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How to modeling the ceiling void ?

asked 2015-04-01 10:36:13 -0600

hugopft's avatar

updated 2015-11-09 07:04:18 -0600

I have been modeling a Building and I would like to know the considerations that I have to take account regard to the ceiling void 0.5m high. I am Considering this ceiling void as a space. The reason for that is because from about 0.2 m of high, we start to observe convection effects, and I think, Considering that ceiling only as air gap (convection effects are not being taken into account), I am not modeling that appropriately. So, which of two considerations above is the better one? Thanks.

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answered 2015-08-25 10:49:20 -0600

As you say, there are two main options for modeling the air space in a ceiling.

A. Model as a separate plenum zone


  • The plenum zone will be modeled with a full heat balance, most likely at a different temperature from the occupied space, and it will not be exposed to the solar and internal gains of the occupied zone below.

  • The occupied zone below will automatically have the correct zone volume for any reporting or inputs that are in air changes per hour (ACH).

  • Allows the plenum zone to serve as an unducted return or supply plenum, and as a source for powered induction units. Actually, this option is required if you have any of those.

Disadvantages and Limitations:

  • More surfaces and zones to describe in the model.

  • The zone air will be mixed (well-stirred) at a uniform temperature. So, while buoyancy-driven surface convection is modeled, there is no circulation or stratification within the plenum zone.

B. Include the void as part of the occupied zone below and add an air gap layer to the ceiling/roof construction


  • Simpler input.

  • Many users take this approach and find it to be adequate for standard dropped ceilings.

Disadvantages and Limitations:

  • The entire exterior surface area (walls that are exposed to the occupied space plus walls exposed to the plenum space) will be exposed to the conditions of the occupied space (temperature, solar, radiant exchange, etc.).

  • You need to adjust the zone volume in the Zone object, if needed, to have the correct zone volume for any reporting or inputs that are in air changes per hour (ACH).

  • The air gap resistance will be constant.

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Thanks @MJWitte, This info was very helpful for me but I hope someone tells me how can I do approach B?I mean how can I Include the void as part of the occupied zone?

4Designer's avatar 4Designer  ( 2016-03-10 10:28:59 -0600 )edit

For example, if the occupied space is 3m high, with a 0.2m void, then the walls of the zone should be 3.2m high. Add the airspace resistance to the roof construction.

MJWitte's avatar MJWitte  ( 2016-03-11 18:08:10 -0600 )edit

I read this discussion and this one but it's still unclear to me how to model large concrete beams (with significant thermal mass effect). To put it into perspective, for instance, imagine there are massive concrete beams with 1m height within a false ceiling with 1.5m height. My conclusion from the 2 above-mentioned discussions is that: I'm going to calculate the volume of concrete beams, then divide it to floor area, and add the resulted number to the height of the slab in construction. Is this approach correct?

Aryan Shahabian's avatar Aryan Shahabian  ( 2018-04-23 07:33:12 -0600 )edit

That is on option, not perfect but it does add the correct amount of thermal mass, but the conduction between zones is not correct. The other option is to add the beams as internal mass in the plenum zone. Again, the correct amount of thermal mass, but not the correct conduction.

MJWitte's avatar MJWitte  ( 2018-05-02 15:37:57 -0600 )edit

Its an old conversation but I try. If the ceiling void is 20/30/40 cm, we set an air gap of 20/30/40 cm ? So it means with a very high thermal resistance and so a very low heat transfer through the roof. It doesn't really represent the reality I suppose. What is the air conductivity in this case ?

Alexis's avatar Alexis  ( 2018-11-30 01:59:36 -0600 )edit

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Asked: 2015-04-01 10:36:13 -0600

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Last updated: Aug 25 '15