Civil Engineering Reference
In-Depth Information
Sawdust and wood
shavings
Loose fill of sawdust and wood
shavings is timber in its most pure
form, and can be used in walls,
floors and ceilings. Sawdust is a
fine-particled, hygroscopic material
which takes up moisture and releas-
es it into the air in the same way as
timber, but at a slightly higher rate.
It also has the same resistance to
fungus and insects as timber.
Experimental buildings investi-
gated by Professor Bugge at the
Norwegian Institute of Technology
demonstrated that after 30 years the
sawdust was in perfect condition, with no sign of any deterioration. The build-
ings stood on the very damp west coast of Norway (Granum, 1951).
Figure 14.12: Thermal insulation made out of sawdust.
Thermal insulation of compressed wood shavings
Sawdust is well dried before use as a wall filler, preferably to less than 20 per cent
moisture. Up to 5 per cent of slaked lime can be added to stabilize the lime and
reduce the possibility of insects getting in, also making it less attractive to mice and
rats. Using quicklime produces a continual drying process, as the lime absorbs plen-
ty of moisture during slaking. This can be a useful solution if the moisture content of
the sawdust is greater than 15 per cent, but quicklime is highly corrosive and reacts
with moisture, emitting a lot of heat. Larger quantities of quicklime can therefore lead
to fire.
To reduce the risk of fire, sand or pulverized clay can be added in proportions of 1:2
and 1:1 respectively. This is approved as non-flammable fill for floor construction with a
thickness of 10 cm. Adding sand reduces the thermal insulation value. Alternative fire-pre-
venting materials are soda, borax and waterglass. Borax, or a mixture of borax and water-
glass in a ratio of 1:1 is used in a proportion of 5-8 per cent. In small buildings the need
for fire retardants is not so great. Experience has shown that damage due to fire in saw-
dust-insulated buildings is no more likely than in other timber buildings, partly because the
sawdust, due to its low weight, does not develop temperatures as high as timber (Granum,
1951).
Both sawdust and wood shavings can be rammed into walls. Loosely filled sawdust
often forms gaps in the insulation, so it should be rammed in hard by hand, making 25 cm
layers of loose fill at a time. Because of settling, refilling with sawdust is necessary every
20 years. Wood shavings, which are slightly more elastic, do not need refilling so often.
Special design details are required, e.g. under windows, to make refilling simple. It is also
an advantage if the vertical spaces within the framework are full height, e.g. in balloon
framing (see 'Structural framework', p. 232).
