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.
By Michel Couvreux
Thanks to the hard work of a few, Natural Hydraulic Lime has become one of the materials of choice for restoration/preservation projects and natural building construction in the United States.
Suddenly realizing the great potential of NHLs in the US, and feeling the economic hardship abroad, several European manufacturers have begun to latch onto the path opened by Saint-Astier.
Unfortunately, all NHLs are not equal in performance and quality.
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 originally appeared in Issue #54. This issue includes a table of straw-bale building codes, guidelines and mandates in the U.S., and links to straw-bale codes, guidelines and supporting documentation; and an extensive review of the status of straw-bale codes and permitting throughout the world.
by Sigi Koko – Pennsylvania, USA
The bottom line is that yes, using straw bales for non-loadbearing infill walls meets existing building codes for both residential and commercial structures throughout the United States. Why is this true? Because building codes are not written to exclude new or alternative construction materials and methods. Rather, each building code begins with an inclusive statement such as the following from the CABO 95 Preface:
“…there are construction materials and practices other than listed in this code which are adequate for the purposes intended. These other methods represent either seldom-used systems or performance-type systems which require individual consideration by the professional architect or engineer based on either test data or engineering analysis and are therefore not included herein.”
The intent of building codes to ensure that materials are used safely and suitably, not to limit the use of appropriate materials. The burden of proof is to demonstrate that an alternative construction method meets the intent of the building code for durability, effectiveness, and safety (including fire resistance). This means showing how straw-bale infill wall systems meet the requirements of the building code for insulation value, flame spread, smoke development rating, and fire rating. Demonstrating compliance with the building codes is possible thanks to many pioneers that have dedicated time and money to sponsor third-party ASTM (American Society for Testing and Materials) tests. The results of these tests show that straw-bale wall systems not only meet the building code but, in most cases, surpass the intent of the code compared to standard stud-and-drywall construction.
Several states and counties throughout the U.S. have adopted building code amendments that specifically address straw-bale construction, though most regions do not yet include such provisions. Obtaining a building permit for straw-bale infill in regions without a specific building code is not impossible, but rather entails a non-standard process. The question is not whether you can get a building permit for infill strawbale, but rather how to best communicate with local building officials that strawbale is a viable method of construction that meets the existing building code.
David Eisenberg has written extensively and with great eloquence about how to communicate effectively with building officials, and I encourage anyone wanting more detailed information to review his writings on the topic. I have used the following strategy with success:
1) Schedule a pre-submittal meeting with the permitting official to communicate your intentions to build with strawbale. If they are not already familiar with straw-bale construction, provide printed information and additional resources. (Don’t overload with information unless it is requested; like all busy people, building officials are less likely to review a daunting pile.) Bring to the pre-submittal meeting:
• drawings of the proposed building
• an overview of straw-bale construction (I use “House of Straw: Straw Bale Construction Comes of Age” by the US Department of Energy, available at www.eere.energy.gov)
• copies of ASTM testing data (fire-related ASTM tests are at www.dcat.net)
For the final permit submittal, my experience is that stamped structural drawings greatly facilitate the speed and ease of the permitting process.
2) Remember that your building official is your ally not your adversary, and has the same goal as you: to ensure that what gets built is safely built.Acknowledge your common interest for occupant well being and safety. You will create connection instead of confrontation and open a dialog on how to achieve your common goal.
3) Be informed or hire an advocate that has experience in straw-bale construction, including how to build appropriately in your climate. The building officials will generally have more confidence in your project when they know someone on your team fully understands this non-standard construction technique. At a minimum, be prepared for the following common questions:
- How does your wall system handle liquid water and vapor?
- What is the fire rating and smoke development rating of the wall system?
- Will the straw bales attract pests, such as termites and rodents?
- What is the insulating value of strawbale?
- How is electrical and plumbing installed?
I have to date not experienced any delays during the permitting process using this method of interaction with building officials. Increasingly, I find that building officials already possess some level of knowledge about straw-bale construction, which was not the case in this region of the country (Mid-Atlantic states) even five years ago.
Finally, I would like to address the issue of adopting existing codes and details in different climates. I design structures in a wet, humid climate with hot summers and cold winters. However, many of the now-standard straw-bale details have mostly developed in arid and temperate climates that are not necessarily durable in this mixed climate. For example, I do not recommend using rebar inside a straw-bale wall in a humid climate, since the cold metal creates an artificial dew point inside the straw wall. The result is elevated moisture around the rebar, which can lead to rotting the straw over time. Instead, I recommend external pinning or using materials that are “warm,” such as bamboo. Similarly, pea gravel at the base creates an artificial dew point, as well as creating a thermal break along the entire base of the wall. My point is not that the originally developed details are inadequate, but rather that they are specific to an arid climate. So when adopting codes and details in different regions with different climatic concerns, ensure that what you propose will perform durably in your climate.
Sigi Koko, the founding principal of Down to Earth, a design and consulting firm specializing in natural building, has obtained construction permits for many straw-bale buildings in her area. With a Masters of Architecture and several years of in-the-field construction experience, she has developed written specifications and architectural details for straw-bale and cob construction. www.buildnaturally.com
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
This article appeared in TLS #43.
by Gernot Minke – University of Kassel, Germany
Note: This article is excerpted from Earth Construction Handbook (by Gernot Minke, WIT Press, Southhampton, Boston, 2000) which contains further information about weather protection, physical and mechanical properties of clayey soils, treatments and additives and modern earth construction techniques worldwide.
1) General. Earth plasters mainly consist of sand and silt with only as much clay as is necessary (usually between 5% to 12%) for developing their adhesive and binding forces. It is difficult to state what the proportions of an ideal earth plaster should be, because not only does the proportion of clay, silt and sand influence the properties, but also the grain size distribution of the sand fraction itself, the water content, the type of clay, the method of preparation and the additives. In order to test the appropriateness of earth plasters, samples with varied compositions should be tested. Earth plasters stick very well not only on earth surfaces, but also on brick, concrete and stone surfaces, if the surface is rough enough.
2) Preparation of substrate. As earth plaster does not chemically react with the substrate, the surface has to be sufficiently rough in order to develop a good physical bond. A good method of getting a strong bond is to wet it sufficiently until the surface is soft, and than scratch diagonally patterned grooves with a small rake or a nail trowel. In order to ensure that the plaster adheres better, it is also possible to use latching in the form of galvanised wire mesh, plastic mesh, reed mats, and such on the substrate before plastering.
3) Composition of earth plaster.
3.1 General. In order to get earth plaster free of shrinkage cracks, the following points must be kept in mind:
- The earth should have enough coarse sand.
- Animal or human hair, coconut or sisal fibres, cut straw or hay should be added (however, too much of these additives reduce the ability of the plaster to adhere to the substrate).
- For interior plastering, sawdust, cellulose fibres, chaff of cereal grains or similar particles can also be used as additives.
- In order to develop enough binding force, the adhesive forces of the clay minerals should be sufficiently activated by adequate water and movement.
- When the plaster sticks to a sliding metal trowel held vertically, yet is easily flicked away, the correct consistency has been achieved.
In order to test the characteristics of an earth plaster, a simple adhesion test can be carried out. The plaster to be tested is applied 2cm(3/4-inch) thick to the flat surface of an upright burnt brick. The plaster has to stick to the brick until it is totally dry, which might take two to four days.
If it falls off in one piece by itself, as seen in the left sample of fig. 3-1, it contains too much clay and should be thinned with coarse sand. If it falls off in portions after the sample is hammered on the floor like the second sample in fig. 3-1, then it has insufficient binding force and should be enriched with clay. If the plaster sticks to the brick but shows shrinkage cracks, like the third sample in fig. 3-1, it is too clayey and should be slightly thinned with coarse sand. However, it can be used without thinning as the first layer of a two-layer plaster. If the surface shows no cracks and the plaster does not come off when hammered, as in the fourth sample in fig. 3-1, then the sample might be adequate. In this case, it is advisable to make a larger test about 1x2m(40×80-inches) high on the actual wall. If shrinkage cracks now occur, this mixture needs either to be thinned with coarse sand or mixed with fibres.
3.2 Exposed exterior earth plasters. Exposed exterior plasters have to be seasonably weather resistant or must be given perfect weatherproof coating. It is important in cold climates that the plasters together with their coating have a low vapour diffusion resistance, so that water condensed in the wall can be easily transported to the exterior. The exterior plaster must be more elastic than its ground in order to meet thermic and hygric influences without cracking. In general, for cold climates, an external earth plaster is not recommended unless sufficient roof overhang, plinth protection and good surface coating can be assured.
Since plastered wall edges are very easily damaged, they should either be rounded or lipped with a rigid element. In extreme climates when the elasticity of large expanses of flat plaster is insufficient to cope with the influences of weather, vertical and horizontal grooves filled with elastic sealants are recommended.
3.3 Interior earth plasters. Interior plasters are less problematic. Usually they create no problem if they have fine shrinkage cracks because they can be covered with a coat of paint. Dry earth plaster surfaces can be easily smoothed by wetting and being worked upon with a brush or felt trowel.
If the surface of the walls demands a plaster thicker than 15mm(5/ 8-inch), it should be applied in two layers, with the ground layer containing more clay and coarse aggregates than the second layer. If the ground layer gets shrinkage cracks, it is not problematic, but could actually help by providing a better bond to the final layer of plaster.
Adding rye flour improves the surface against dry and moist abrasion. The author has proved by testing that this resistance can also be built up by adding casein glue made of one part hydraulic lime and four to six parts fat-free quark, borax, urea, sodium gluconate and shredded newspaper (which provides cellulose fibre and glue). The mixes in the accompanying chart worked well.
Lime reacts with the casein within the fat-free quark forming a chemical waterproofing agent. A similar reaction is obtained with lime and borax (which is contained in the shredded newspaper). Sodium gluconate acts as a plasticizer so that less water needs to be mixed for preparation (thereby reducing the shrinkage). Urea raises the compressive and the tensile bending strength, especially with silty soils.
Waste paper shreds lead to better workability and reduce shrinkage. The mixes B, C and E showed best workability. When using mixes A and E, it is preferable to first mix the casein glue and the shredded newspaper together with the water, and then, after an hour, add earth and sand.
With all mixes, it was found that the final smoothing of the surface, which was done by a felt trowel, was best done after several hours or even a day.
4) Guidelines for plastering earth walls. As pure earth plaster does not react chemically with the substrate, it might be necessary to treat the substrate suitably so that sufficient bonding occurs. The following guidelines should be kept in mind:
1. The surface to be plastered has to be dry, so no more shrinkage occurs.
2. All loose material should be scraped off the surface.
3. The surface should be sufficiently rough and, if necessary, moistened and grooved or the mortar joint chamfered, as described in section 2.
4. Before plastering, the substrate should be sufficiently moistened so that the surface softens and swells and the plaster permeates the soft layer.
5. The plaster should be thrown with heavy impact (slapped on) so that it permeates the outer layers of the ground and also achieves a higher binding force due to the impact.
6. If the plaster has to be more than 10-15mm(3/8-5/8-inch) thick, it should be applied in two or even three layers in order to avoid shrinkage cracks.
7. To reduce shrinkage cracks while drying, the mortar should have sufficient amount of coarse sand, as well as fibres or hair.
8. To improve the surface hardness, cow dung, lime, casein or other additives should be added to the top layer.
9. In order to provide surface hardness and resistance against wet abrasion, the surface should be finished with a coat of paint[Editor’s Note: breathable paint].
10. While using plasters, the change of physical properties caused by additives and coatings should be kept in mind especially with respect to vapour diffusion resistance.
5) Sprayed plaster. A sprayable lightweight earth plaster with high thermal insulation containing shredded newspaper was successfully developed by the author in 1984. This plaster can be applied in a single layer up to 30mm(1-1/4-inch) thick using an ordinary mortar pump. In order to get a shorter curing period, some high-hydraulic lime and gypsum was added to the mixture.
6) Thrown plaster. Fig 6-1 shows how a traditional African technique, which consists of throwing earth balls on a wall, has been adapted. Here, this technique is used on a wood-wool board for a winter garden wall. In order to increase the adhesion, bamboo dowels were hammered halfway into the board.
7) Wet formed plaster. As loam plaster retains its plastic state for a long time and is not corrosive to the hands like lime or cement plasters, it is an ideal material for moulding with the hands. Fig. 7-1 shows an example of an exterior loam wall stabilised by a lime-casein finish.
Professor Dr.-Ing. Gernot Minke is a professor at Kassel University and a consultant structural engineer since 1967. He has a keen interest in earthen structures and low-cost, low-impact housing. He numerous publications include the Earth Construction Handbook (WIT Press, Southhampton, Boston, 2000). Contact: <[email protected]>
This article was submitted by Friedemann Mahlke, a student of Dr. Minke and a straw-bale builder and researcher. Contact <[email protected]>
This article appeared in TLS #53. The topic of this issue is Moisture. It contains an extensive article about Moisture Basics and Straw-Bale Moisture Basics (by John Straube, edited by Bruce King) it also includes articles on moisture meter accuracy, moisture sensors, seismic resistance, and plaster testing.
by Rene Dalmeijer – The Netherlands
In June 2003, Jasper van der Linden, a building engineering student at the Eindhoven Technical University, Eindhoven, The Netherlands, tested the sound isolation of an earth-plastered straw-bale wall. Rob Kaptein of RAMStrobouw and I assisted in carrying out the test. The test was executed in a true acoustic test chamber according to ISO 140-3. We were able to execute a consistent test giving a good indication of how well a plastered straw-bale wall retards sound.
Based on the outcome of the test, it is to be expected that a reasonably well-designed and built straw-bale wall without acoustic defects (like protruding post-and-beam members) will perform in the region of 53dB and upwards (55dB with A weighting; “A-weighting” means the impedance is corrected to approximate human hearing sensitivity, which varies depending on frequency). The 2dBA increase in performance when compared to the test is mainly because we used very thin (worst case) plaster thickness in the test sample. Normally earth plaster finishes would be thicker. This puts the performance of a straw-bale wall at more or less the same level as a decoupled brick cavity wall and even exceeding it in the critical low-frequency region.
Most everyone who has been in a straw-bale building has had the sensation that interior sounds somehow seem louder, because interior sounds become more distinct for not being drowned out by background noise coming from the outside. This is a clear indication that straw-bale walls work very well as an acoustic insulator. Normally built structures depend on high mass for good sound insulation. But there is also another way of achieving good sound insulation, which depends on a damped cavity surrounded by two not-sostiff membranes with sufficient mass. A straw-bale wall, specifically with earth/clay plasters, is an excellent example of this alternative way of achieving good sound insulation, as the test result clearly illustrates.
The test was executed in the acoustic lab of the Eindhoven Technical University. The test and test facility is according to ISO 140-3 which is to test the sound isolation of building aperture of two acoustically separated chambers (the test sample is placed in an aperture between the chambers). Although I am aware of the limitations of the test facility for testing a wall system, we have endeavored to make this test as accurate and as representative as possible. The aperture’s size (ISO 140-3 std) is 1.88m2 /20 ft2. The tested straw-bale wall section had the following configuration:
- Two-string (460mm wide building quality bales laid flat density 120-130kg/m3)
- Earth/clay straw plaster 25mm and 35mm (intentionally asymmetrical cover)
- No reinforcing plaster netting or mesh or any form of pinning
The chosen sample structure was to be as representative as possible of a normal earth/clay plastered straw-bale wall structure as used by the experienced straw-bale builder Rob Kaptein of RAMstrobouw. Rob was also responsible for manufacturing the test sample. The graph and table summarize the test result.
[Rene’s comment on the measured performance: The result can be expressed as 53dB according to A-weighting. Actually expressing the sound isolation value in one number (i.e., 53BA) is a simplification. In actual fact, giving the performance at each of the various frequencies is much more meaningful.]
Generally this is done at either one octave intervals (1/1oct) or at one-third octave intervals (1/3 oct), the last giving even more detailed information.The graph and table show both measurements (not A-weighted). The dip at around 250Hz is due to the transition between the masws and damped cavity odes of operation of the test sample and should be largely disregarded as part of the vagaries of a test.
The 53dBA test result might seem low but in fact is very good. Most conventional wall systems including a brick cavity wall with much higher mass have a lower performance. Specifically interesting to note is the 2-3dB better performance at very low frequencies of the straw-bale test sample when compared to brick-wall systems. Nearly all wall systems, including stick frame, are able to sufficiently subdue high-and mid-frequency sound, but low-frequency sound is problematic. In practice, better performance at low frequencies is worthwhile because it means that the ever-present background noise in suburban areas is perceptibly reduced.
Recipe for Straw-bale Wall Acoustic Isulation
Besides sheer mass, low stiffness with sufficient mass and acoustic decoupling are very imortant for acoustic sound insulation. The relatively low stiffness of a straw-bale wall with earthen plasters is ideal. The fact that the cavity between the two plaster shells is filled with straw provides excellent acoustic damping. Beware and be careful to fill all cavities and voids with very light straw/clay. Avoid any direct mechanical contacts between the inner and outer plaster shells, as these will seriously degrade sound damping performance. Contrary to what you would expect, loosely packed bales will perform better than very tightly packed bales. Extra thick (>35mm) earth plaster specifically improves low-frequency performance. Cement and lime plasters perform almost as well but earth plaster with lots of straw is the best due to a lower modulas of elasticity (stiffness). Applying significantly asymmetrical plaster thicknesses helps to avoid coincident reverberation of the inner and outer plaster layers. The thicker plaster layer should be on the sound source side of the wall. Pay a lot of attention to all openings and edge details; these are the weak points. An air leak of only one sq. mm will seriously degrade performance. Door openings and windows are literally acoustic holes in the wall; these need special detailing and attention to even remotely approach the acoustical (and thermal) performance of the surrounding walls. Even double doors generally show poor performance compared to the wall. The gaskets and seals in the doors should be double or even triple, but even then there is a problem as, over time, the seals will degrade and leaks will occur. The type of door you are aiming for is more like a steel watertight door in a ship than a house door with multiple closing bolts and tightening clamps. (All of this only if acoustical performance is essential.)
In conclusion, I would like to emphasize that, due to the nature of a straw-bale wall (an excellent sound barrier), the wall is not the problem; the connections between the
wall and all other elements incorporated or surrounding it are. In other words, it is the same issue as with thermal and moisture performance. I strongly suspect that most sound isolation tests executed on straw-bale walls are measuring the defects of other structural components or mistakes in the test procedure (a non-calibrated sound source, background noise, and such).
Here are some simple rules of thumb depending on the type of acoustics you want, e.g., very lively to very well damped. Soft acoustic instruments require a “live” (reflective) room. Loud amplified sound needs a “dead” (absorbtive) room. The single most important parameter is the reverberation time and level. The harder the surfaces, the livelier the sound. A bathroom is lively, hence your strive to sing even if you can’t. The opposite is standing on top of a snow-bound hillock [small hill or mound] – virtually no sound reflects back to your view. The bigger and harder the room, the longer the reverberation time, e.g., a cathedral. Next the relative dimensions: an oblong box (like Concertgebouw Amsterdam) approaches the ideal. Preferably the relative dimensions are approximately 2 to 3 to 5; this ratio will avoid the formation of dominant harmonic resonance and standing waves. The exact ratios needed for a given acoustical requirement depend on the size and acoustic reflectivity. I personally prefer rooms without parallel surfaces, thus avoiding standing waves. I think if you finish a room with earth/clay plaster on straw-bale walls, with wooden flooring and a well-pitched ceiling, you will have quite acceptable acoustics for musical performances. If it’s too lively, you can always add some damping afterward by placing soft furnishings in the room or hanging curtains on the windows. A bigger audience also helps.
Good acoustic isolation is definitely one of good merits of straw-bale walls. It should be seriously considered for purposes where sound isolation is of importance. It would be hard to find a more affordable solution to building sound studios, quiet houses in noisy neighborhoods, or noisy workshops in residential surroundings.
Rene Dalmeier has been interested in straw-bale building since 1998. In June 2005, he finally took the plunge and turned his hobby into a profession by becoming a full-time straw-bale builder.
A whisper = 15 dB … Normal conversation = 60 dB. dB: Abbreviation for decibel(s). One tenth of the common logarithm of the ratio of relative powers, equal to 0.1 B (bel).