Re: [EnergyPlus_Support] Venting Wall Cavities

["Michael J. Witte" <mjwitte@xxxxxxxx>]

If you really wanted to get complicated, you could model the wall cavity as a 
separate zone similar to a trombe wall except with an opaque outer surface.  A 
reasonably good model, as you suggest would be to model the brick layer as an 
external shading surface, or you could simply set the "Sun Exposure" field on 
the exterior walls to be "NoSun".  EnergyPlus will not automatically alter the 
convection coefficients on the outside surface due to the presence of a 
shadowing surface.  The next step would be to set the "Wind Exposure" field on 
the exterior walls to be "NoWind".  This will set the convection coefficients 
to a natural convection value instead of the forced convection value induced by 
the wind.  You answer is probably somewhere between the "WindExposed" and 
"NoWind" case, because I would guess that the ventilation in the air gap 
increases in windy conditions.  

You say that the mass of the brick veneer is not important, but I am not 
convinced that this is true.  If the brick is exposed to the sun, it will heat 
up during the day, and then it will radiantly heat the outer surface of the 
timber wall for some hours into the evening.  If you model this as a shading 
surface or use "NoSun", then all of this solar heat will be lost.  With this is 
mind, you may find it most conservative to model the full wall construction, 
igoring the venting of the air gap.  So, you have several options available, 
and the best choice will depend on the ultimate use of the results.  

Mike


On 2 Feb 2004, at 13:05, Simon Gowland wrote:

> Hi all,
>       I am trying to model a wall with a construction
> consisting of..
> 
> -brick veneer
> -airgap
> -building paper
> -timber framing with insulation
> -plasterboard
> 
> I have a question.
> 
> 1/ The airgap between the brick veneer and the
> building paper is vented to aviod moisture problems.
> How would this be modelled in Energy Plus. i.e do you
> have to model it as a seperate zone. My understanding
> is that the thermal resistance of the brick veneer is
> approximatly halved due to the venting of the airgap.
> Since the mass effects of the brick is not of
> relevance since it is outside the insulation layer,
> this wall might as well be modeled as a standard
> timber framed wall. However then the shadowing effects
> of the veneer are not recognised. If the veneer is
> modelled as an external shading device, I wonder how
> the wind sheltering of the timber framed wall would be
> calculated?
> 
> Has anyone had experience with modelling cavity walls
> like this type before?
> 
> Thanks
> 
> Simon Gowland
> 
> 
> 
> 
> 
> 
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========================================================
Michael J. Witte, GARD Analytics, Inc.
EnergyPlus Testing and Support      
EnergyPlus-Support@xxxxxxxx


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