Lime Plaster; Making My Hempcrete Build Feel Finished

I love talking about hempcrete building.  However, it is a little leap for most, including me, to think of eliminating all the layers in a traditional build.  I get lots of comments like, how will you keep the moisture out without a vapour barrier?  Will the walls look industrial-like when we’re all done?  And my personal favourite, can you paint the exterior and interior walls different colours?

First of all, you want the walls of your hempcrete home to breath.  This is really important for the insulation factor of the building.  This counterintuitive measure is what actually increases the R-factor so you don’t want to seal off the building with a traditional vapour barrier or you actually compromise the efficacy of the hempcrete.

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Also, if you take a look and traditional homes in England, where the climate is very wet, you’ll see post and beam construction with lime finishes everywhere.  Lime is actually a calcium based slake that can be crushed to a powder and applied as the smoothest of finishes with a little water and a fine trowel.  And yes, with some stunning milk paint, you’ve got a gorgeous finish.  Take a look here:

Lime Plaster 101: the basics

by Sigi Koko

Lime is a confusing term, because it can refer to various chemically different (but related) materials.  (Not to mention the citrus fruit!)

For example, cured lime plaster, chemically speaking, iscalcium carbonate…basically limestone.  But theuncured material that goes on the wall, is also called “lime plaster”…but it is calcium hydroxide to a chemist.  Yikes!

So let’s go through some of the basics of lime to give you a great understanding of the ins and outs of how to use it.

What’s the big deal about lime plaster?

Lime has been used for thousands of years as a fabulous binder in mortars, plasters, and paints.  It wasn’t until the post-World War II housing boom that quick-setting cement products eclipsed lime in construction.  Lime cures more slowly than cement, but it holds many advantages because it is a workable, self-healing, breathable, nearly carbon neutral material…making it a great choice for natural building.

Why is lime plaster aligned with natural building?

First, lime-based products have a smaller carbon footprint than their ubiquitous cement counterparts.  Cement production creates 1.25 pounds of CO2 for each pound of cement produced, whereas lime is nearly carbon neutral.

Second, and perhaps most importantly, lime is what’s called “breathable”.  Breathability refers to a material’s ability to allow air-borne vapor, ie humidity, to pass through it.  Think Gortex…water-repellent and vapor permeable.  The breathability ensures that moisture will not build up inside the wall system.  In turn, this ensures that any biodegradable materials, such as wood or straw, are protected from decomposing.

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How is lime made?
(a little chemistry…)

  1. Limestone, shells, or other material that is high in Calcium Carbonate is burned in a kiln.  The heat drives off Carbon Dioxide, leaving Calcium Oxide. This is also called Quicklime.
  2. Quicklime (Calcium Oxide) reacts with water in an extremely heat-producing reaction, a process called “slaking”.  The result is Hydrated Lime, or Calcium Hydroxide (since hydrogen from the water bonds to the Calcium Oxide molecule). This reaction can be quite dangerous, so it is common to purchase Hydrated Lime (Calcium Hydroxide) instead of Quicklime (Calcium Oxide).
  3. Once Calcium Hydroxide is exposed to air (whether it’s in powder or putty form), the lime reacts with Carbon Dioxide in the air and ends up where it started…as Calcium CarbonateSo except for the energy of the kiln, the lime is carbon neutral.

Because lime plasters react with carbon dioxide from the air in order to harden, you can easily keep the calcium hydroxide form of lime in its putty form indefinitely by storing it with an inch or so of water on top of it (or in a completely air-tight container).  This effectively prevents the lime from getting into contact with air and thus prevents curing until you are ready to use it.

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How is plaster made?
(tips for mixing, & applying)

THE PROPORTIONS

For general lime plasters (especially on the exterior), I use 3 parts sand to 1 part lime (calcium hydroxide).  This is a great all-around mix that is sticky enough to work and cure strongly, yet with enough sand to prevent lots of cracking.  If you intend to work your finish to tighten and smooth it out, you can use a more “lime rich” ratio of 1:2 (lime:sand) or even 1:1 for very very finely worked plasters.

THE MIXING

Lime putty increases plasticity and workability the longer it is mixed.  So the longer you mix it, the creamier and easier to spread it gets.  (Magic, right?)  I mix in a mortar mixer (not a cement mixer!) for at least 20-30 minutes.  Only add water (a small amount!) if your mix is extremely thick.  The plaster should be stiff but should spread easily, like cream cheese.  Allowing the mixed lime plaster to sit overnight improves workability, but remember to remix the plaster again before using.

 

THE WALL PREP

To prepare strawbale walls for lime plaster, first shape your walls exactly how you would like them to look once plastered.  It is time-consuming to build up the lime plaster to fill in large voids (since it must be applied in thin coats).  Next, install expanded lath (not chicken wire!!) to cover any slick surfaces, such as wood…anything that is too smooth for plaster to hold onto.  Make sure your lath bridges across the wood and at least 6″ into the straw so you don’t get a crack right where the lath ends.  I do NOT recommend using lath over all of the strawbale, unless you live in a seismic region and your code requires this.

 

Be sure to dampen your walls down well before applying each coat of lime plaster.  For the first coat, this means soaking the strawbales until they are damp and the straw is pliable.  For each subsequent coat, soak the wall down the day before you will plaster, again the morning of plastering, and throughout the day keep the wall damp as you work.  Otherwise the wall steals moisture out of your plaster quickly, and can pop the bond that holds your plaster on the wall.

 

APPLYING THE PLASTER

I generally use 3 coats of lime plaster for exterior walls or showers.  You can use 1 or 2 coats for decorative interior finishes.  The first coat can be up to 5/8″ thick if it is applied to strawbale, otherwise each coat should be a maximum of 3/8″ thick.  Any thicker and the lime cannot absorb carbon dioxide adequately for curing to fully take place.

 

I apply the plaster with a wooden float to create a well-shaped wall that has decent texture.  For the finish coat, I smooth the final surface using a flexible pool float.  You can continue to buff or polish the lime as it is curing for a very smooth sheen.  There are many highly refined finishes that can be achieved with simple lime plaster.

 

Score the surface of each coat (except the finish plaster) to create lots of surface area for the next coat of plaster to key into.  And allow at least 7 to 10 days between coats to give each ample time to cure.  (Also see the next section for curing tips.)

 

NOTE: I do NOT recommend lime plaster over clay plasters for exteriors in wet climates.  The clay substrate shrinks and swells depending on moisture content.  The cured lime cannot shrink and swell with the clay and so it will be more susceptible to cracking when used over clay plaster in a wet climate.  Lime can be used over solid clay walls, such as cob & adobe, because there is so much more clay present to absorb ambient air moisture without measurable swelling.

 

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THE CURING PROCESS

You want the lime to cure…NOT dry out.  That means it needs to react with carbon dioxide from the air before all of the moisture evaporates.  If it dries out before it has cured (and converted into calcium carbonate), the resulting plaster will be weak and possibly crumbly.  So protect the plaster from wind and sun until it has cured, and it helps to dampen the wall daily as it is curing.

 

Do not apply exterior lime stucco if there is any risk of freezing, otherwise moisture in the plaster can freeze, expand, and cause critical failure of the plaster.  The temperature needs to be above 40 F for at least a week to keep the curing process going.

 

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Some nitty gritty details
(and where to find materials…)

SAFETY FIRST!

Note that lime is highly alkaline, and can severely burn your skin.  Unlike acid burns, you generally do not feel an alkali burn until the damage has been done.  So please use full protective gear whenever working with lime, including elbow-length rubber gloves, long sleeves, eye protection, etc.  If your clothes get lime putty or lime water on them, change, so the lime is not in contact with your skin through your clothing.  I always keep a bucket of water & vinegar nearby to neutralize my tools, gloves, and hands as I’m working.

 

WHAT KIND OF LIME TO USE?

I use fresh hydrated powdered lime and then soak it on site from the very beginning of construction (ideally several months).  The longer you soak it, the creamier and easier to trowel your plaster will be.  I have had most consistent results with vertical kiln products fromMississippi Lime.  The vertical kiln operates at a lower temperature and so there is less inert material in these products, meaning they are very high in purity and total calcium content.

 

I ask for bags that are date-stamped less than 6 months prior to purchase.  This ensures the lime is fresh.  If it has been in the bag for a long time, it gets exposed to CO2 in the air and begins to carbonate and become inert.  Powdered lime that has converted to calcium carbonate looks identical to calcium hydroxide (hydrated lime), but when you soak it, it will not get very thick and when you put it on the wall it will dust or crumble.

WHAT KIND OF SAND TO USE?
Choosing the sand for your plaster can seem mundane and unimportant.  But that is not the case!!  The key variable is that the sand must be “angular”, which means it has a lot of surface area to bond with the lime.  I use “toothy” or “angular” mason’s sand for all three coats of lime plaster.  You can also use concrete sand (which is larger)…just remember that your plaster needs to be thicker than the largest particle in your mix (otherwise the pieces will drag around with your trowel).  Note that the color of the sand will impact the final color of your finish coat of lime.  If you want very white plaster, experiment with white sand.

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CAN LIME BE PIGMENTED?
Yes!!  Any pigment that can be used in concrete will work with lime.  The pigments must be able to handle the alkalinity of the lime.  Mineral pigments generally are fine, plant-based pigments generally will not work (they change color and fade due to the alkalinity).  In any case, do several test patches to confirm how much pigment to add to achieve your desired color.

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