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RE: [EnergyPlus_Support] Roof modeling

To: EnergyPlus_Support@xxxxxxxxxxxxxxx
Subject: RE: [EnergyPlus_Support] Roof modeling
From: "Michael J. Witte" <mjwitte@xxxxxxxx>
Date: Wed, 10 May 2006 10:45:24 -0500
Cc: "Aboye, Merid" <Merid.Aboye@xxxxxxxxxxxxxx>
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No!  Surfaces cast shadows only in the direction which they face.  A roof faces 
up and does not cast shadows downward.  Apparently we need to add some 
explanation of this to the Input Output Reference.  Would you please tell me 
what specific text led you to believe a roof extension would shades walls?  We 
will add an obvious note there.

If you use FullExterior as the shadowing option (in the BUILDING object), then 
shadows cross zone boundaries, that is automatic.

For roofs which extend beyond the walls, there are two issues to consider:

1)  How much of the roof area transfers heat into the building?

2)  How to model shading impacts.

In the case of a gable roof with an attic, the full roof area transfers heat 
into the attic zone, and there needs to be additional heat transfer surfaces 
(facing downward) to model the soffit heat transfer.  (view this with a fixed-
space font to understand the little drawings)

  /   <--- Roof heat transfer surface, sloped, facing upward
 /
/___  <--- Soffit heat transfer surface, horizontal, facing downward
   !
   !
   !


In the case where the roof extension does not transfer heat, the extra roof 
area must be a shading surface.  Shading surfaces are mirrored automatically, 
so they *do* cast shadows in all directions.  In this case, extending the roof 
heat transfer surface area would also overestimate the heat transfer through 
the roof.

   /
  /   <--- Roof heat transfer surface, sloped, facing upward
 /!
/ !   <--- Overhang shading surface, sloped, facing up or down, mirrored
  !
  !
  !


_____ ____________  <--- The right-hand portion is is heat transfer surface
     !                   The left-hand portion is shading surface
     !
     !   Zone Interior
     !


This can be confusing.  Please ask more questions if this is not clear.  My 
apologies for not noticing this last week.

Mike




On 2 May 2006 at 15:42, Aboye, Merid wrote:

> Thank you for the quick response.
> 
> Also, the roof has sizable overhangs on all sides. According to the user
> manual, roof extensions/ overhangs do not need to be modeled as separate
> shading surfaces. If I understand correctly, EnergyPlus determines the
> portion that extends beyond the walls (using entered wall and roof vertices)
> to calculate shading effects. Since the roof will be divided into sections
> covering several zones, will an overhang associated with a particular roof
> section be limited to shading only the wall(s) of the zone it belongs to?
> Or, depending on the Sun's position, can it also shade the walls of other
> zones? 
> 
>  
> 
>   _____  
> 
> From: EnergyPlus_Support@xxxxxxxxxxxxxxx
> [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx] On Behalf Of Griffith, Brent
> Sent: Tuesday, May 02, 2006 3:22 PM
> To: EnergyPlus_Support@xxxxxxxxxxxxxxx
> Subject: RE: [EnergyPlus_Support] Roof modeling
> 
>  
> 
> It needs to be divided into surfaces covering each zone. 
> 
> 
> -----Original Message-----
> From: EnergyPlus_Support@xxxxxxxxxxxxxxx
> [mailto:EnergyPlus_Support@xxxxxxxxxxxxxxx]On Behalf Of aboyemeplus
> Sent: Tuesday, May 02, 2006 1:16 PM
> To: EnergyPlus_Support@xxxxxxxxxxxxxxx
> Subject: [EnergyPlus_Support] Roof modeling
> 
> 
> 
> Can a roof spanning over several zones be modeled as one big 
> rectangular heat transfer surface? Or, does it need to be divided into 
> surfaces covering each zone? 
> If the roof can be modeled as a single surface covering several zones, 
> which zone name is entered under the  "InsideFaceEnvironment" field ? 
> 
> Merid Aboye
> 
> 
> 
> 
> 
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========================================================
Michael J. Witte, GARD Analytics, Inc.
EnergyPlus Testing and Support      
EnergyPlus-Support@xxxxxxxx





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- RE: [EnergyPlus_Support] Roof modeling
  - From: Aboye, Merid

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