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GSBN:Moisture Problem


Early in our bale wall moisture monitoring I noticed the initial high moisture spike in the standard  five moisture sensors immediately after the stucco was applied. The building grade bales generally arrived on site with a moisture content between 9% to 12%. A few days after plastering the moisture content at the sensors would be around 18% then begin to decrease steadily. Nearly all these homes were finished immediately after the stucco work and the families moved in. 

Sensor readings after the initial Canada Mortgage and Housing Corporation study were then randomly taken as requested by the owners. This summer I tested one home prior to its sale. The house has been occupied since completed in 1997, the five moisture readings were between 7% to 9%. The house is featured on pages 166-171 of Catherine Wanek's "The New Straw Bale Home."  It sold for $450,000CND.

Two years ago master plasterer Gordon Askey elbowed his way into our stucco work. His 35 years experience as a stucco contractor and innovator has proved invaluable to our clients. He has concerns about late season plaster work in unheated buildings due to the moisture load he feels the plaster puts on the building. I mentioned this discussion to him this morning and he was very clear about the need to gently heat the buildings in the fall and early winter, particularly when the night time temperatures drop to minus 5 degrees Celsius. Below 5 degrees he does not work outside. In the fall, if there is any delay in installing the heating system in the newly plastered bale home, he uses portable electric heaters and two dehumidifiers per house to protect the bales. He did this for his first two bale home projects and was astonished at the amount of water collected daily from the dehumidifiers. Gordon observes the steady drying of the exterior plaster, even in cool conditions, he worries about moisture trapped within the building.

The standard stucco product in this region by Lafarge; it has a composition of 70% Portland cement and 30% lime. It dries significantly over night and often the walls need dampening before the second coat is applied. To avoid cracking of the second coat by the suction of the first due to the drier initial layer, Gordon "double backs" the plaster, a technique he learned as an apprentice. We divide the job into workable segments with expansion joints on the outside walls and use the interior posts as dividers. We spray this section with the carrousel pump, level and edge it with trowels, scratch it with a broom, then stop for lunch. We respray the same wall section again in the afternoon, often with integral colour and float finish it. This system has drastically reduced the cracking in the surface layer. 

Rain splashback off scaffolds has been a problem which we now avoid since we hang tarps from the eaves of the building to prevent extreme weather, either rain or  direct sun from damaging the new plaster. Our experience with conventional stucco is it often dries quickly and has to be moistened between coats. This is not the case with earthen plasters, which often take weeks to dry between coats, allowing the bales to absorb increased amounts of moisture.

Yesterday an architect friend called from Washington state and I mentioned this discussion to her. She too has concerns about clients who do not heat their homes after the plaster is applied in the fall. She has seen earthen plasters mould over winter due to this excessive moisture. 

Good ventilation, dry materials, safe placement of the building on the land, best construction practices, special considerations to the conditions we create with late season plastering, all important aspects of this work. 

My thanks to you all.


Sustainable Works
Habib John Gonzalez
RR#1, S-4, C-12
Crescent Valley, British Columbia
Canada, V0G-1H0
tel/fax 250-359-5095
"Better the kindness of imperfection than perfection without kindness"

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