To the original post, I would use the line of the wall and ignore the columns. If you feel that there is a considerable shading effect, then use side fins or a building shade. Look at a section of wall the width of one column spacing as calculated the combined U value and thermal mass of the wall to see if the columns will have enough of an effect to make it worth modeling. I don't know how large the columns you are referring to are but it is likely that they would have a negligible effect on the model as a whole, especially when you consider the uncertainty of other values that are input. There is a line in Table G3.1.5 saying that other envelope assemblies that cover less than 5% of the total area need not be explicitly model and can be averaged into other wall assemblies.

As to Jeremiah's post,

* ASHRAE 90.1 does not require thermal bridging to be modeled in the baseline.
Thermal bridging should also be taken into account when calculating wall and floor U Values. External un-insulated assemblies must be explicitly modeled.

* ASHRAE 90.1 does not require internal mass to be modeled in the baseline.
I don't think any internal constructions are addressed in 90.1. Generally the effect of any internal walls will be negligible compared to the external walls unless your typical wall is heavy weight like a concrete block wall. Some schools are like this. Either way I would say internal mass would not need to be included in the baseline since the baseline is a metal framed building, which implies GWB internal walls.

* ASHRAE 90.1 does not require self shading to be modeled in the baseline.
* ASHRAE 90.1 requires a light weight wall assembly for the baseline.
The baseline should used a metal framed wall, which still has some mass effects that are required to be modeled. The way I understand it, a "light weight" wall refers to neglecting the mass effects of walls in your calculations.

Brendan Hall

Brendan,

I expanded on some of the points you made below.

Mike

Sent from my iPad

FYI.

ASHRAE RP1468, just finished, which provides BIM-to-thermal modeling examples, contains example modeling DOE2 input files and BIM files for concrete thermal bridging, and other examples like curved windows, etc.

The final report can be I obtained from Mike Vaughn at ASHRAE.

Jeff S. Haberl, Ph.D., P.E., FASHRAE, FIBPSA

Patrick,

I frankly think that you're looking at the modeling of building geometry
in simulation programs too literally. Please keep in mind that in the
end all
the simulation programs are just doing hundreds of one-dimensional heat
flow calculations through thousands of time steps. Wall thickness is needed
only to compute the heat flow characteristics (U-value, response factor,
transfer function, etc.) but is not modeled explicitly in the
simulation, i.e.,
walls have no thickness in the simulations. Similarly, the space volume
is used only to determine the amount of air participating in the space
heat balance, so therefore you should always input the actual enclosed
volume, even if it does jive with the dimensions of the enclosing
surfaces. If you're concerned about the shading effect of the columns,
they can be model them as narrow vertical building shades with widths
equal to the protruding part of the columns, but if the walls are
opaque, the effect must be extremely small.

However, I think you're ignoring what I think is the most significant
heat transfer aspect of these columns, which is the two-dimensional
conduction through them, because the columns act like stubby fins on the
wall, particularly if they're solid concrete with no insulation. That's
where I would concentrate my energies if I wanted to get the modeling right.

To do that, you would really have to model the wall-column-wall assembly
with a 2-D conduction program like THERM (available from LBNL), although
that was written for modeling window frames and can only output a
steady-state U-value. I still have a 2-D conduction program written in
Fortran called WALFERFN (sounds German, doesn't it ? but the acronym
stands for WAll Finite Element Response Factor New :-) ) that I use to
calculate response factors for composite walls, but that can only handle
planar surfaces.

Joe Huang

Patrick,

Those are some mighty columns!

I think it depends on what you're trying to achieve with the model. If you're looking at radiant temperatures, local thermal comfort or similar, then I would agree that taking an area weighted approach is going to neglect important local differences due to thermal inertia and other factors. However, if you're looking at the energy use of the entire building, and it's conditioned 24 hours, or in a climate with negligible diurnal swing, or with limited thermal mass generally, I would still say that a simplified approach would give you results that are accurate enough. I concur with Joe on the point about thermal bridging. Particularly if the remainder if the fa?ade is insulated. Most construction inputs in energy modelling software don't accurately account for thermal bridging, so if you think they're going to be significant, then use a separate 2D calculation software and adjust your u-values appropriately.

That said, as they're relatively large and it's easy enough to divide up the walls into separate constructions, as it is with software like IES, then by all means model them separately and apply a different construction.

Much of the importance of the above is also dependant on the climate your build is in. E.g. Un-insulated concrete columns in a northern European climate are a terrible idea and will come with a big energy penalty. In warmer climates, their contribution to the overall energy of the building will be much smaller and your efforts on the accuracy of their modelling may be better spent looking at something like the fresh air or building leakage.

Regards,

Edwin Wealend