[Strawbale] Pressure-treated Wood Foundations ( PWF) (was re: Frost heave)

[Robert Tom]

Shody Ryon qi4u at yahoo.com wrote:

> It seems that for a super insulated building a wood
> floor and foundation would be a better choice than
> concrete;


>  with good moisture management

And therein lies the rub.

Wood set below grade is put into conditions where rot naturally wants to  
occur.

So they treat the wood with chemicals in an attempt to prevent that rot  
 from occuring.

That treatment is forced into the wood under pressure and for PWF, the  
depth of penetration of those chemicals is (I forget the exact depth but  
close enough for this discussion) only something like 10mm or less.  The  
depth of penetration for lumber intended for decking etc is leven ess,  
only something like 4 mm or so.

That might be fine if wood didn't do things like split/check or get dinged  
during construction, all of which could create a path through the treated  
outer layer by which the microbes and bugs which eat and cause wood to  
decay, can get into the untreated portion. And once they get in, why would  
they not destroy that untreated core ?

"But PWFs are usually wrapped around the outside by a waterproof membrane  
so that water from the surrounding earth can't get at the wood " you say.

Right.

And moisture from the heated interior wants to try and escape and if it  
gets past the interior air barrier, and into the insulation of the PWF, it  
bangs up against the backside of the exterior moisture barrier whose  
location in the thermal profile violates the first Rule of Thumb for  
insulation ... a vapour barrier must not be located on the cold side of  
the insulation because it will result in condensation at that plane and  
that condensation will not have any means of escape.

And that's why you see, with many (if not all) PWFs that are between 10  
-20 years old, big heaps of mouldy drywall and insulation piled up outside  
after having been ripped out because people in the house were getting sick.

The moral of the story is that using moisture-susceptible materials in an  
environment where there is no escaping the consequences of moisture  ie  
foundations (which is not the same thing as the moisture that is conducive  
to creating conditions in which frost heave will occur)
is seldom a "better choice".

Concrete is a good material because it does okay in such environments but  
concrete's high embodied-energy (EE) makes it a less "good" choice from a  
Greenie POV.

But concrete masonry units (CMU) unlike monolithic cast-in-place concrete)  
can be easily deconstructed at the end of the building's life and re-used,  
any number of times, with no real degradation to speak of, so when their  
EE is distributed over the lives of several buildings, they look pretty  
good from a Greenie POV.

But there are other choices, other than wood or concrete. It's not a "one  
or the other" proposition.

-- 
=== * ===
Rob Tom
Kanata, Ontario, Canada
< A r c h i L o g i c  at chaffY a h o o  dot  c a >
manually winnow the chaff from my edress in your reply