Stucco Notes for Our Straw Bale House
We originally hoped to do a first coat of stucco inside at the same time, but it has been clear for some time that we could not afford to delay exterior stucco. We need to do wiring, plumbing, gas lines, insulation and drywall before we stucco inside! Maybe we can be ready by December!
The first coat of exterior stucco was applied with a stucco spray gun and a crew of five (Sager Plastering, of Snohomish). It went on in four and half hours (one hour of which was really completing our preparation). Altogether amazing to watch: the crew were like dancers! The man on the stucco mixer never stopped moving, and the others didn't miss a beat either. One for the spray gun, one full time with a long darby, and the another artist with a trowel did finish shaping. The fifth crew member divided his time between moving the hose and darby work.
The brown coat took a little longer; a full nine hour day for a six person crew. A lot more trowel work, and everyone pitched in with the finish sponge trowels at the end, to bring up a nice sand finish. We prepped eyebrows over doors and windows, and half columns beside the doors; the stucco crew took these in stride, and did a fine job on them! Thank you Joe Sager Plastering!
Now we need gutters (next week!), Tyvek on the gables, two more windows, and we will be secure. The cedar shingles for the gable ends and the shed dormers are here, and we are priming the trim boards.
So why stucco? Well there are limited choices for "siding" on a strawbale house. The bales must be protected from weather and rodents. Wooden, aluminum, or vinyl siding did not make sense or appeal to us. Siding would require a separate support structure to allow airspace between siding and straw. Moisture condensation is the main issue. The accepted wisdom for strawbale is to protect it, but don't try to seal it. Have "breathing" walls that reduce incoming moisture but do allow the inevitable moisture to escape. Our architect recommended stucco: traditional cement w/lime stucco does provide some breathability. Acrylic stuccos are not recommended! Earthen plasters seem more suitable for dryer climates. Lime plaster is sometimes recommended; but there are very few construction people with knowledge of lime plasters, and lots of confusing and even contradictory information about lime. It sounds tricky to work with, and we found the information and choices a bit overwhelming.
Led by an experienced, but not expert, stucco man (a carpenter/contractor with great enthusiasm and interest in all materials and aspects of construction) we stuccoed for three long days with four other paid workers. None of us were expert, three had some stucco or plaster experience. Our inexperience showed in the time we took, and in the rather rough results. Most of the stucco looks OK, but some walls are only acceptable because they are in closets (or the laundry room...)! Rough stucco is both delicate (small protusions get knocked off easily), and unpleasant to the touch (it scratches you!).
We did use a higher ratio of lime for the interior stucco, vs. the exterior. The thickness varies considerably, according to the skill and style of the individuals doing the work. It seemed to dry fairly quickly in the hot weather, but some of the moisture certainly wicked into the straw. Moisture readings were within bounds, but higher than previously.
After a few weeks, I started experimenting with a cement/limewash recipe or two. I did not get far down my list of recipes, due to schedule pressures I stuck with the my first one when I found I did not like working with the very thin pure lime wash that I tried next. I used a 2:1 lime/cement mix, with salt and color. The real benefit of my cement lime mix for my walls was the smoothing and filling of tiny holes and flaws in the rough stucco. While we lost some of the shadowing and texture of the trowel marks (very attractive on the better walls), we gained a lovely soft smooth feel and look.
I started with standard grey cement, and dry La Habra stucco color powder. I quickly switched to using white cement, sometimes tempered with some grey. For several walls I used various proportions of a "buff" Quickcrete liquid color. My kitchen is a very light grey achieved without color, just with a small proportion of grey cement. My living room is a light violet colored by latex paint pigment and the same small proportion of grey cement in the mix. Gorgeous.
The cement color creates variation in the limewash; as the wash dries there is separation and the brushmarks show clearly with light streaking. It is quite attractive and somewhat manageable, but I have not learned to use this in a controlled way. The more color, the stronger the contrast of the pale streaking. The latex pigment does not appear to have this separation, but as I used a very light color, I cannot say there is no variation or streaking at all.
I have had two small patches of my "lime"wash crack and peel off, both next to windows. The the fresh wash is very delicate at first, but seems OK after several days. It does not seem to be "chalking" when rubbed. It can take some wiping, but stains easily.
Our not very consistent or proper use of mesh did not help us get the stucco on. We often did not orient the expanded metal correctly, so that it was harder to get the stucco to stick! However the curves, niches, and arch that we created have proved worth the effort; we both love the effect. We can sit for hours (after a few beers or a bottle of wine) staring appreciatively at our home's fine curves :-)