Rice Hull Bagwall Construction
The southern part of the United States has plenty of rice straw.
However, the process for shredding it is expensive. Fortunately,
the industry has another by-product that does not require preparation,
and can be obtained year-round at a moisture content of 12-percent.
Rice-Hull Bagwall Construction |
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The rice hull contains opaline silica in a concentration of
20-percent, combined with significant amounts of lignin. This
combination of lignin and silica makes excellent insulation when
a house is wrapped in a blanket of rice hulls. In order to do
this, a large floor is needed. The roof and wall cavities are
created by combining roof, wall and floor trusses.
The floor truss often used is a 12-inch open-web structure, also
referred to as a posi-strut or spacejoist. The walls are made
of 12-inch trusses, and conventional trusses are used for the
roof. It is vital that the floor joist be open-web in order to
avoid creating back pressure when the rice hulls are blown into
the floor cavities.
The spacejoist (with OSB on the bottom) creates a floor cavity
of 12 inches. A rice hull structure cannot be built on a concrete
slab, since in hot, humid climates concrete attracts condensation.
This means that the house would have to be equipped with costly
air conditioning.
The wall trusses are built
using two 2x4s, with the outer edges 12 inches
apart, fastened by three gusset plates made of plywood located
at the top, bottom and middle. These trusses, spaced 16 inches
apart, can withstand even hurricanes. The exterior wall should
be constructed of 4x10-foot fiber-cement panels.
OSB is not needed.
The panels are much easier to mount than lap siding. Since there
is a gap of five inches between the two studs of the wall truss,
you do not need to drill holes in the studs in order to run electrical
lines. The gap also breaks up the transfer of sound and heat,
and the truss acts as a unit, allowing the wall to handle high
winds.
You can insert the rice hulls by hand or you can use a powerful
blower. Now you have easy, cheap insulation, but you still have
to be concerned with the sun's radiant energy. This is achieved
by covering the attic with barrier foil. The foil is installed
over the roof trusses, face-down.
Then horizontal 1x4s are
anchored to the trusses. This is followed by sheets
of corrugated metal. Again, OSB is not used. Then the attic is
filled with 12-16" of rice hulls. Ridge vents and soffits
cause heat to dissipate, so even in the absence of air conditioning,
there is no condensation.
Rice hulls, unprocessed, are a Class 1 or Class A insulation
material. They emit no odor, so they pass the odor emission test
easily. When tested for moisture vapor, they gain only 3.23% weight,
so this test is easily passed as well. They are also non-corrosive,
creating no perforations or holes in copper, aluminum or steel.
They are highly resistant to fungal growth. On the ASTM E84 surface
burning test, the results are nothing short of amazing. The building
code requires a fire spread index of 25 or lower, and a smoke
development index of 450 or lower. Rice hulls have a FSI of 10,
and an SDI of 50.
Conclusion
In the United States, over a million metric tons of rice hulls
are produced in any given year. They are often available at no
cost.
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