LIME STUCCO PLASTERING
WHY USE LIME STUCCO INSTEAD OF CEMENT STUCCO?
Lime sticks extremely well to the surface of the straw bales, and chicken wire is therefore not required as it is when using a cement stucco. Portland cement will not stick well to the bale surface and chicken wire must be used and in fact is required by most building codes. Lime plaster perhaps mixed with adobe plaster is more durable and waterproof than adobe alone. Straw and lime (also straw and adobe) have been used together for centuries -- marriages made in heaven.
Lime retards the growth of mould (a serious problem in straw bale homes if there are any moisture problems). Portland cement will not retard the growth of mould.
Lime is somewhat breathable (letting air and moisture pass through the wall) and will therefore let moisture escape out of the wall if it gets trapped in the bales. Portland cement is not breathable, thus trapping the mosture which then rots the bales away.
LIME STUCCO FORMULAS:
White Portland (or Universal) Cement: 1/2 bag
Type-S Lime: 2 1/2 bags
Masonary Sand: 90 shovels
Chopped Strand Fiberglass Fiber: lare handful
White Portland (or Universal) Cement: one shovel
Type-S Lime: 2 bags
Masonary Sand: 65 shovels
Ferrous Sulfate Powder (water soluable type): 16 cups
SHOOTING LIME STUCCO
A lime stucco was shot professionally on all interior wall surfaces of our straw bale house... a base coat and a finish coat on two sucessive days. (The adobe plaster on the external walls which was applied by hand took over a year, and we just didn't have the time or the energy to do it again on the inside.)
NOTE: A small amount of Portland Cement was added to increase drying/curing time. Lime plaster takes a minimum of two to three weeks to cure otherwise. Since we were under time constraints and had to shoot the walls within a two day weekend, adding approximately 20%cement (one to five ratio of cement to lime) produced sufficient drying/curing that the second finish coat could be applied within 24 hours. Only a small amount of cement was added to the final coat. Adding more than 20% cement is not recommended as it significantly decreases the breathability of the stucco which is so crucial for straw bale walls.
White cement was used since coloring added later to the finished walls will take and cover better than over the grey of the more common form of Portland cement.
The surface was lightly trowelled for the right texture using a float sponge. The light tan coloring of the plaster was achieved by adding Ferrous Sulfate powder (a soil additive obtained from fertilizer stores) to the raw plaster mix.
A RARE AND VALUABLE RESOURCE:
You can now buy you own sprayer unit at
THINGS TO LOOK OUT FOR WHEN SHOOTING LIME:
Somewhat more water than is used for traditional cement stucco was needed in order to keep the pumper/mixer and supply hose from clogging up. Picture below -- grumbling stucco crew getting their pumper unclogged.
We had some cracking as the stucco started to cure. This was easily repaired with a wet float sponge before the plaster was dry. A warm (but not hot) and humid environment is best to allow the stucco to cure slowly and not dry out too quickly.
Lime is very alkaline and caustic -- producing a terrific case of "dishpan hands" in just a few minutes. Don't breath the dust as you shovel it or get it in your eyes. Wearing gloves, a dust mask, and eye protection is highly recommended. Also the ferrous sulfate not only stains plaster, it stains your hands so wear gloves.
Serious Straw Bale by Paul Lacinski and Michel Bergeron, A Real Goods Solar Living Book, 2000 - pages 244-261.
The Straw Bale House by Athena and Bill Steen, A Real Goods Solar Living Book, 1994 - pages 207-210.
The Last Straw Journal, Issue 29, The Green Prairie Foudation for Sustainability, 2000 - an entire issue devoted to the use of lime in straw bale construction. www.thelaststraw.org
"Lime-Based Renders" by Bob Bennett, artical on the Lime Centre web site: www.thelimecentre.co.uk/Library/renders.pdf
"Staining With Ferrous Sulfate" by John Swearington, artical on the Skillful Means web site: www.skillful-means.com/strawbale/papers/staining.htm
Page 4 of Construction Pictures / New Straw Bale
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