quicklime slaking is violently exothermic: the water can boil, spatter and release steam, so add the lumps gradually to the water, from upwind, in goggles, heavy gloves, boots and full cover. If lime gets on skin: brush off dry lime, remove contaminated clothing, then flush with large amounts of water for at least 30 minutes, the alkali figure used in Chemical Burns. If lime gets in an eye: hold the eyelids open and flush with clean water at once for at least 30 minutes, then get medical care. Do not rub the eye. Do not neutralise with vinegar or any acid, because the heat from neutralisation adds to the injury; plain water is correct. Dry lime dust irritates the nose, throat and lungs: wear a dust mask when handling it, and never plaster overhead without eye protection.
What’s Next
- Learn next: Plaster as Fire Barrier
- Combine with: Earth Plaster (Interior)