Civil Engineering Reference
In-Depth Information
strings on a bale and make a loose pile of
the straw, it would burn very easily as it
contains lots of air. Therefore it is essential
to clear loose straw from the site daily, store
straw bales safely, have a no-smoking policy
on site, and protect the site from vandalism.
If a wall is to be unplastered for a while, be
sure to trim it, getting rid of the 'fluffy bits'
that would encourage the spread of flames.
The straw bales hold enough air to provide
good insulation value but because they are
compacted firmly they don't hold enough
air to permit combustion.”
Report to the Canada Mortgage and Housing
Corporation by Bob Platts, 1997
“ASTM tests for fire-resistance have been
completed. The results of these tests have
proven that a straw bale infill wall assembly
is a far greater fire resistive assembly than a
wood frame wall assembly using the same
finishes.”
Report to the Construction Industries Division
by Manuel A. Fernandez, State Architect and
head of Permitting and Plan Approval, CID,
State of New Mexico, USA, 1997
Once the straw is built up into a single bale
wall it tends to behave as though it were
solid timber, particularly when it is load-
bearing and therefore under compression,
but also when used as infill. In a fire, it chars
on the outside and then the charring itself
protects the straw from further burning. It's
like trying to burn a telephone directory - if
you tear loose pages from it, they will burn
easily, but if you try to set fire to the whole
topic, it's very difficult.
Structural performance
When the wall is plastered both sides, the
risk of fire is reduced even further, as the
plaster itself provides fire protection.
The requirements laid down in 'Document A:
Structure' are for brick, concrete or timber
walls. You will find no guidance here for
building strawbale walls. This does not mean
it cannot be done! Research in the USA has
shown that structural loadbearing strawbale
walls can withstand loads of more than
10,000 lbs/sq ft, equivalent to 48,826 kg/m
2
.*
For the purposes of Building Regulations,
a wall built of any material that is covered
with half an inch of plaster has a half-hour
fire protection rating, which is the require-
ment for domestic buildings. All the fire-
testing research done on strawbale walls
concludes that this type of wall-building
system is not a fire risk. A list of research
documents can be found in Appendix 3 and
on
The Last Straw
website.
There is no doubt that loadbearing straw
walls can withstand greater loads than will
be imposed on them by floors, roofs and
possible snow loading. It is the design of
associated timber work, the even spread of
loads around the walls, and the quality of
building that is crucial here, not whether the
straw can do it.
Research in the USA and Canada reached
the following conclusions.
“The straw bales/mortar structure wall has
proven to be exceptionally resistant to fire.
With infill walls, in timber-frame structures,
the straw does not take weight anyway
* Ghailene Bou-Ali (1993)
Results of a Structural Straw Bale Testing Program.
MSc thesis published as a summary by the
Community Information Resource Centre, Tucson, Arizona.
