Re: [EnergyPlus_Support] Overhanging / Canopy like roof.

How are you modeling the shading object? Â Is it one big shading object that covers the entire roof? Â If you have one large shading object, and your first model (the one with no air gap) has the shading object directly over the normal roof, it will shade the normal roof. Â In my understanding, in EnergyPlus, shading objects do not affect the outdoor airflows. Â Remember that EnergyPlus is not a computational fluid dynamics simulation engine. Â It uses simplifications in order to simulate buildings as quickly and accurately as possible. Â However, the dynamics of air heated under a thin shading layer is not something that EnergyPlus will do simply with a shading object and a building surface.

The simplest way that I can see to model the two alternatives (ventilated air space vs not ventilated) would be to do the following:

1. Â No Air Gap - model the overhang only as a shading object, with no shading over the main roof. Â Incorporate the shading object construction as additional layers in the roof construction. Â Make sure that the solar and thermal absorptance values are correct for the outside layer material.

2. Â 6" Air Gap - model the entire shading object suspended 6" above the roof. Â In this model, the shading object will block solar gain from reaching the roof surface. Â However, the outside air temperature seen by the roof surface will be the current outdoor temperature in the weather file. Â This is a simplification, but as long as the shading object is sloped and vented at the top and bottom (in real life), the temperature under the shade should be pretty close to the exterior temperature. Â If you are not comfortable with this simplification, explore use of the "SurfaceProperty:ExteriorNaturalVentedCavity" object.

Karen

On Wed, Aug 27, 2014 at 9:36 AM, Mathieu LAMOUR lamour.mathieu.energie@xxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:

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Hi all,

I'm quite interested in the subject, because i posted a similar answer a few month ago.
I tried to find a good way to know what is happening in the air gap between this separated canopy and my conditioned space roof, in order to check that the fact to model the canopy as a big shade was that efficient. Changing reflectivity of the canopy, surface coefficient algorithm, etc.

Another option i thought, and that Ned Lyon seemed to confirm, was to model this air gap as a totally ventilated space. This last thing, i still didn't try. Even if it sounds good, isn't it too "complicated" to see what happens...?Â

At last, when i change the air gap height, i have some differences, but it's only due to the fact that solar radiation is touching more or less of my conditioned space roof.

By experience, does anyone think that the air gap temperature could increase so much that this big shading device would be unproductive (depending on its height, for example)...?

Sorry for not answering, but keeping asking...

When i model the air gap as a ventilated space, i'll let you know.

Regards,

2014-08-26 22:37 GMT-05:00 Asit Mishra asitkm76@xxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx>:

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It would be useful to know if you are employing any other passive measures for reducing cooling load parallely. In my limited experience, I have seen that a certain combination of passive measures would lead to a particular level of reduction in cooling load. To clarify, say there are 3 measures, A, B and C. When you employ measure A, you may get a large reduction and small reduction for B and C. Next time, if you employ B first, you may again see a large reduction followed by small ones for A and C. So, it is not just the particular passive measure you are employing at a certain step, rather it is also useful to consider what other measures have been already put in place to reduce loads.

regards,

asit

On Wed, Aug 27, 2014 at 8:38 AM, Chandan Sharma chandangsharma@xxxxxxxxx [EnergyPlus_Support] <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:

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How does the heat gain from the walls and internal load, including infiltration/ventilation etc. look like?

On 8/27/2014 7:34 AM, Chandan Sharma wrote:

Benjamin,

I agree that shading with an air gap above the roof should reduce the cooling load and also agree that expect more than ~2.54% reduction in cooling load which is seen here for 6" air gap. May be someone else can enlighten us with the results.

Thanks,
Chandan

On 8/26/2014 11:48 PM, benjamin_khuong@xxxxxxxxx [EnergyPlus_Support] wrote:

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Chandan,

Yes I did just that, I ran simulations with an air gap of 0, 6", 1', 1'6", and 2'. As the overhang went higher the annual cooling load decreased. I'm assuming that is due to the loss of shading over the south side of the house.

But shouldn't there be a bigger difference from having no air gap to a 6" air gap.

With no Air gap the roof of the home is being directly heated by the sun but with a 6" air gap the roof of the entire roof of the home is being shaded.Â