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Re: [EnergyPlus_Support] Venting Wall Cavities
I would suggest that if you think the vented cavity R-value change is
significant for your study, that you create a normal unvented wall and
then alter the layer properties to simulate the effects of venting (I
think Barnett among others has some recent test information). However,
this task is probably beyond the current wall modeling capabilities of
EnergyPlus and many other generally available programs because the wall
properties are actually dynamic with moisture and solar exposure
changes.
If you are interested in studying wall systems, I would recommend using
another program such as WUFI. EnergyPlus estimates the effect of wall
construction on space energy use, it does not have detailed wall
analysis routines.
I would not recommend treating the brick as shading device. It would
be better to keep it as a layer and alter solar absorption to change the
load on the cavity air space. Note also that moisture transfer
estimates for this (or any) construction in EnergyPlus are not as
refined as those calculated by a program such as WUFI. When the brick
is wet by rain and then the sun shines on it, there is a very strong
water vapor flow toward both the inside and outside. Also probably
significant energy dissipation in evaporation. EnergyPlus does not
calculate how wet the brick may get for this transfer process because
the necessary calculation detail would likely dramatically slow down a
calculation process for building energy use. Just one on the
compromises of life until the next generation of PC's duplicate super
computer power for everyone.
Ned Lyon
Senior Project Manager
Simpson Gumpertz & Heger Inc.
41 Seyon Street
Building #1, Suite 500
Waltham, MA 02453
Phone: 781-907-9000
Fax: 781-907-9009
>>> mjwitte@xxxxxxxx 2/2/2004 2:50:46 PM >>>
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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