Boundary Conditions

[Lubna Al-Tameemi]

I am modeling wall to foundation in THERM, the exterior side of my section is soil (ground) what is the Boundary Condition on the exterior side in terms of the film coefficient ? Should I omit the soil in my section? or Should I put the exterior  BC between the soil and wall ? Thanks

[fostertom]

Is that ground surface, at top edge of the brown?

We're told not to try to model soil in Therm, because its temp changes dynamically with the seasons, and under the influence of the heat input from the building.

But I don't see why not, if you're sure you know that the soil holds a stable, unchanging temp, at a certain distance down from the interior floor surface.
Or distance back in from the interior wall face, if it's your retaining wall.

For example, if you know there's a water table below, or permeating water in the soil outside the retaining wall, then what temp is that water? Say 5C seasonal average?

In the case of the underfloor water table, say seasonal average 1.2m below floor level, then that could be your boundary condition - 5C at 1.2m below FL. This would extend 'to infinity' beyond the external walls - along with the boundary condition that is the ground surface, close above.

In the case of the retaining wall built against soil full of permeating water, perhaps the entire ground would be 5C, right up to back of the lower part of the retaining wall - but as you come up to the surface, ground temp against the upper part of the retaining wall will increase (or decrease), more influenced by surface temp. I could prob explain how to model that?

[Robin Mitchell/LBNL]

Here is LBNL's recommendation about how to model slab on grade construction. I have also attached a PDF with this and another example. The main issue will then be deciding what value to use for the ground temperature. 

Robin
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Robin Mitchell
Windows & Envelope Materials Group
Lawrence Berkeley National Laboratory

[Lubna Al-Tameemi]

Thanks for reply. I have a basement and it is not on grade slab. The slab is 1.4 m under the grade. If I followed example 2 from the attachment, how can I calculate the Psi factor?  I am doing details to prepare a report for a course.

Thanks

Lubna