Civil Engineering Reference
In-Depth Information
Airtightness
Not only that, but strawbale builders are
usually very conscientious people who have
chosen this type of construction for very
sensible reasons, and when we run courses
to help them build, we use a process that
involves each person in a thoughtful and
responsible way. Building with this sort of
awareness can create much better-quality
buildings. We pay attention to the process
of building as well as the function, and it
produces amazing results!
The airtightness test is used to determine
a building's air leakage, and basically
determines whether your building has
been designed well, and then built well.
Obviously, heat will escape from anywhere
that air does. For the test you have to
close all the windows and doors and use a
machine that sucks air out of the building.
The rate at which air moves, and is therefore
being sucked into the building through
unintended gaps, is then measured. Most
problems in standard designs occur at the
junction between the roof and walls, around
windows and doors, and around electrical
boxes. Buildings larger than 15m
3
need to
be tested and their air leakage proven to
be no worse than 10m
3
/hr/m
2
at 50Pa. As
far as we are aware, only one UK strawbale
building has been tested to date, and the
tester said it was the most airtight building
he had ever tested, at 1.56m
3
/hr/m
2
! We
are not at all surprised, and would expect
the same results from all our strawbale
buildings. We have some very good design
details for straw that are now being used in
mainstream construction.
Log book and energy ratings
New buildings are now required to have a
log book. This details lots of information
about the building, including the type of
construction. It also shows the calculations
based on type of insulation, floor-to-window
area ratios, U-values of different components,
etc., which combine to produce a figure
showing how energy-efficient the building is
- the SAP (Standard Assessment Procedure)
rating (or similar). The log book for a straw-
bale building would also include details of
how to maintain the walls and plaster/render,
what to do in case of accident, e.g. water
penetrating the bales due to storm damage
on the roof, etc., how to fix things to the walls,
and how to make extensions or alterations.
Most problems with airtightness occur
because of poor-quality work and lack of
understanding of the reasons for following
designs accurately, coupled with using out-
dated designs that can't really be improved,
such as the cavity wall system. It's common
for electricians installing electrical back
boxes to break through the internal concrete
block skin into the cavity of the exterior wall,
thus exposing the building to air leakage via
the cavity. Monolithic wall systems such as
strawbale ones can never suffer from this
defect and are therefore inherently better,
reducing the potential for poor-quality work.
And finally . . .
Lastly, a word of caution about Building
Codes, Building Regulations and building
practice in general.
You need to be careful about what you
read in topics and on the internet about
