[Strawbale] Self draining plinth wall?

Dave Howorth dave at howorth....uk
Sat Oct 15 01:02:15 CEST 2011

On Fri, 2011-10-14 at 08:35 +0200, beisteiner at gmx... wrote:
>  Dear All I am in the process of designing a family house, built in the
>  load bearing compressive frame technique in Austria. So far the design
>  meets the requirements of the local building regulations. However,
>  there is one detail where there is still discrepancy. It is the issue
>  about the self draining plinth wall. Some experts (or countries) do
>  not and others make it a requirement for strawbale buildings (in
>  countries with cold climates). Does anyone know why?

I'm not quite clear about your exact concern. Is it whether the bottom
of a straw bale wall should be self-draining, or whether there is a need
for a plinth wall?

A plinth wall can fulfill several functions:

(1) In the UK, and perhaps elsewhere, there is a requirement for a
horizontal waterproof layer ('damp proof course') that normally has to
be at least 150 mm above the ground surface, to prevent moisture from
the ground entering the building by capillary action. The plinth wall
provides a convenient support for this, and clearly the wall underneath
the damp proof course must be resistant to the continual presence of

(2) There's also usually a requirement to keep timber 150 mm above
ground level though that can be avoided in some circumstances, I believe

(3) The bottom of a wall is subject to splashing by rain bouncing off
the ground, so it is useful if the low part of the wall is not sensitive
to rain.

(4) Some sites suffer occasional flooding. In such places, it is
desirable that the wall does not disintegrate!

(5) A plinth wall provides space for insulation under the floor but
above the ground, if that is the desired style of construction.

>  I am aware that  there is condensation in the straw wall during cold
>  periods, but is it  that much that drainage is necessary? How much
>  condensate is to be  expected at the base plate (ladder construction
>  with insulation)  during winter time? In case water is accumulating,
>  does it need to be  drained off or can the water sit there until it is
>  getting warmer.

I'm not sure what construction you have in mind that would cause water
to sit there? Why can't some drains be incorporated that make the
question irrelevant?

Raft foundations can also provide the necessary height above ground
level, for example.

>  For  example, Amazonails just do self draining foundations. One
>  argument  for installing those is to make sure, that water entering
>  the top of  the wall trough a leaking roof, run trough to the bottom
>  of the wall  and then drain off. So far I have not found other
>  arguments supporting  the self draining plinth wall. 

Hopefully you will understand from my explanation above that Amazonails
are complying with the local law, among other reasons.

Apart from leaks through a roof and the reasons given above, the wall
construction also needs to be able to drain other sources of water such
as leaks in the kitchen or bathroom or from water tanks or pipes, as
well as dealing with wind-driven rain if present. So a self-draining
bale wall is a very good idea, I believe.

HTH, Dave

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