In the first of a two part series article, Jacob Deva Racusin explains the differences in monitors for your walls. This is “must know” information for every builder and owner.
By Chris Magwood
An important concept to understand when considering moisture and building materials is adsorption. Moisture in vapor form infiltrates any and all materials. The surface of most materials will offer individual water molecules an electrically charged attraction, and the water molecules will “stick” to all available surfaces. The makeup of plaster and of straw bales offers a vast amount of surface area for this adsorption. Plasters are full of micro-pores and straw has great deal of available surface area as well as micro-pores in the hollow stems. Together, these materials allow a surprisingly large amount of moisture to safely adsorb onto/into the materials without the water molecules accumulating in sufficient layers to become drops of liquid water. Bales and plaster can hold a remarkable amount of moisture in adsorbed form. “For a 8 pcf (pounds per cubic foot) bale, more than 1 pound of water (approx. 1/12 gallon or 0.46 liters) in vapour form can safely be stored per square foot of wall area” according to John Straube in Building Science Digest BSD-112. This explains why the walls can perform so well as “vapor open” or “vapor permeable” systems.
by Chris Magwood
The straw bale revival of the 1990s reintroduced builders to a pioneer building method that showed remarkable potential for building in a modern context. The nature of the basic components of a straw bale wall system – bales with plaster applied directly to bales – combined several obvious advantages over other wall systems while simultaneously raising some serious questions.
The advantages are familiar to TLS readers: low environmental impact, simplicity and well placed thermal mass. The serious questions and attempts to answer them were the lifeblood of TLS: What about moisture? Air tightness? Cold climates? Humid climates? Longevity of the straw in all these conditions?
There was a lot more going on in the construction world of the 1990s than the straw bale revival. The housing industry was recovering from multiple moisture-related disasters, many caused by overly airtight but under-ventilated homes and/or the use of un-vented “waterproof” exterior cladding systems. The research into these issues, among other factors, led to a rise in prominence of “building science” as a distinct engineering discipline. The straw bale revivalists were extremely lucky to have the attention of some of building science’s leading practitioners, in particular John Straube, who authored several key articles and studies about straw bale walls that were essential to answering the question of why bale walls were so resilient, despite some seemingly obvious moisture concerns.
Simply put, building science attempts to quantify the movement of heat and moisture through building assemblies and help designers and builders to make decisions that lead to structures that adequately deal with the moisture and temperature regimens of their particular climate and use.
The Straw Bale Alternative Solutions Resource is a document prepared by the Alternative Solutions Resource Institute (ASRI) addressing, obviously, bale construction. While the goal of ASRI is to “foster and facilitate the use of natural materials and systems in the construction of buildings…” this document is meant to focus specifically on bale construction and how these buildings can be permitted under the Alternative Solutions section of the British Columbia Building Code (BCBC). For those of you familiar with alternative solutions, or alternative materials as described in the 2009 International Building Code (IBC), this document literally lays out the framework and arguments for the use of bale construction under the alternatives section of the BCBC. While this is obviously geared toward the British Columbia provincial building code it has as much applicability in the context of other model codes around the world. The complete and comprehensive nature of this document is a real lifeline for anyone requiring a permit in a jurisdiction with many questions about straw bale construction.
What makes this document most impressive is its coverage of all aspects of building science related to bale construction. Moisture, plaster materials, fire, structural design, storage of bales, foundations, openings and box-beams are all covered in enough detail to lay a solid enough framework for anyone to permit a bale building. The comprehensive nature of the document makes it required reading for all architects and engineers working on bale buildings.
While it covers pretty much every aspect that could come into question about bale buildings, it is geared for the seismically active maritime climate of British Columbia. Expected rainfall in much of B.C. is heavy and they do not mince words when it comes to flashing windows, how far you should keep the bales above adjacent grade, and the role of roof overhangs. While they do make sure that seismic design is addressed, they do not include any examples or give minimum requirements. They do expect an engineer to be involved for the earthquake stuff. One item to note that probably comes from being in a seismically active area is that all of their illustration show mesh being used in the plaster. This conflicts with many purists view in parts of the world with low seismicity and moderate wind loads.
For the plaster junkies out there, it even has a section that will keep you interested. It does a great job summarizing the basic concepts that we have come to terms with over the years and how a bale wall with plaster should perform. As with the rest of the document, they do very well strongly discouraging the use of pure cement plaster due to the wet climate. However, they do allow for cement-lime in appropriate ratios.
One important thing this document does really well is deal with terminology. The basic premise first introduced in Bruce Kings book, Design of Straw Bale Buildings, is that the terms we use to categorize bale walls have been inaccurate and are widely misused. According to this document the two types of bale walls are Structural and Non-Structural (much like all other wall types). If you are going to use this document, you should get used to not using the terms “load-bearing” and “post-and-beam” with the building officials. Either the walls are intended to withstand vertical and lateral loads in excess of holding themselves up, or they are not. How they are framed or stacked is of less relevance than how they are intended to perform, from a classification point of view.
Other notable items are clear statements such as the following:
- “A minimum insulative value of R-28 may be used when calculating the thermal performance of a plastered straw bale wall using standard bales.”
- “Conventional vapour barriers are not necessary or advisable.” (Polyethylene barriers are listed in a section titled Incompatible Materials, which also includes embedded rebar or metal and cement stuccos.)
Due to being in a wet region, one interesting inclusion is the following:
- “In areas of high rainfall or high relative humidity, consideration should be given to making exterior walls “rainscreen-ready” in anticipation of the need for addition protection.”
In summary, the ASRI solution for bale construction is an impressive, well-written, comprehensive document that all professional practitioners of bale construction should have on their shelves. While it does not go into detail as much as some books on the subject, it covers everything in a way that shows the authors did their research and left us with a worthy tool in our quiver. The document is available from the ASRI Website for $25 to help offset costs and maintenance over time, as well as give ASRI a budget for their next projects, which, get this, includes the following:
- rammed earth
- adobe block
- light clay
- earthen plasters and floor systems
- thermal mass
- on-site grey-water and black-water treatment
- alternative healthy electrical technologies
- passive and active solar integration, and
- living roof installations
We should support them if they can do something similar for these other building materials and systems.
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.
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!
No matter what type of wall you build, the driving forces of moisture will be:
- Air pressure difference (gradient)
- 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!
1. Rainscreen Cladding: A Guide to Design Principles and Practice.Anderson, J.M. & Gill, J.R. Butterworth-Heinemann, 1988.
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 <[email protected]> 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.
This article appears in issue #57 of TLS. There have been other articles about moisture sensors in recent years.
by Habib John Gonzalez – British Columbia, Canada
This article appeared in a slightly longer version in TLS#22/Spring 1998.
Here are the simple steps and materials needed to build your own bale wall moisture sensor:
1. Determine what depth of the bale you want to monitor and cut the 3/4-inch PVC pipe to that length.
2. Make the white pine sensor disk 1/8-in. thick to fit snugly into one end of the pipe.
3. Solder two lengths of telephone wire to two pairs of small bolts. One end of the pair of wires is bolted to a PVC pipe cap so the tips will protrude from the finished interior wall. The other end of the wires will be bolted to the sensor disk.
4. Use epoxy to glue the disk to one end of the pipe; run the wires through the pipe and fasten the other pair of bolts to the interior wall end cap. Glue the cap to the pipe.
5. Glue a perforated pipe cap over the sensor end of the pipe.
6. The sensor is ready for installation in the bale wall.
7.The TimberCheck moisture meter is available from www.leevalley.com
8. A number of bale wall moisture studies were sponsored by the Canadian Mortgage and Housing Corporation (CMHC). You can get a summary of all of the CMHC moisture work on their web site www.cmhc-schl.gc.ca/publications/en/rh-pr/tech/dblist.cfm?mode=year. Scroll down to the bottom of the list for 00-103 (year 2000, document 103) on straw-bale moisture monitoring.
1. Outer end-cap
2. Perforated PVC pipe
3. Wood disk with screws
5. PVC pipe
6. Inner end-cap
7. Screw contacts
Included in TLS #49 (Myths and Realities, Spring 2005) was a discussion of ways to deal with moisture at the bottom of windows. David Eisenberg shared a written design detail for a pan under the window to carry water away from rather than down the wall. We wanted to share a drawing of this detail and David kindly provided one for us to share in Tech Tips.
Here’s the portion of the discussion in which David details this design idea.
“Protecting the bales beneath the windows requires that you catch the water under the window and make sure it gets all the way out of the wall. In other words, ideally, you would have a pan of sorts under the window, sloped slightly to the outside, extending a bit beyond each side and with a lip at the back and on each end (so water can’t just run off the ends), and extending out beyond the exterior wall surface, with a drip edge – so that any water that leaks through or runs down the sides of the window ends up in this pan and is shown the exit. You can make these pans out of metal, plastic, ice and water shield, cast this shape into a concrete sill, anything that will keep the water from leaking through it, but the principal thing here is to make sure that the water can’t get into the wall below the window. You can put your window sill material, whatever it is, on top of this pan flashing being careful not to punch unsealed holes when you install the sill. It can take a little thought and ingenuity to do this, but it assures you that, when the windows leak, the water leaves the building.
“That old practice of just putting roofing paper or plastic over the top of the bales and setting your windows on it and then plastering over it just leads the water down inside the plaster to the bales wherever the water protection ends unless it runs continuously down the wall under the window to below the bales (and we don’t recommend doing that). It just temporarily moved the problem down, didn’t solve it.”
This article appeared in TLS #50.
by Chris Newton – Queensland, Australia
Can you design and build straw-bale homes for a hot and humid climate? Living in Queensland, Australia, I am frequently asked to identify an invisible line on the map where “she’ll be right” applies on one side of the line and “don’t go there” applies to the other. The part of me that fears litigation wants to respond with “ask me in 20 years time,” the technical part of me feels it has to be evidence based, and the logical part knows the answer already exists in the local environment. So I take on board here these three points and discuss how I attempt to find that line on the map in our building history, current research and the observation of the environment we live and build in.
Queensland extends from 10 degrees south to 29 degrees south of the equator, covering more than 1.72 million square kilometres. Queensland is more than twice the size of Texas. Within Queensland, we live in monsoonal, tropical, subtropical, grassland and desert climate zones.
The table below represents summer (December though March) in the climate zones of Queensland. Summer is dominated by the monsoons making this a hot, wet and humid season. All zones in Queensland have mild and dry winters.
We can create a microclimate in and around our homes. Changes in air movement, moisture load or sunshine can significantly change the wetting and drying potential of a section of the building. When designing the house and gardens in a humid climate, we need to be aware of creating microclimates that cannot dry out.
Humidity is the water vapour held in the air. This is the ratio of the actual amount of water vapour in the air to the amount it could hold when saturated; it is expressed as a percentage. The capacity for air to carry water vapour increases as the air temperature increases. Air with a temperature of 30°C/86°F can hold more than three times as much water vapour as air at 10°C/50°F.
The dew-point temperature is temperature in which air must be cooled in order for dew to form. Droplets of water can be deposited within the straw-bale wall when air cools below the dew point and water vapour condenses.
Wood can absorb moisture content up to 25% from a relative humidity 98% (See Straube report in Resources at end of article). Straw is hygroscopic with its large surface area and internal pores having the ability to absorb moisture. A bale whose moisture content is at 8% will weigh less than the same bale with a moisture content of 20%.
We have a copy of an 1860 encyclopedia. It’s only damage is some yellowing and a few small brown spots (mold). This book had no special storage other than to sit on a bookshelf in subtropical Brisbane. So it seems that humidity alone may not be enough to cause decomposition of straw bales. However, I know through talking to people from Cairns that it is the norm to have molds growing on curtains, furniture and shoes throughout their summer. Newspapers and photos curl from the moisture they absorb. So humidity alone is enough to support mold growth in the tropics.
Historically, bathrooms have remained an area with high failure rates from moisture; this is true in any building type. Protection for straw-bale systems in wet environments exists. This can be in the form of vapour barriers, water barriers, design considerations, and attention to detail. It would be fair to say that, over the life of a building, some houses despite best efforts will experience elevated moisture levels in part of the wall system. Concentrated moisture only becomes a problem if the ability to dry is not timely for the given climate conditions. Remember that molds grow rapidly in hot and humid conditions, and are dormant in cold conditions.
Drying is the balance for wetting. The measure to ensure this includes a capillary layer below the bottom straw bale and a render with high permeability. Water vapour moves from low concentration to high concentration. High humidity will reduce the ability for the wall system to dry. In the tropics, rain may persist over several days. Attempting to dry clothes in the shade will take a long time during which they will acquire a moldy smell. You can not expect a wall system on the south side of the building to dry as efficiently as those on the north. High humidity will further compound this. (Note that we live in the southern hemisphere.)
Can you build with straw bales in a high humidity climate?
The line that removes high risk for straw-bale construction is unlikely to be a latitude line. Maybe it is a line that farmers have already identified. Grain farmers look for a climate dry enough so the grain dries adequately before harvest. The dry grain is then suitable for storage. Humidity is not a problem for the sugar cane growers who harvest the crop with high moisture content and send it straight to the mills where the juice is squeezed from the cane. So maybe the invisible line is found on an agricultural plan.
How Straw Decomposes, Matthew D. Summers, Sherry L. Blunk, Bruan M. Jenkins. www.ecobuildnetwork.org/pdfs/ How_Straw_Decomposes.pdf
Straw Bale House Moisture Research, CMHC (Canadian Mortgage and Housing Corporation). www.cmhc-schl.gc.ca/ publications/en/rh-pr/tech/00-103-E.htm
Moisture Properties of Plaster and Stucco in Strawbale Buildings, Dr. John Straube. www.ecobuildnetwork.org/pdfs/ Straube_Moisture_Tests.pdf
Monitoring the Hygrothermal Properties of a Straw Bale Wall, Dr. John Straube and Chris Schumacher. www.ecobuildnetwork.org/pdfs/Monitoring_Winery.pdf
Bureau of Meteorology–Australia. www.bom.gov.au/ weather/qld/
Chris Newton, Earth-n-Straw, Queensland, Australia, 0413 195 585, <[email protected]> www.newtonhouse.info. Chris, an owner/builder, educator and trainer in strawbale, plasters and other aspects of natural building, is the new President of AUSBALE, the Australia and New Zealand straw-bale building association.