We are doing a project in a pretty cold climate. The project design has a really big glass fa?ade facing south. One of the ideas to reduce conductive heat loss is to create a double skin fa?ade on the south side.
Now I realize that this is going to take a fair amount of analysis work to design it properly. What I was wondering is if there is a quick analysis tool/methodology to tell if this is even feasible to go into more detailed analysis.
Any help would be much appreciated
Vikram Sami, LEED AP
Maybe modelling the cavity as sunspace might work - it will not take care of
the cavity ventilation part but will probably work for the solar radiation
part (not sure if sunspace allows for daylighting - you will have to check
on that.)
Another option would be to use EnergyPlus - I think it has an example file
for double facade.
-Rohini
Hi Vik,
I hate to trample over anyone's little flower patch, but I've read some scathing assessments of double-facades as an energy-saving strategy. You might want to check this and the associated references out: http://www.buildingscience.com/documents/insights/bsi-007-prioritizing-green-it-s-the-energy-stupid.
That said you might not be satisfied until you develop your own conclusions. Proponents of this approach repeatedly cite benefits of CFD, so to give a "fair/proper" assessment any tool you work with would probably need to be modeling convective airflows between floors.
NICK CATON, E.I.T.
Nick and Vikram, double skin facades or climate walls or windows have been around for many years now. Vikram's question is really how do you produce a back of the envelope calculation to see if a double skin fa?ade is a viable option. Well here goes, the basic improvement by using a double fa?ade is the improvement of the U value decreasing energy loss. Take a look around the world and the successful applications of double facades is mostly in regions where heating is the dominant energy consumer. One of the first things were learnt more than 30 years ago was the U value was improved so long as air was ventilated through the cavity, but what happened to the U value when the air system was switched off after occupation? which consumed more energy, the energy loss of the building without the function of the double fa?ade or the energy consumption of the air system to maintain the functionality of the double fa?ade. Beyond the improvement of the U value we can then start looking at improved shading coefficients and surface temperature control of the inside surface of the double fa?ade. For these different aspects of double fa?ade design I believe the design criteria was simplified in a series of tables issued in a couple of ISSO publications in the 80's. These documents will get you rough data to be able to assess whether or not the double fa?ade is feasible. As far as deluxe simulation tools probably the best is the ROOM program by Mike Holmes and Andrew White and then there is also TRNSYS.
Getting back to Nick's comments, I have found over the years that the most comments regarding double facades come from people that have never designed them, here I am referring to the article and not you Nick.
I first started designing double facades in the late 70's in the Netherlands and over the years have designed quite few and completed quite a few repair jobs on attempted double facades. One thing I have learned is that if it isn't cold then the double fa?ade is there for show only!
Peter Simmonds, Ph.D.
The Manulife building in Boston has a double facade
http://www.som.com/content.cfm/601_congress_street
Hi,
you may want to use one of the following two online tools:
http://www.bestfacade.com/textde/BestFacadeDSF_InternetTool/index.html
http://designadvisor.mit.edu/
Both should give you a good estimate with respect to energy, daylight and
comfort.
Good luck.
--
best regards
Matthias Haase
Another tool to use to analyze double fa?ade buildings is IESVE. You could
possibly pull it off during your free 30 day trial period.
Carol
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Hi
Using En+ for DS may lead to unrealistic results. A student of mine made
is MSc comparing En+ results to the monitoring results of a naturally
ventilated DSF test cell and the air gap air temperature/ventilation
flow was significantly out of reality (unfortunately it's in
portuguese). Similar results obtained by Kim et all may be found at
http://www.iaqvec2010.org .
You can check at IEA Annex 44
(http://www.ecbcs.org/annexes/annex44.htm#p.) what is available and a
set of pros and cons, but DSF is not an easy task to properly model.
Hi Matthias
How are you?
Do you really think so about bestafacade "model"? I think it's so over
simplified that it becomes useless.
I was expecting you to advise the use of TRNSYS...
Fernando
One of the most important issues on DSF (maybe the most important) is
gap ventilation. So care should be taken into account when using sun
space models.
Hi Peter
I think you are only considering DSF gap forced ventilation (/when the
air system was switched off after occupation?/). This not an issue is
it's naturally ventilated. My opinion, and I am not a designer but I've
been monitoring a few occupied DSF fa?ades in the warm climate of
Lisbon, is that as for any other building system it has to be properly
designed having in mind location and use. I monitored really bad fa?ades
but also fa?ades that behave quite well allowing the interior
temperature to remain within comfort levels during weekends when HVAC is
switched off.
I don't know ROOM but TRNSYS is a good option for modelling DSF (see the
work of Haase et al).
I agree that cold climates are preferable when considering a DSF option,
but there are other issues like, for example, tall and wind exposed
fa?ades, specially if glazzed, where inside shading is thermally a bad
option and for outside positioning fast damage is certain. Anyway owners
may allways impose a look for their building...
Peter, Marques et al:
Thank you so much for sharing your collective insights! My initial comments were intended only to open the discussion, as I have heard contrarian opinions in the past and didn't want a friend to be unaware of them =). Consider me an interested student, certainly not an authority on the subject!
I for one have been motivated by this discussion to do some homework, and now have a much better grasp of the fundamentals. Apparently ASHRAE Journal put out a good primer roughly a year ago for others in the same boat of curiosity to understand the concepts and challenges present in DSF design better - I found this handy: http://www.rwdi.com/cms/publications/76/200904_Ventilating_Facades.pdf
Thanks again,
NICK CATON, E.I.T.
Vikram,
There is a number of tools that allow you to assess the DSF performance.
Simple, non commercial (freeware) tools you can use are WIS 3 (for steady state boundary conditions) and Parasol for dynamic calculations (tool also that implements - if I am not wrong - the 15099 standard).
As to dynamic thermal modelling tools you can use Energy Plus, or other commercially available tools; I have worked in the past with IDA ICE 3.0 (there is the newer version IDA ICE 4.0 available) that will allow you to assess DSF performance. In fact, I found IDA ICE very useful since I could assess the performance of several DSF types (naturally ventilated, mechanically ventilated, hybrid, airflow windows) and geometries (box windows, multi storey facades (see PhD thesis "Single and Double Skin Glazed Office Buildings. Analyses of Energy Use and Indoor Climate" http://www.ebd.lth.se/fileadmin/energi_byggnadsdesign/images/Publikationer/Bok_Dokt_avhandling_HP_G5.pdf).
DSF software validation was carried out under the IEA Task 34 Task (http://www.iea-shc.org/task34/).
A description of methods and tools to assess DSF performance is extensively described under a literature review (http://www.iea-shc.org/publications/downloads/Task34-Double_Skin_Facades_A_Literature_Review1.pdf).
I hope this helps.
Best regards,
Harris Poirazis