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, <firstname.lastname@example.org> 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.