[Cob] rubble trench foundation questions

[Amanda Peck]

I don't have a good handle on what you need structurally.  The time I did 
one, the drain--only about 18 inches down--did drain to daylight, which is 
the whole point of having a drain there.  On top of that we tamped smalish 
gravel until the ground rang, then foundation/stemwall, then building.  But 
our posts were a couple of feet OUTSIDE of the building so the overhang was 
well supported.  So far, it's OK, and it was log, not straw bale, and we are 
having fewer and fewer serious winters (now that I've said that, I may be 
able to try out the snowshoes next year!).

My soil was a mixture of silt blown many hundreds of miles from the Dust 
Bowl (according to the septic tank inspector) and rocks.

Check on AGS or PAHS.  AGS isn't so space-consuming.  The idea is that you 
use up summer heat in the winter, when you run cold in for the summer.

www.greenershelter.com



.........

James wrote:

Hi, here in N central Washington we are building a post and beam straw bale 
house with cob interior walls and floor. Local inspector okayed a 2'X2' 
rubble trench foundation with 4"drain pipe, with the posts on 10"X34"X34" 
footings integral to the inside edge of this trench, but when we started 
digging we found one corner of the house 2' lower (so our "trench" ended 
being at grade!) We plan to berm it with 2' of loam up against the 3' of 2" 
rigid foam on the outside surface (top 1' being a 12" X16" bondbeam), and 
have gravel on the inside (sub floor - insulated only for the first 8' on 
the southside of the living rm. with 2" foam under the floor). My question 
is; should Can you run we dig deeper on the low side (hard to do at this 
point, and just increasing the thermal mass more - see concern below)? or 
insulate more (1"X24") with a winged/skirt around the whole 
perimeter(expensive but doable)?

Some factors; the site is dry but on fine clay below 2' of loam, I'm 
conerned about the trench and the post footings being just on this loam. I'm 
also conerned about the increased thermal mass that the extra gravel fill 
has added to the house. Should I insulate under the whole house to reduce 
this mass (with 1.5" foam)? - we have a great passive solar site but it is 
the great NW after all and often snowy/cloudy in the winter with temps down 
to -35 F.... I'm worried about the house becoming a cold heat sink, only a 
good thing in the 100+ degree summers.

Any thoughts, or similar experiences? Thanks for your help - in advance! 
James