[Bldg-sim] C-factors

Tue Mar 30 05:32:16 PDT 2010

Sounds right to me.   I've wondered about the relatively low "U" for
basements, too.  My best guess for ASHRAE's rationale is that the additional
heating load is balanced by reduced cooling load..

The Building Performance Team
James V. Dirkes II, P.E., LEED AP
1631 Acacia Drive NW
Grand Rapids, MI 49504
616 450 8653

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of James Hansen
Sent: Tuesday, March 30, 2010 8:18 AM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] C-factors

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!

GHT Limited
James Hansen, PE, LEED AP

Senior Associate

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Arlington, VA  22201-4749

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