This Instructable shows how to work with hydraulic lime plaster on a strawbale building.
Lime Plaster refers to a mortar using only pure lime as a binder and sand as an aggregate.
Lime has a long history of use, serving both functional and decorative purposes.
This is what I'm using as a exterior stucco and interior plaster on this project. I use the St Astier Natural Hydraulic lime and I'm very happy with the result. This is a very old building method, in fact Saint-Astier Natural Hydraulic Lime was used to build the city of Perigueux in France in 30 B.C. I purchased it from PA Limeworks in Allentown, PA.
I used Hydraulic lime plaster because it is one of the "greenest" materials you can use in construction. This is due to its purity, its calcium carbonate composition, it's CO2 absorption, its longevity and potential for allowing the materials to be reused or recycled, and the result of a low energy production process.
Below are some key advantages to using Natural hydraulic lime as a stucco on strawbale .
Elasticity-Important in minimizing shrinkage and cracking.
Allows for minor movements.If the building moves, a lime mortar will move with the building. Through crystalline bridging, using the free lime content, the lime mortar will heal those cracks that occur. Portland cement can become brittle as a result of it's high burning temperatures
Permeability- Good vapor exchange qualities allow for condensation dispersion.
Allowing structures to breathe reduces or eliminates condensation and rot . Bugs don't like it either.
Hydraulic lime plaster will combine the ability to resist water penetration while allowing the structure to breathe freely
Self healing- small cracks will heal up because of the amount of free lime in the product.
If some cracking occurs in the brown or second coat, you can mist the crack and close it up the next day.
Insulation- The breath-ability of NHL mortars reduces moisture in walls, therefore improving insulation levels.
Reworking- All St Astier mortars can be reworked (8-24 hours), reducing wastage and increasing work rate. This is due to the absence of cement, gypsum, pozzolans or high aluminates.
Recycling- Materials built with hydraulic lime mortars can be reused. the NHL mortar itself may be recycled in a number of ways, such as an aggregate for new lime mortars, fertilizer (NHL is calcium carbonate), or it can be used for water purification to adjust pH levels.
CO2 absorption- The most eco-friendly feature of using limes. CO2 is reabsorbed during the carbonation of the free lime. Also the amount of energy used at the production stage is one half of what is needed to produce cement. Consequently, the release of CO2 into the atmosphere is much less.
Step 1: Tools Needed
You will need trowels. The finish trowel is the main unit for applying the plaster. It can really move a lot of material quickly.
The pool trowel is the best for smoothing the irregular shapes you encounter in strawbale walls.
Pointing trowels are for getting into tight places. same with the small flat end trowel.
a Hopper sprayer and the compressor to run it, especially for exteriors, if you can afford it it will really speed things up. You will need a compressor that can push 18 CFM's
A cement mixer is essential on a project of this scale. The one I have is a cheapo unit and I'm always having to jerry rig something to keep it working.
For smaller plaster or concrete jobs you can use a drill powered mixer.
Buckets and tarps and rubber gloves.
ladders, scaffolds and other medieval devices.
scratching or scaring tool which can be as simple as a piece of hardware cloth or you can spend 30 bucks on a designer tool.
A big mortar pan is nice when you're up on the scaffold.
A hudson sprayer for misting. Keep it clean and for water only. I got mine for around 18 bucks new.
you might want a set of Coveralls so you don't get plaster everywhere.
Eye protection: if you're like me, if plaster can go in my eye it will, and it burns.
Helpers! sure make things go faster
Step 2: Final Prep the Walls
Strawbales by nature are not perfectly smooth, but, the smoother you can make them the easier your plastering job will be. You can use chicken wire to sculpt your curves and arches for door ways and windows. I use a big pair of Tin snips to cut the wire.
If you have any wood framing or bucks you can use a Pneumatic stapler to attach the wire or you can make pins from a decent stiff wire and jab it into the bales.
Wire also helps to smooth any big lumps or bumps on a flat wall. Check to make sure that you don't have any lose straw and that any holes are filled.
If you have any exposed posts or other woodwork it is a good idea to oil them with boiled linseed oil prior to plastering. Linseed oil is a good general purpose finish, pigments can be added to the oil as desired.
Step 3: Mix the Plaster
For the first coat I used 1 part plaster to 1.75 parts sand. I had the sand delivered from a local source. Be careful not to use too much water at first or you'll get a slurry that is hard to keep on your hawk or in the hopper sprayer. I run the sand through 1/4 inch hardware cloth screen to get out any pebbles or clay that may be in the sand. For the first coat on the exterior I also added a small handful of fiberglass for extra strength
Step 4: First Coat
It's something that will come with practice.
If using the hopper sprayer for the first coat spray on a 3/4" thick "pricking-up" coat of NHL and 1-1/2 parts sharp, well-graded sand which was mixed for 10 minutes in your concrete mixer and
then after being spray-applied pressed into the straw with a stucco trowel
to seal everything up. Straw will most likely still be showing through in
many places. Because of the slower set time of NHL you have plenty of time to work with your plaster.
Step 5: Scratch the Plaster
pattern with a scarifying tool. (or hardware cloth)
Wait 7 days and keep the first coat dampened to slow cure the lime
plaster protecting the exterior with burlap from wind, sun, driving rain and by not
doing any plastering when temps will fall below 40 degrees F for 72 hours
Use the hudson sprayer on the interior coat and mist it a several times a day.
7 days may seem like a lot ,but, by the time you work your way around the walls you'll be ready to start the 2nd coat.
Step 6: Second Coat
with no standing water remaining spray or trowel on a 1/2" thick Brown or second coat of 1 part NHL
and 2 parts well-graded sand with the same mix and cure conditions as the first coat.
After the plaster firms up enough scratch the pressed-in second coat in a
cross-hatch pattern with a scarifying tool.
Step 7: Finish Coat
NHL 3.5 and 2-1/4 parts sharp, well-graded finer sand with the same mix and
cure conditions as the first coat.
Interior: Trowel apply an 1/8" thick finish coat by hand, (troweled smooth
and tight) with 1 part NHL 2 and 2 parts sharp, well-graded finer sand with
the same mix and cure conditions as the first coat adding no more than 8% of
the binders's weight of your own pigments.
Step 8: Additional Notes:
- If you want flatter or square walls in your strawbale building, you can repeat the second or "brown coat" after another 7 day cure on a scarified (scratched) second coat.
- Mist with water and use a trowel to rub out any shrinkage cracks on the final base coat in either case within 24 hours of its application.
cure properly for 10 days .
- Before you plaster any interior walls it's a good idea to install a plaster stop. For the top of the wall I used a strip of wood I ripped on the table saw. I attached it with 18 gauge brads. What this does, is give you a consistent top edge to plaster up to. I planed it smooth and after attachment oiled it with linseed oil. It looks good and goes with the rest of the wood.
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