Green Means

Natural and Stunning Lime Rendered Walls

By Shannon Scott

            Bare naked, lime plastered walls are beautiful.  Properly mixed and applied, lime rendered walls offer striking appeal and structural integrity far superior to ordinary painted and textured wall board. 

            Lime’s greenest property is that it’s carbon neutral.  It gives off CO₂ during manufacturing, but reabsorbs CO₂ after it sets.  Lime is recyclable and biodegradable.  It is resilient, anti-bacterial, and long lived - still in great shape today on buildings plastered hundreds of years ago.

            Lime renders allow straw bale and other natural substrates to breathe, transpiring air and moisture.  Transpiration increases negative ions into indoor atmosphere.  Negative ions make people feel better - as if standing near a waterfall – so an indoor environment feels more soothing.

            There are two common lime renders, plaster and wash.  Lime plaster differs from wash in that it contains sand or other aggregate to create a mortar or concrete-like consistency.  Often natural fibers such as hair or chopped straw are added for additional bond strength.  Plasters also contribute to a wall’s structural integrity as they cure to form a solid sheathing. 

Lime washes are applied as thin slurries. They have no sand or fibers and do not add texture or structural support.  Lime slurries are commonly referred to as white wash, yet they can be pigmented for delightful appeal.

Lime needs three factors to adhere well to a substrate or underlying surface:  mechanical bond or “tooth”; absorbency; and chemical compatibility.

Mechanical bonds help lime to grip solid porous surfaces such as straw bales, bricks, masonry blocks and other substrates.  Smooth surfaces may be lime washed by roughing them up using an abrasive like steel wool or wire brushing, to create tooth, but not plastered.  Smooth surfaces may include glass, dense stones, or wood.

Absorbent substrates allow lime plasters and washes to maximize adherence.  If a few drops of water soak into the substrate the surface is a good candidate for lime.  If a substrate has been treated, perhaps with paint that repels water, it will not support lime.  

Chemical compatibility also contributes to adherence.  Lime wash can be applied to glass, and most all natural course materials, but not plastic, painted surfaces, or treated metals.  Traditional cement stuccos take lime plasters and washes quite well.  However, tilt-up smooth concrete panels often used in commercial buildings do not.

Hydrated type S lime, common in the U.S., is available at hardware and building supply stores.  This lime is “hydrated” because calcium oxide has been hydrated with just enough water to turn into calcium hydroxide, which is the powder you are buying.  Industry reports state that type S lime once it is mixed with water is ready for immediate use, but I recommend aging it for better consistency and workability.

            Making lime slake is easy.  Powdered hydrated type S lime is mixed with water to the desired consistency using a paddle attachment on an electric drill.  Then it slakes or cures in an air tight environment.  Large rubber garbage cans with tight fitting lids work great for the entire mixing and aging process.  The longer lime is slaked the better.  It will develop a creamier texture as particles become smaller, and be easier to work with.  Three months has been a recommended minimum, but we used it after only weeks – not ideal, but necessary when under construction time constraints.

If lime slake is exposed to air it will begin to set up and turn back into limestone.  Keep the lid on tight!

For lime plaster, mix aged lime slake with sand and perhaps fibers.  Lime-sand mortars have some elasticity and better accommodate settling or movement in walls than other heavy wall finishes that may contain cement.

If you want to make ordinary textured sheet rock walls look richer use a clay product such as American Clay, over sanded primer.  It comes in a variety of colors and finishes and offers the same breathability and luster as lime mixes.

Note: Wear gloves and eye protection when mixing or applying lime plaster or lime wash.  The chemical reaction in lime plasters and washes is exothermic, meaning it exudes heat and can impart severe chemical burns on skin or other soft tissue.  I have scars on one wrist from allowing lime plaster to remain there for hours as I worked.  I should have stopped and flushed the lime off with cold water for at least 15 minutes to force granules and chemicals from the surface.  As it turned out I ended up with an infection and 30 minutes of an antibiotic I.V. drip in the M.D.’s office.  Be careful. 

                                                                                              

                           

Making lime slake with a drill paddle attachment

A second coat of lime plaster applied over straw bale walls.

The author applies American Clay Loma over 2 coats of lime plaster.  Though pigmented lime would have been adequate, the clay added thickness and richness.

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