Semi-exterior spaces

[Helena_BS]

Hello,

We are simulating a building with lots of semi-exterior spaces. This means that, in those spaces there is no air conditioning system neither mechanical ventilation and there is a façade compound aternativelly by glass – voids in a proportion of 0.6m of glass - 0.1m of void.
If we use a WINDOW - VOID composition (like defined in the project drawings) for defining this wall, how will affect to the energy balance and temperature of this space?
How is the void’s effect when using it as an exterior wall?

[drajmarsh]

Hi Helena,

Using VOID objects is the recommended way to define a completely bounded zone but still include openings either between two adjacent zones or to the outside.

A VOID tells Ecotect that there is a free exchange of air and heat through that boundary. As with all thermal analysis algorithms, they are an approximation of reality so what you get from a VOID is pretty close, but not exact. For example, the effects of bulk air-flows that are likely through that opening are included in the calculation, but because Ecotect doesn't do any detailed computational fluid dynamics during the thermal analysis, complex convection currents that may be caused by the exact location of the opening will not be included (such as placing the VOID as a small slit at the top of the zone to vent warm air).

If you have VOID openings on opposite sides or adjacent sides, some effects of cross ventilation are also included based on hourly wind speed data. However, you can either control or over-ride this by specifying your own values for the zone's infiltration rate and wind sensitivity. If by adding VOIDs as boundaries to the outside you want that zone to be effectively the same temperature as the outside, then I would also adjust the infiltration air change rate of that zone to something like 50ach to simulate effective ventilation.

Also, you can manually adjust the effective thermal resistance of the VOID openings by adjusting their U-Value (which defaults to 20W/m2K, but can be as high as 60W/m2K). Whilst possibly not applicable in your case, this does mean that you can create two materials, one with a high value and the other with a low value, and then use an hourly schedule to move between them at different times (useful for simulating blinds, open/closing louvres, etc). Obviously this involves a bit of guessing as to the relative effects of the open and closed states, but you need to think of these kinds of analysis as a test of how your building is likely to perform under specific conditions rather than being a 'solution' to its exact performance.

Hope that helps...

Kind regards,
Andrew

--
Dr. Andrew Marsh
Principal Software Engineer
AEC-Simulation
Autodesk, Inc. Edited by: drajmarsh on May 19, 2009 8:10 AM