[Bldg-sim] C-factors

[Keith Swartz]

In basement rooms that I have modeled, additional insulation usually has little impact if the space has internal loads that require cooling. The reduced heating energy is offset by additional cooling, since you can no longer get as much cooling from the "cold wall." However, if the room is only heated (no cooling), then insulation makes a big difference. I discovered this when I noticed heating during the summer. (The summer heating went away when I lowered the thermostat setpoint from the default setpoint. It was a utility room.)

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Keith Swartz, P.E., LEED AP

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From: James V Dirkes II, PE [mailto:jvd2pe at tds.net]
Sent: Tuesday, March 30, 2010 7:32 AM
To: 'James Hansen'; bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] C-factors

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......

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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!

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