change roof construction to glass

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All,

Does anyone know how to change an existing roof in detailed mode of the
model to a glass roof? It is an irregular shape (from a polygon) and
therefore creating child windows all over it will be very hard because
they will not fit in the shape very easily as rectangular windows. Can I
change its properties in the .inp file?

Thanks in advance,

Lauren Rosenthal

Lauren Rosenthal's picture
Joined: 2011-09-30
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Lauren,

I don't think you can do what you are trying to achieve in eQuest/DOE2.
There's a couple things to think about. You could effectively assign the
thermal properties of your roof construction to that of your glazing (i.e.
U-value, reflectance, etc.) but you will not be able to simulate the solar
gain properties or VLT unless the object is specified as a window or
skylight. If you go this route you will also not be able to simulate
daylight harvesting from the roof glass either. DOE2 needs to know that it
is a window or skylight in order for the daylighting controls to work.

I would suggest maybe 2 options to try: 1) You can input skylights to cover
the greatest percentage of your roof area that will fit within the geometry
that you describe and still get some amount of glass roof effects accounted
for in your simulation although we know it won;t be 100% accurate for the
proposed design. 2) you can do all of these thermal effect calculations
outside of eQuest and use your calculated data to integrate these numbers
with your simulation output---this is a much more tedious approach and you
will have to rely on your manual engineering analysis to describe the
anticipated thermal results that can't be simulated within the program.

Hope this helps.
Pasha

Pasha Korber-Gonzalez's picture
Joined: 2011-09-30
Reputation: 400

Pasha,

I may be missing something, I think you may have incidentally answered
how this can be done without realizing it =)?

1. Define a roof construction to match the thermal
massing/conductive properties of the glass framing.

2. Define and assign roof skylights (check all inputs against your
ceiling glass info) as required to match the percentage coverage (minus
the framing) of the glass for the construction. You can do this quickly
in the Wizards by defining % or define and locate with more precision in
detailed.

3. I don't' believe exact positioning on any given roof section is
necessary if you have no daylighting controls to model or elements
shading these skylights.

Thermals and daylighting controls should be accounted for and affected
correctly using skylights - if you have any building shades/elements
above the glass roof plane be sure to locate them and the skylights
accurately to model their shading effects correctly.

In Lauren's case with an irregular polygon, I don't believe you have any
recourse but to specify a series of rectangles. I think you'll find
however creating and positioning skylights in detailed to be easier than
you might think, if you haven't tried already. I always suggest saving
often and separately when going through any detailed window relocations
- things can sometimes go awry if you have a frame falling outside the
parent construction boundaries, for example.

NICK CATON, E.I.T.

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Joined: 2011-09-30
Reputation: 805

Any portions of the roof that are transmitting light and solar energy will have to be defined as windows in eQuest (or most other programs). I'm assuming when you say glass roof that you are considering basically a full-size skylight, with irregular shape?

Defining a roof surface with the properties of glass will still result in an opaque surface, although the thermal resistance and mass would be as-specified, and transmission of solar radiation would be neglected. This won't work if you have any spaces below with daylighting controls, let alone the heat transfer from light transmission.

Suggest that you give it your best shot with child windows, depending on your daylighting needs. Add the majority of the area, take stock of results, add a few more windows with finer detail, check results, and repeat. Eventually adding the finer detail isn't going to provide meaningful changes to the outputs. (As a previous poster mentioned be sure the interaction with any daylighting controls is accounted for.)

David S. Eldridge, Jr., P.E., LEED AP BD+C, BEMP, HBDP

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