I wanted to run something by everyone to see if this is how most people
model underground walls:

For my climate zone (4A), the required C-factor for Underground Wall is
1.140 (no U-value listed).

If you go to Appendix A of 90.1-2004, specifically sections A4.1 and
Table A4.2, I *think* that you come to the conclusion that the baseline
underground walls should be modeled as 8 inch medium weight concrete
block with a density of 115 lb/ft3, solid grouted cores, and NO
INSULATION. This is what A4.1 says, and the published C-factors in
Table A4.2 indicate that a C-factor of 1.140 is obtained using this wall
and no framing / no insulation.

If you then refer to Table A3.1C, for that wall type, sure enough a
C-factor of 1.140 is listed with a corresponding U-value of 0.58.

When I run my baseline model with an underground wall construction
having a U-value of 0.58, I end up with way more cooling hours and
energy that my proposed model, which has R-13 insulation. This is
presumably because those underground spaces are constantly losing heat
thru the poor wall construction to the 55 degree earth heat sink (or
whatever eQuest calculates it as).

I'm not complaining, as it makes the proposed system look that much more
efficient, but I just wanted to double check that I'm doing this right.

On a side note, I'm having a tough time reducing my unmet load hours for
the underground VAV system, but that's besides the point...

Thanks!

James Hansen, PE, LEED AP