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.

Macro Climate

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.

Microclimate

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.

Relative Humidity

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%.

Wetting Potential

Table Daily Humidity in relation to Temperature Changes Source: Australian Bureau of Meteorology

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.

Resources

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, <chris@newtonhouse.info> 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.

Load-bearing walls waiting preparation

by Tony Caniglia – Colorado, USA

This technique was developed to reduce the amount of fill with loose straw
or straw/clay required when the bent (rounded) sides of the bales are butted
together. The purpose is to remove the bulge on the ends of the bales so
that the bales are squared up and fit right up tight together.

Prepping the bales before stacking them can help make them nice and square.
Do this somewhere away from the house or building for fire safety, to keep
the dust away from other workers, and to collect the loose straw that will
be created.  Start with a large number of bales. Use a couple of other bales
to help hold one bale stand up on end. With your chainsaw, cut downward a
few inches next to the strings on the end of the bale and move the chainsaw
out toward the edge of the bale.  The bales may have a little roundness
between the strings, so clean that area up, too. Try and keep your chain saw
level, and don¹t hit those strings! Step back to eyeball it to see if the
bale looks square. Clean up 6 to 10 bales, then set the chainsaw down and
flip all the bales over to stand them up on the other end, and do the other
side. You may have to lay the bales on edge and, with a little jump, put
your knee into the bale or hit it with a sledge hammer if it has a curve to
it. You could also lay the bales flat on top of a bench, if you don¹t want
to bend over or want to keep the bale stable (another person working with
you can help make this work easier, too).

You may occasionally hit a string with your chainsaw, say one out of ten,
but it is easy to restring the bale. Just tie another piece of string about
16 inches long to the cut string and make a loop knot on one end. Put the
other end through the loop, crank it down (pull it tight) and tie it off.
Once square, the bales push together better and will help make the walls
more stout with less voids. This means little or no stuffing with loose
straw. When the bales are stacked, grab a 4-ft level, a couple of sledge
hammers (or other ³bale bangers² as you prefer) and get another person to
help. One of you should stand on the inside of the wall and the other one on
the outside of the wall. Smack the bales so they don¹t overlap one another
too much. Focus on getting one side as plumb as you can (for example, work
on getting the inside plumb). Now trim the surface of the bales on both
sides of the wall (inside and out) with a chainsaw or weed whacker. Be sure
to do the whole wall, top to bottom. That will help to finish cleaning up
any overlapping bales and any humps, bumps and lumps. This nice, plumb wall
will make your lathing, netting, plastering and troweling process easier,
not to mention the money you will save in stucco materials! And these
beautiful, straight walls may make your building easier to sell in the
future!

by Joyce Coppinger – Nebraska, USA

Tips about Bales
Why wait until you have the framing done and the roof on before finding
your bales. Find your bales during the planning process and well before you
begin construction. Knowing the size of the bales before you design the
building will help you determine wall spans and wall heights, perhaps saving
some of the cutting and retying of the bales, and can help you decide how to
stack the bales­ flat or on edge. Placement of windows and doors may be
easier to determine. You will even have time to select the best bales to
use, eliminating those that might have weeds and seeds, signs of moisture or
mildew, or aren’t shaped or tied well. For help in finding and buying your
bales, try these web sites:

www.strawlocator.com – At this web site, you can list the specifications for
the bales you need for your project, and you can search the listings of bale
suppliers.

www.hayexchange.com – Remember “hay” is not “straw” when searching this web
site.

www.thelaststraw.org – Click on Back Issues & CD-Rom, then scroll down to
2003 Resource Guide. The highlighted article titled Bale Wisdom-Bale Buying
101 lists 20 tips for buying your bales, information on bale orientation,
bale storage and handling bales.

When you know the size of the bales in the design process, you can
calculate wall heights so that you have full bales in each course,
eliminating the need to fill flakes and cakes at the top of the wall. You
can also calculate the placement of windows and doors so they fit readily
into the bale courses as they are stacked and/or the framing for the windows
can be spaced so a full bale fits under and above the windows and above the
doors.

Trimming the bales to eliminate the bent or folded (rounded) ends will
give you a rectangular unit to work with. All sides of the bale will have
cut stems and, when the bales are stacked, will lock together better­top,
bottom and sides. The triangular hole between bales that occurs when bales
are not cut will be eliminated, so you won’t have to stuff as much loose
straw or light straw/clay fill between the bales and bale courses. (See Tech
Tip, pg 23)