[Bldg-sim] Where should you choose your wall location forsimulations, inside, outside or center of wall

[Chip Barnaby]

All,

I generally agree with Nick's approach.  I said the same thing in the 
ASHRAE HOF Residential Loads chapter (which I revised in 2005).  For 
residences, this scheme reconciles with US practice for determining 
floor area (a nominal 1000 ft2 house is 1000 ft2 only if you include 
the exterior wall thickness) -- that makes it easy to check input.

Volume is overstated, as noted, but generally only minimally.  I've 
also seen procedures for doing blower door leakage tests that 
recommend estimating volume from outside dimensions.

All in all, then, the outside dimension method is simple to apply 
consistently, easy to check, and accurate enough.

On the other hand, it does NOT agree with other standards and 
procedures that are in use.  There is an ASTM standard that defines 
floor area (I think) and uses inside dimensions.  Also, I think 
rentable space is sometimes measured from the window inside surface 
(which I guess is the real estate version of split the difference?).

As automated data exchange procedures become more ubiquitous, 
definitions like this will have to be rigorously standardized so 
building model fidelity can be retained as data is passed among 
applications.  There is an ASHRAE project getting underway related to 
consistent extraction of a thermal model from a BIM.  That is a step.

Chip Barnaby

At 01:14 PM 9/11/2009, Nick Caton wrote:
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>My practice with all load calculation/energy modeling software has been:
>
>All floors/roofs: Top of surface
>Exterior glazing/walls: Outermost surface
>Interior walls/partitions:  Center of construction
>
>I know this is the advice offered by IES-VE's support staff, but 
>that doesn't mean all modeling engines behave the same.  As you may 
>gather, I've always assumed energy modeling and HVAC load 
>calculation models "subtract out" the thickness of constructions 
>from calculated space volumes.  I don't know whether this is a 
>"universal standard practice" between software packages / modeling engines.
>
>Since I would ultimately much rather slightly oversize than 
>undersize an HVAC system, I'm of the mindset that the potential for 
>a little extra surface area / conditioned space would if anything 
>bump my results in a desired, conservative direction regarding space 
>loads.  If absolute accuracy is your goal, then in lieu of a 
>"universal standard," I'd imagine you would have to ask the 
>developers/users of the specific program you're using at the moment 
>to determine its true behavior in this regard.
>
>~Nick
>
>cid:489575314 at 22072009-0ABB
>
>
>NICK CATON, E.I.T.
>PROJECT ENGINEER
>25501 west valley parkway
>olathe ks 66061
>direct 913 344.0036
>fax 913 345.0617
>Check out our new web-site @ <www.smithboucher.htm>www.smithboucher.com
>
>From: bldg-sim-bounces at lists.onebuilding.org 
>[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Ryan Del Balso
>Sent: Friday, September 11, 2009 11:50 AM
>To: bldg-sim at lists.onebuilding.org
>Subject: [Bldg-sim] Where should you choose your wall location 
>forsimulations, inside, outside or center of wall
>
>Are there standard practice recommendations for where to place your 
>wall for a simulation?  Knowing every wall has thickness, but the 
>simulation engines are generally just using no thickness walls with 
>a calculated U/R value, where should you define your walls for the 
>simulation?  Should they be defined at the exterior face of the 
>wall, the interior face or the center of the wall.  It seems the 
>exterior face can greatly increase the zone volume, thus incorrectly 
>determining the energy usage, but the interior face underestimates 
>the exterior exposure of the wall.  The center of wall seems a good 
>compromise, but is difficult to identify and draw.
>
>For interior walls/zone delineation the same question applies.  From 
>an energy standpoint is it better to place the zone boundary on the 
>face of the higher load zone or the lower load zone or split the difference?
>Thanks for any advice or direction to standard practices.
>
>
>154683_logo_final
>
>
>Ryan Del Balso
>Building Performance Engineer II
>ryan at ambient-e.com
>130 W. 5th Avenue, Denver, CO 80204
>t 303.278.1532x210 | f 303.278.8533 |
><http://www.ambient-e.com/>ambient-e.com
>
>
>
>
>
>
>
>
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---------------------------------------------------------
Chip Barnaby                   cbarnaby at wrightsoft.com
Vice President of Research
Wrightsoft Corp.               781-862-8719 x118 voice
131 Hartwell Ave               781-861-2058 fax
Lexington, MA 02421         www.wrightsoft.com
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