Civil Engineering Reference
In-Depth Information
Technology) in 1973 the issue of condensation and sizalation was discussed at length.
There were several houses in the western suburbs that had been wrapped in sizalation to
control airflow, which would in theory reduce the energy consumption of the houses.
Undoubtedly it did reduce energy consumption, however, that was not the end of the story.
When the houses were still quite new, about two or three years old from memory, it
was discovered that there was serious deterioration of the bottom of the external wall
timber framing. Condensation from humidity produced inside the houses had condensed
against the sizalation and pooled on the bottom plate (the piece of timber that the
uprights or studs are nailed to). The bottom plates and the lower section of the studs had
rotted and had to be replaced. The roofs had to be supported and the plaster removed to
facilitate the replacement of the affected timber framing. Humidity produced inside a
house is going to go somewhere. In simplistic terms, warm air caries more moisture and
will travel toward the cooler air. Since the discovery of the condensation problems caused
by wrapping the house with sizalation, brick veneer and weatherboard houses are now
wrapped in a product called builder's wrap. Builder's wrap is not a waterproof sheeting,
as it is designed to allow limited airflow through the sheeting, thereby reducing the risk of
condensation on the inside of the wrapping, which leads to timber deterioration.
Obviously, this product is not suitable for installation in the roof where a nonporous
membrane (sarking) is required.
Given that the only enemy of straw bale houses is water, I would not even consider
trying to seal the outside walls. As the moist air travels towards the colder air outside, the
moisture will be trapped within the straw bale wall, which is undesirable to say the least.
Cement stucco
Cement stucco is ideal to give a double fronted brick veneer a new lease of life. Many of
the cream brick double-fronted homes of the fifties and sixties have been given a new
lease of life with the application of cement-based render. Even weatherboard homes have
had a render finish applied to them. Wire netting is stapled to the weatherboards and the
cement render is applied over the weatherboards without the added cost of new
foundations and brickwork. The wire netting is the key to hold the render to the house.
The results can be striking.
Cement stucco has its place
Cement stucco has its place, but I do not believe that it is on a straw bale building. I
recommend that you read an article by Gary Kruithof, 'Moisture, materials and walls that
breathe' (
Owner-builder
, Issue 117 June/July 2003). Gary raises the issue of the natural
moisture content of materials and appropriate considerations when combining different
materials, as is the case in a straw bale wall with render. I have included an excerpt from
the article below.
Typical equilibrium moisture content of different products.
Concrete 18-20 %
Wheat straw 7-9 %
Mud brick 4-6 %
(Note: the material in a mud brick is comparable to earthen render.)
