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.
Ryan Del Balso
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 CATON, E.I.T.
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
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