[TRNSYS-users] How to model a cavity zone under a building

Fri Jun 11 10:47:37 PDT 2021

If you assume that the cavity has a very high infiltration rate (which seems
to be the case here) and you can accept the assumption that radiative
transfer with the ground is negligible, a simpler option would be to remove
the cavity from the model. 

The floor above the cavity would become an outside surface, you would just
have to make sure it does not receive solar radiation. You can do that by
forcing its exterior absorptance to 0 or by defining a special orientation
for which you do not use automatic solar radiation calculation in Type 56
(and you leave the inputs unconnected with constant 0 value). You could also
define that outside surface as a boundary wall with a boundary temperature
equal to the ambient temperature. 

This is (I think) acceptable if the floor is reasonably well insulated, so
that the assumption on the cavity temperature has a minor impact on the
model. One advantage is that it saves you from having to use a ground
coupling model.

Of course, if you are specifically interested in what is happening in the
cavity, then David’s suggestion to model it as a zone and to model
infiltration (rather than assuming a constant rate) is what I would choose.
And I think you would have to add a  ground coupling model, to be consistent
with the level of detail in the rest of the model. 

Michaël Kummert

Polytechnique Montréal

From: David BRADLEY via TRNSYS-users <trnsys-users at lists.onebuilding.org> 
Sent: June 11, 2021 10:33
To: TRNSYS users mailing list at OneBuilding.org
<trnsys-users at lists.onebuilding.org>
Cc: David BRADLEY <d.bradley at tess-inc.com>
Subject: Re: [TRNSYS-users] How to model a cavity zone under a building

Seyedmostafa,

 You're on the right track. At least your method is the same that I used
when I was modeling a similar situation. In my case there was a thin vinyl
skirt around the building so I used its thermal resistance for my massless
walls. I think the way to assess the infiltration into the cavity is to
couple the Type56 thermal model either with CONTAM (using Type97 or Type98)
or with TRNFlow. Both are designed to estimate airflow between pressure
nodes in buildings.

kind regards,

 David

On 06/11/2021 05:06, Seyedmostafa Mousavi via TRNSYS-users wrote:

Hi all,

Hope you are very well.

I’m modelling a cabin in SketchUp using the TRNSYS3D plugin. In the real
cabin structure, several steel posts were used to elevate the cabin and
separate its floor from the ground. The air easily flows in this cavity
under the cabin, see Fig. 1. To model the cavity, I’ve considered a bottom
zone in which the walls were defined as Massless Layers with R-value of
0.044 h m^2 K/ kJ (equal to the air gap), see Fig. 2.

However, I’m not sure if this modelling process is correct. Do we have any
option in TRNSYS to consider such cavity under buildings?

BTW, if this model is correct, how should I calculate the infiltration rate
for the cavity section? Any idea is highly appreciated.

Thanks a lot for your help.

Warmly,

Seyedmostafa Mousavi

The University of Melbourne, Victoria 3010 Australia 

E: sm.mousavi at unimelb.edu.au <mailto:sm.mousavi at unimelb.edu.au>  

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David BRADLEY
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