Finishing Bale Walls with Siding – TLS #57
Jeff | Jun 29, 2009 | Comments 5
This article appeared in TLS #57.
Loose Strings: Technical Discussions
by Jeff Ruppert – Colorado, USA
T e c h T i p s
A little known fact in the bale building realm is that a handful of people scattered across different continents have experimented with the idea of finishing their bale walls with wood or some type of manufactured siding. The technical term for siding over a bale wall assembly is a “rain screen.” The use of a rain screen (sometimes referred to a “multiple defense assembly”) on a bale wall plays the role of keeping rainwater off of the bale portion of the wall. This is in contrast to the standard way of finishing a bale wall with plaster and allowing moisture to come into contact with it on a regular basis (also referred to as “faceseal” walls). In fact, almost all of the literature to date on bale-wall construction makes the assumption that they are faceseal assemblies.
In this article, we are going to take a look at the pros and cons of in-stalling siding over a bale wall. To some people the idea of not having a plaster finish on a bale house would seem weird, mainly due to aesthetic reasons. However, for those who have chosen to use siding, aesthetics take a backseat to function due to high rates of rainfall throughout the year, as well as constant high humidity. The option of allowing bale walls to even get wet in the first place is not an option and therefore other systems must be considered.
For those of us who live in drier climates, the consideration of moisture is not as dire, therefore giving us more choices. However, doesn’t the siding option make sense if you are concerned about moisture at all? If you would like to design a building with mixed finishes, such as a combination of plaster, masonry and siding, this would open up the opportunity to include bale walls as an option on those projects. In fact, by installing a rain screen over bale walls are we not greatly reducing the potential for moisture damage, as David Eisenberg puts it, by “designing problems out of the project” from the start? We will explore these issues and hopefully offer you another choice in your search for solutions.
Rain Screens
In the old days, a rain screen was simply an exo-barrier that was attached to a building to catch rainwater and shed it before it could hit the structure behind it. The Norwegians titled this approach the “open-jointed barn technique,” since originally it was used in conjunction with the construction of barns1.
With tighter construction and newer forms of finishes, the technology of rain screens has evolved into a science. One of the advantages of using a rain screen on a bale wall is that, no matter
how you do it, it will probably add a significant layer of protection that would otherwise not exist. This assumes that you do not install the siding to accidentally direct water into the wall. The potential exists for this to happen, so just like any other type of finish, pay attention to the details!
Siding over bale walls
No matter what type of wall you build, the driving forces of moisture will be:
- Air pressure difference (gradient)
- Gravity
- Surface tension
- Capillary action
- Rain drop momentum.
The dominant force acting on your walls will be the difference in air pressure across the siding itself. As the wind blusters around your house, there are pockets of less and more pressure ever changing within and around your wall assemblies. The main goal is to minimize any pressure differences so water is not accidentally driven into the wall assembly. By minimizing pressure differences, the main force acting on nearby moisture will then be gravity, drawing water down to the ground where it belongs, before it reaches your bales.
In order to equalize pressure, an air gap behind the cladding (siding) needs to be well ventilated to the atmosphere. This can be achieved through different methods, but whatever you do, make sure not to create a gap for wind to blow rain behind the cladding. This means providing ventilation behind the siding so air can pass through easily, but including a barrier at the points of ventilation to keep wind-driven rain from entering.
The advantages of using a rain screen are:
- Adds another option for finishing bale walls (aesthetic),
- Keeps moisture completely off the bale portion of the wall assembly,
- Provides replaceable/changeable finish,
- Has low or no maintenance (depending on material),
- Uses local materials in northern climates near forested areas.
The disadvantages of using a rain screen are:
- Plaster finish is not an option on a bale wall,
- May not be as durable as some types of plaster,
- Materials may not be sustainable or even available in your area,
- Aesthetic of siding may not match your project.
Rain Screen Concept on Bale Walls
It is important to remember that no matter how we finish bale walls, they must be sealed with plaster. This means that even if we choose to use a rain screen, we must apply at least one coat of plaster. One way to install siding on bale walls is to first install nailers for the siding. These can be in the form of 2-in.x2-in. wood strips attached to the sill plate and beam at the top of your bale wall.
We recommend attaching the nailers before stacking the bales, but you can do it afterwards if you like. Once the nailers and bales are in place, one coat of plaster is applied between the nailers. A rough coat of plaster over the bales is all that is necessary. Little or no troweling is required because no one will ever see the results. After plastering, building paper is stapled to the nailers and the siding is then installed, leaving a gap behind the paper for ventilation and drainage.
One issue of concern with this method is the gaps that can occur between the plaster and nailers as the nailer wood shrinks over time. These gaps can allow air to ?ow in and out of the bale wall, creating a loss of insulating value, as well as a path for insects and/or rodents. Extra care and/or the application of caulk can take care of these gaps. Also, these gaps can be eliminated if the nailers are installed after plaster is applied. Whatever you do, be sure that a gap remains between the back of the siding and the plaster.
This is but one way to install siding on to a bale wall. There are variations to this concept, but the goals remain the same – keeping rainwater and back-splash off your bale walls. Pay attention to the details and remember the forces that are acting on water that comes into contact with your walls. Holding these basic concepts in mind will help you design your wall system. And most important, do your homework first!
Happy wall building!
Resources
1. Rainscreen Cladding: A Guide to Design Principles and Practice.Anderson, J.M. & Gill, J.R. Butterworth-Heinemann, 1988.
www.shildan.com/Rainscreen/History.htmlhttp://irc.nrc-cnrc.gc.ca/pubs/ctus/17_e.htmlwww.greenhomebuilding.com/pdf/RainScreen.pdfwww.cmhc-schl.gc.ca/en/inpr/bude/himu/coedar/loader.cfm?url=/commonspot/security/get?le.cfm&PageID=70139
Ed.Note: Jeff encourages TLS readers to send in questions and comments to The Last Straw. There may be outstanding issues that builders are dealing with that most laypeople may not aware of. There are always many questions from people new to straw-bale construction. With this in mind, this column is offered and intended to encourage everyone to educate themselves to the fullest extent regarding building construction, and we are here to help in any way we can. This forum endeavors to offer the best of our knowledge, with no claim to its completeness, but to the spirit of bale building as a continuing evolution of one form of habitat within the larger realm of natural building. We offer this forum for dialogue, with no implication of being right or wrong. This forum is for you, the learner, artisan and teacher.
Jeff Ruppert, P.E., Principal, Odisea LLC, Ecological Building, Engineering and Consulting, P.O. Box 1505, Paonia CO 81428, 970.948.5744 <jeff@odiseanet.com> www.odiseanet.com
Jeff has been in the construction trades for over 25 years, beginning as a laborer and draftsman on his father’s construction projects. He has spent many years working on construction projects he designs, and is a licensed engineer in Colorado.
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Filed Under: Details • Moisture • Rainscreens • Tech Tips • TLS #57
[...] finishing your building with plaster at all! You have the choice of finishing the building with a rainscreen assembly, or some form of siding. In this case you will still need at least one coat of plaster behind the [...]
I think that when you are remodeling you want to be around the project as much as possible!
If you can do a lot of it yourself that helps on quality and money.
Usually the best job done is the one you do because you know how you want it done and you
want it done nicely because you’ll be seeing it everyday – the contracter wont!
Thanks for this blog it had a lot of great information!
Jeff,
We use rain screens over SB quite a lot in the Netherlands due to our climate. We can have long wet spells where Lime plasters can have difficulty in dealing with the amount of water. Besides this we often are forced to space restriction to build more stories thus increasing the amount of exposure. Sometimes we just have to resort to a rain screen.
As jeff says a rain screen requires careful detailing to prevent it from becoming a source of moisture damage. Beside this I would like to add the main drawback of a rain screen. With a rain screen if there is moisture build up in the plaster or bales it is not so obvious as with a visible plaster. The rain screen hides moist spots which makes problems hard to detect.
Rene,
Thanks for the very good point regarding a lack of visual connection to the wall. It makes a good case for installing moisture sensors.
Jeff
Reading through the comments on moisture penetration, I thought this may be of interest:
“WATERWAY”
RAINSCREEN DRAINAGE MATS
HEALTHIER WALLS THROUGH RAINSCREEN & VENTIATION
CREATE SPACE
BETWEEN YOUR BUILDING AND THE ELEMENTS
Fundamental theory in cladding integrity prevents liquid water from penetrating vertical walls, ideally through appropriate materials selection, architectural detailing and professional execution consistent with specifications and construction documents.
We can all attest to the increased frequency of problems arising from water infiltration and deterioration in exterior envelopes. Particularly, wood framed buildings constructed to more stringent codes, appear increasingly susceptible to problems unlike older, drafty and un-insulated walls built decades ago. Avoiding deterioration in older buildings is not a result of better resistance to water intrusion, but rather a better ability to dissipate the moisture. Forensic evaluations note compromise from moisture penetration increase significantly if water is unable to evaporate or drain, causing accelerated consequential damage. Absent perfect design, execution, materials performance and continued maintenance, inevitably problems can develop.
If we are more susceptible to decay, let’s consider requirements for mold growth:
#1. Spores – In wet or humid climates, mold spores are present everywhere.
#2. Temperatures – Generally above 40 degrees F.
#3. Food Source – Most organic materials including wood and paper products.
#4. Oxygen – Even with low permeance building materials.
#5. Water – Varies by species, but growth occurs at levels between 20% – 40%.
Depending on project location and climate, the moisture content of even protected building products may fluctuate at levels just below the minimum required for fungal growth. And if one considers four of the five elements needed for mold development are inevitably present at all times, even a small amount of additional water can result in decay, causing deterioration.
If modern construction techniques reduce water vapor diffusion and agree moisture will likely penetrate outside walls, it is important to incorporate a defensive strategy to mitigate the effects of water intrusion and prevent rot.
“CREATE SPACE WITH VERTICAL WALL DRAINAGE MATS”
WaterWay Rainscreen Drainage Mats provide that additional space for drainage. They also contribute to air circulation and ventilation when properly designed. The combination of water drainage and increased air flow will enhance drying and in turn reduce the damaging effects of water penetration. Building papers and house-wraps are well established in providing some protection against substrate damage. Traditional barriers offer a marginal safeguard, but they do not always provide an adequate path for drainage. With siding materials tightly installed against weather resistive barriers, opportunities for drainage and ventilation can be inadequate or even blocked.
The Code (ICC and IRC) Mandate:
“Exterior wall envelopes are required to be weather- resistant and provide a means for draining water that enters the assembly”
Engineers and designers are exploring various configurations to accommodate the code and remove water from the exterior walls while incorporating some mechanism of ventilation. Enhancing air flow and managing entrapped moisture make sense; the challenge lies in a simple, efficient and cost effective approach.
“WaterWay Drainage Mats” consist of a Nylon / Polymer core of fused, entangled filaments in varying thicknesses from a nominal ¼ inch to ¾ inch, depending on desired cavity space and cladding selection. Certain profiles are available with a protective filter fabric bonded to one side, even our “Patent Pending” polyolefin-based house-wrap is available with drainage mat attached. “WaterWay” Rainscreen mats are equally effective for stucco, EIFS, manufactured stone, fiber-cement, brick, standard lap and cedar sidings and other wall system materials.
This technique achieves desired results with limited additional expense. The incorporation of WaterWay Rainscreen Drainage Mats can run as little as $.65 a square foot for materials. With varying labor rates it’s difficult to offer a specific cost for installation, although it should not be more that $.95 – $1.50 (labor & materials) per square foot. With over 60 million feet in service and an exponential increase in demand, you can feel confident in this effective concept.
For additional information on the “WaterWay Rainscreen Drainage Mats” including product data sheets, profile samples, test reports, detail drawings, specifications and an animated DVD presentation.
Contact Stuc-O-Flex International, Inc. @ 1-800-305-1045
http://www.stucoflex.com or e-mail techservice@stucoflex.com