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Solar time to determine the length of the overhang

asked 2016-06-29 11:27:03 -0500

updated 2016-06-29 15:23:59 -0500

Hi everybody,

sorry if my question is so basic, I would like to know the length of my overhang on the East facade of my building, but I do not know which day and time that I have to choose as a solar time?

my case study is a primary school which is located in Florence (43°47′N 11°15′E), Italy.

Thanks in advance for any help and suggestion.

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Are you trying to come up with a formula to determine the target length for an overhang based on shading strategy? For example on south facade you could have overhang long enough to shade window at mid day in summer. Horizontal shading on the east facade won't be able to be long enough to shade the window when the sun is low in the sky in morning (updated from afternoon, thanks @rpg777).

David Goldwasser gravatar imageDavid Goldwasser ( 2016-06-29 12:03:49 -0500 )edit

@David Goldwasser , Yes David. I wanted to find the length by formula.But rpg777 answer is showing that energy modeling can be very useful to find it as well.

4Designer gravatar image4Designer ( 2016-06-29 12:48:48 -0500 )edit
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P.S. You can still find the old Pilkington calculators on eBay. They -- and the online tools mentioned in the first url above -- are useful for quick ballpark estimates of how large an overhang might need to be, and also for illustrating that late afternoon sun is not a factor on an east facade, ever. =)

rpg777 gravatar imagerpg777 ( 2016-06-29 15:22:24 -0500 )edit

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answered 2016-06-29 12:05:09 -0500

This is a passive solar design question, but I guess one that could be definitively answered via energy modeling, so...

In general, the size of the overhang should be such that it blocks direct sun from entering the space (or at least a significant portion of it) for the summer and the better parts of the spring and fall. There is no specific time you would use for this analysis. You should be looking at the effect of the overhang size on all solar profile angles that impact the facade on which the overhang is going to be applied. There are tools (even free online ones) that you can use to get in the ballpark. Once you've done this, you can apply the appropriate size overhang to your model and run an annual analysis in OpenStudio or other energy modeling tool, and see the impact on cooling/heating, daylight availability, glare probability, etc. It's the effect on all of these things that determines the "right" size for the overhangs.

You could use OpenStudio measures to automate this somewhat. Using the "apply overhangs" measure in concert with the Parametric Analysis Tool (PAT), you could do a series of simulations looking at a range of overhang sizes and review the results.

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I would advocate the use of parametrics or better still optimisation. This is time-consuming to do manually given the multitude of inter-related factors inside and outside the building, including the trade-off between passive winter heating, summer cooling and daylighting controls. The DesignBuilder team included shading overhang length as an optimisation variable in last year's ASHRAE competition and found that the optimum length of the overhang on the East façade was shorter than the other facades. I don't think we would have spotted that using "traditional" methods.

Dave gravatar imageDave ( 2016-06-30 11:16:22 -0500 )edit
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...which is why I mentioned the use of OpenStudio's PAT, @Dave. The question was essentially "what hour should I use for overhang sizing", and the answer to that question -- to quote Nigel Tufnel -- is "none". There's a lot more to it, and as you point out, system interaction and lag can influence things non uniformly. It's why simulation (and parametric analysis and optimization) can be useful (BTW an old salt would know from experience that east and west facades have different answers).

rpg777 gravatar imagerpg777 ( 2016-06-30 11:42:52 -0500 )edit
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Agreed, most modellers would intuitively know that you often (not always) need different overhang lengths on different façades, but optimising them is another matter. Commercial modellers often don't have time to analyse every viable combination of design variable. Likewise, the team had very little time but used optimisation to automate the analysis, and (in the context of the question asked) quantify the differences in optimum overhang length on different facades. The effect of external shading and reflection surfaces also means you don't necessarily get the results you might expect.

Dave gravatar imageDave ( 2016-06-30 13:39:14 -0500 )edit

@rpg777 , Thank you for your really informative answer, I have seen your really helpful video as well. Its about five days that I am still dealing with this question and now I have a related question here . It would be really appreciated that if I get any guidance from you?thanks in advance.

4Designer gravatar image4Designer ( 2016-07-05 05:53:50 -0500 )edit

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Asked: 2016-06-29 11:27:03 -0500

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Last updated: Jun 29 '16