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,
Joshua W. Talbert P.E., LEED-AP
See attached.
Timothy Howe, MS, LEED? AP
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Joshua -
Confining my comments to eQUEST:
There appears to be a quick shortcut we've tried on some models:
eQUEST appears to create virtual materials with virtual R-vals to turn the UA product into the appropriate F-Perim product anticipated by ASHRAE.
This step occurs when exiting the Wizard mode - ie pushing the "Finish" button.
Floor slabs specified as slabs-on-grade will often have a virtual material in their layer definition - this virtual material's resistance will vary, and I believe it may be varying based on the ratio of perimeter to surface area.
If you've done all your zoning within the wizard, then it is possible to get eQUEST to automatically generate the different "virtual material" resistance factors for each of the different room's slab surfaces, if you need to vary slab insulation, and if the slab insulation options available in the Wizard include the option you're looking for.
Assuming you've left wizard mode, you can save the project as a separate file name (so as not to over-write any detailed edit changes), re-enter Wizard mode, reselect the slab insulation you would like, and "Finish" the wizard. The virtual material resistances will have changed. Then these virtual resistances can be copied to the virtual materials you have in the "real" detailed data edit file.
A caution - I haven't validated that this produces the exact correct factors to translate eQUEST's modeled area into the
Has anyone else on the list looked into this procedure?
Aaron Dahlstrom , PE, LEED(r) AP