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

[Nathan Miller]

Unless you are looking at very small spaces, I just don't see the extra
volume being added to the space as introducing that significant of an
inaccuracy to the model compared to all of the other approximations,
estimations, and outright guesses we have to make to construct an energy
model.

 

Just think of any wall that is a curved surface that most programs have to
approximate as a series of planar surfaces. You can't perfectly match the
proper volume there either.

 

Nathan Miller

Senior Energy Engineer/Mechanical Engineer

direct: 206.788.4577

fax: 206.285.7111 

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Chip Barnaby
Sent: Friday, September 11, 2009 10:37 AM
To: Nick Caton; Ryan Del Balso; bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] Where should you choose your wall location
forsimulations, inside, outside or center of wall

 

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.com 
 
From: bldg-sim-bounces at lists.onebuilding.org [
<mailto: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 |
ambient-e.com <http://www.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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