|Straw Bale Building (SB)|
What is the insulation value of a straw bale wall and would it really save you in heating costs?
R-value varies from 30 to 40. Canadian Society of Agricultural Engineers, Halifax, NS
"Straw bale construction, along with appropriate building conservation technologies and simple passive design could provide up to a 60 percent reduction in building heating loads over current practice." US Department of Energy 1995.
Won't mice and other pests eat the walls?
No. The bales you should be using should be straw, NOT hay. Straw refers to the chaffs of the grain. Hay still has the grain on them. Do not use hay bales!!
Won't it burn easily?
Fire needs oxygen to burn. Bales are generally packed so tight that fire cannot catch. Also, the plaster often 'bakes' like a clay pot, preventing the fire from even reaching the straw.
"Bale walls withstood temperatures up to 1,850F for two hours." Fire safety tests, National Research Council of Canada.
"The bale panel was tested for over two hours and withstood temperatures that reached 1,942F. The temperature rise on the unheated side averaged less than 10F." ASTM E-119 Fire Test, SHB AGRA, New Mexico, 1993.
Chris Magwood - Straw Bale Guru.
the walls rot?
"Straw bale walls do not exhibit any unique propensity for moisture retention. It is clear that straw bale walls can function, without incorporating an interior vapour barrier, in northern climates." Straw bale Moisture Monitoring Report, submitted to the Canada Mortgage and Housing Corporation (CMHC) by Rob Jolly, 2000.
Are straw bale homes cheaper to build?
Loaded question. Its all about what you want from your home and what you are willing to do (physically) to get it. As with all other types of construction, much of the cost of building is in the labour. If you, family and friends do most of the work, that would cut down costs. Also, finish materials effect the final cost of any house. I have seen straw bale homes that costs as little as $35/sq.ft. and as much as $150/sq.ft.
Can straw bale really make a difference to the environment?
"The embodied energy for the conventional frame house was 509,000 Btus. The embodied energy for the low-impact straw bale house is 41,000 Btus, or about one-twelfth that of the frame house." Investigation of Environmental Impacts, Straw Bale Construction, by Ann V. Edminster, University of California, Berkeley, 1995.
Bales being placed on foundation. Bales have been 'French dipped' meaning that the first slip-coat of clay/sand has been pre-applied by dipping the bales in the slip before stacking. The slip is usually applied after the walls are dry stacked.
construction is not new. Within North America, one of the first documented
SB buildings was erected in Alliance Nebraska over 100 years ago. SB is
part of a larger group of earthern construction methods that include cob,
adobe, rammed earth and light straw-clay. All these methods use the same
materials, namely straw, clay and sand in varying degrees and mixes. Earthern
construction is part of the larger natural/alternative building movement
which would also include timber framing, cordwood, earthships, earthbags,
hempcrete, papercrete and waste-product bales such as paper bales and
plastic bales. SB construction is still in its infancy with a lot of potential
and new methods and ideas that are just beginning to sprout!
Straw refers to the stalk of a cereal plant which is found between the roots and the grain head of said plant. Straw is different from hay in that hay refers to the straw plus the grain head. This is a very important distinction that often confuses people and their assumptions about SB (addressed later).
Straw is a very strong material and has a higher tensile strength than softwood. As for the type of grain, just about any kind of cereal will do: wheat, barley, oats, rye, rice and hemp are popular choices. The debate continues unabated as to which type of grain is "better". But it is generally agreed that you should use whatever is grown in your area to reduce energy use due to transportation. Generally, it is not specifically the type of straw that matters but its condition and how it was baled.
Bales refer to the mass of compressed straw that are bound in rectangular blocks made from baling machines that cereal farmers use to harvest the straw from their fields for animal bedding and mulch. After a combine cuts, thrashes and collects the grains, the baling machine gets to work in clearing the fields. Most farms produce more straw than they know what to do with and they are often stacked away in a barn or an outdoor shelter. Because of the lack of economic use for bales, they are considered an agricultural waste product.
by baling machines are tied together length-wise by polypropylene string,
sisal, hemp or wire. Depending on the size and the setting of the machine,
they may be two-string bales or three-string bales. These different sizes
of bales will depend on the farmer, the size of his/her operation and
they type of baling machine used. Other than the size difference, there
is no advantage/disadvantage in using one bale over another. Two-string
bale dimensions are roughly 14-16" in height by 18-19" in depth
and 24-42" in length. Three-string bales are roughly 16-18"
in height by 24-26" in depth and 36-48" in length. On average,
a bale would weigh 40lbs.
For the actual construction of a SB house, the bales are stacked in what is called a running-bond. This is the same way that bricks are laid with the bricks/bales alternating in their courses. Bales can be stacked flat or on-edge. Flat stacking refers to the strings facing up and hidden within the wall while on-edge would place the bales on its side with the strings exposed to the interior and exterior. There is also a debate as to which style is "better" but I personally think it a matter of what works for a particular situation. Flat stacking supposedly allows for an easier keying-in process during the plastering of the walls because the end of the straws face out and allows for the plaster to adhere easier. Conversely, on-edge bales expose the smoother face of the straw (the surface of the stalks as opposed to the ends of the stalks) and may present a more difficult plastering situation. However, on-edge bales are dimensionally thinner which saves space although no one is quiet sure what difference it makes in reference to insulation value (on-edge vs. flat). Personally, I think that given that straw bales have been tested for insulation in the range of R-30-40 the difference between a 14" thick bale (on-edge) and a 18" bale (flat) is not that significant only because its thermal capacity is so much greater regardless of size than a conventional building system (stick frame with batt insulation).
Second coat of plaster being applied to walls.
construction can be load-bearing or be used as in-fill in a post and beam
frame. And yes, they can be two stories (or more). The preference for
load-bearing or non-load bearing (in-fill) will depend on local building
codes, experience, your environmental impact and what your needs are.
Load-bearing SB uses the bales and plaster to transfer all loads, live
and dead, from the roof to the the foundation. This was the method that
the Nebraskans used and is sometimes referred to as Nebraska-style. No
supporting frame work is used (except to frame-in openings like doors
and windows) and the bale walls are compressed between the sill and a
top-plate. The appeal of this type of SB is in its ecological use of materials
(i.e. very very little lumber), its ease in construction for the average
person (i.e. stack it like you did with lego! You do not need a specific
set of skills to stack bales) and its relatively quick pace of erection.
SB is ideal for in-fill in post and beam types of construction (whether timber framed, mechanically fastened post and beam). Bales are used in the same way that exterior sheathing, insulation and interior sheathing would be used in a conventional stick framed home (but replaces all these highly processed materials with straw, clay and sand).
Other than bales, the second key component in SB construction is plaster. Plaster doesn't seem to get its due credit within the building industry. Perhaps its seen as more of a finish material and ornamental in its use but within SB construction, the key is in the plaster. Plaster in a SB building performs various functions including moisture control, air permeability control, additional thermal mass, fire protection, pest protection and as the main load carrying component of the wall system.
is interesting about straw bale construction is that it is the bale and
plaster assembly that makes the walls strong. Just like structurally insulated
panels (SIPS) made from orientated strand board (OSB) and expanded polyurethane,
bales and plaster act as a structural sandwich where the plaster acts
as the stiffest part of the assembly which carries the vertical and lateral
loads to the foundation. The bales act as a reinforcement for the plaster
Plaster, as opposed to stucco, refers to a wet-applied, mineral-based coating applied to the interior and exterior of walls (stucco refers to the exterior finish which is not structural and contains cement binders). Plaster may be made of something as 'simple' as clay, sand and straw (earth plaster and sometimes also with animal dung) to more processed materials such as gypsum, lime and cement. The plaster in a SB building is applied three-fold: base coat, brown coat and finish coat. Each successive layer gets thinner and finer in its composition of materials. Plaster can be applied by hand or with a trowel pump (depending on the type of plaster as lime is caustic and can damage skin).
Alas, the Three Little Pigs is modern construction propaganda!
Evolved from German wattle and daub; known as leichtlembau. Wattle and daub was a commonly used infill system between timbers in Europe and Asia. Wattle refers to thin wooden strips weaved between staves and daub or plaster, was troweled over the wattle. In light straw/clay, straw is coated with a slip of clay and packed into forms. The walls average 10 to 12 inches thick. At 12", tests have shown the walls to have an R-value of R-19. This value is equivalent to standard 2x6 walls with batt insulation. However, these walls perform better than the standard system because it is installed as a stand-alone ("out-sulation") monolithic envelope that surrounds the timber frame (therefore, there would be no thermal bridges like those found in stud construction and clay in the system acts as thermal mass that would retain and radiate heat over time). Light straw/clay in its current manifestation in North America was pioneered by Robert Laporte.
Packing light straw/clay in forms.
More Straw Bale Building
Chris Magwood, Peter Mack and Tina Therrien
New Society Publishers. 2005.
Design of Straw Bale Buildings
Bruce King et al
Green Building Press. 2006.
Creating Sanctuaries of Clay, Straw & Timber