[Bldg-sim] F-Factor

[Talbert, Joshua]

Hello Bldg Sim Folks,

Could anyone comment on how they "translate" the F-factor requirement for on grade slabs into a U-value / material for a floor construction in Energy+ or eQuest

My method has always been to create a fictional material with negligible thickness that approximates the equivalent R-value for the whole floor area of the zone based on the edge requirement.  This results in me creating a separate one of these fictional materials for each zone.  This is because even though a zone might have the same F-factor requirement the difference in perimeter to floor area ratio effects the actual heat flux through the floor, or so I think.

Here is an example of how I have modeled this, see attached picture for a layout:
[cid:image001.png at 01CC6249.A570F5C0]

F-Factor = 0.64 (equivalent to R-10 insulation for 48" and for an "Unheated Slab" per ASHRAE 90.1 Table A6.3)

Zone 1
Area =                                   25 sqft
Edge Perimeter =             10 ft

Zone 2
Area =                                   375 sqft
Edge Perimeter =             70 ft

Perimeter * F factor = Area * U value equivalent

Zone 1 equivalent U = 0.256
Zone 2 equivalent U = 0.119

This process has made sense to me, but in reviewing this technique with a colleague we wondered what does everyone else do.  Am I under/over predicting slab loss for certain zones with this method?  Your comments and insight are always appreciated.

Thanks,
Josh


Joshua W. Talbert P.E., LEED-AP
Mechanical Engineer
Hallam///ICS
38 Eastwood Dr. Suite 200
S. Burlington, VT 05403
P: (802) 658-4891
F: (802) 658-1457



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