slab on grade answer

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i just received a clarification about the f-factor for 4" slab on grade
vs 6" slab on grade for the purposes of leed review. the usgbc response
(through the website/contact section) is:

"Thank you for your technical customer service inquiry regarding LEED-NC
v2009: EAc1. It is assumed that you are following the Performance
option (Option 1) of EAp2 & EAc1, using Appendix G of ASHRAE 90.1.

Yes, the 4" slab (modeled & specified) should meet the requirements for
LEED purposes. Please note that there is no F-factor requirement listed
as part of the Mandatory provisions of Section 5.4; it is only listed
for the Prescriptive approach requirements of section 5.5, which is not
necessary for LEED EAp2/EAc1 compliance.

If you model the slab as a 4" slab with 12" of earth (0.75 soil
conductivity) underneath it in the Proposed Case model, the energy model
should indicate the U-value of this slab. You could simply state in
your submittal (4" slab modeled with 12" of earth with 0.75 Btu/h-ft-?
soil conductivity, U-value calculated within energy model as x.xx).
Please be careful to avoid the common mistakes of modeling the slab
without any earth beneath it, or modeling the Baseline slab "U-value"
using the "F-factor" values from Tables 5.5-1 - 5.5-8. F-factors and
U-values are not interchangeable, of course. Also, if the energy model
cannot reflect an F-factor, please model the Baseline slab-on-grade
floor as a 6 inch concrete slab over earth with a soil conductivity of
0.75 Btu/h-ft-degree F, in accordance with A6.1.

We trust that this will help you with your process and we thank you for
your interest in LEED Rating Systems.

If you have any further questions, please feel free to contact us via
the GBCI contact page:


GBCI Technical Customer Service

Patrick J. O'Leary, Jr.'s picture
Joined: 2011-09-30
Reputation: 200