[BLDG-SIM] thermal modeling of air cavity

Griffith, Brent Brent_Griffith at nrel.gov
Tue Oct 2 11:43:00 PDT 2007

If you want to use Material:Air, then you can get thermal resistance values for various air gap configurations from ASHRAE Handbook of fundamentals, in HoF 2001 its Table 3 in Chapt 25.  But these are for sealed cavities, not ventilated.   


For an opaque exterior ventilated cavity, there is also a special model in EnergyPlus - see the Surface:HeatTransfer:ExteriorNaturalVentedCavity object.  But that model isn't really intended for such a thick, heavy outer veneer.  



From: BLDG-SIM at gard.com [mailto:BLDG-SIM at gard.com] On Behalf Of Josh K
Sent: Tuesday, October 02, 2007 11:11 AM
To: BLDG-SIM at gard.com
Subject: [BLDG-SIM] thermal modeling of air cavity


Hi all;

I'm attempting to model a building that's facade has a ventilated 25mm air gap between a 100mm brick veneer on the outer side and an EPS/steel stud construction on the inside. I'm curious exactly how I should account for the conductance/resistance of the air gap. This will obviously depend on the inner and outer surface temperatures and the natural convection currents that results from them, and further to that the outdoor wind speed; I'm wondering if anyone is aware of any methods that have been developed to approximate the equivalent thermal properties of an air gap in such a situation. 

The software that I'm using (EnergyPlus) allows one to specify an "air" material object (Material:Air) to be used analogously with conventional material objects (ie gypsum, EPS, etc) in forming wall constructions. The only physical property that one can specify for the Material:Air object is its thermal resistance in m²·K/W. Ultimately, what I'm trying to figure out is the best way to specify this field. 

My thanks in advance for any assistance that may be offered to me on this matter...


Josh Kjenner, EIT, LEED AP
Manasc Isaac Architects Ltd.
10225 100 Avenue :: Edmonton, AB T5J 0A1

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