[Bldg-sim] f-factor & table a6.3 (90.1-2004 & 2007)

Nick Caton ncaton at smithboucher.com
Mon Mar 28 02:41:43 PDT 2011



The heart of your query isn’t really crystal clear to me, but I’ll take a stab:


Section A1.2 says if a building official feels sections A2 through A8 do not “adequately represent” the proposed construction, Section A9 is to be used, but A.9.2.e. curtly says no alternate procedures are permitted to find alternative F-factors for SOG floors.  At first glance, it would appear the 90.1 committee might be giving any extremely meticulous building officials/reviewers/designers the run-around.


The way I see it, Section A6 in its entirety is suggesting concrete slab thickness varying from 6 inches “doesn’t matter much.”  


More precisely: “Real-world” SOG design within the confines of any 90.1 calculations shall be restricted to the following variables:

1.       The R-value of any insulation, if present

2.       The configuration of said insulation, if present,

3.       Whether the slab is heated


Note there are a series of variables are not brought up or either explicitly held constant, beyond slab thickness, and I understand these all can have a decent effect on perimeter conductivity:

1.       What’s on top of the slab

2.       Slab height relative to grade / footing exposure

3.       Soil conductivity/moisture properties


For further reading, I know there are multiple white papers out there going well into depth if that’s what you’re seeking – LBNL comes to mind as being part of some research…  I personally haven’t charged myself with crunching the numbers up to this point, rather letting my software of choice do the legwork.


The reasons I would speculate that section A6.1 specifies a 6” thickness would include

1.       It helps ground/quantify from what the numbers in the table are derived

2.       It gives a reference to the base construction’s thermal mass – necessary should you wish to model something different for your proposed constructions within the context of, say, an Appendix G performance rating.

3.       While a strict reading of appendix A might lead one to believe all constructions used in all calculations must follow the prescribed values, I’ve only ever run into one reviewer who called into question the use of custom constructions and ASHRAE Fundamentals-derived materials/properties which do not appear within the (relatively abbreviated) Appendix A.  I think the real intent is to provide a description of those constructions as defined earlier in the code – namely envelope constructions in the context of describing either a “prescriptive minimum” or in the context of energy modeling as the “baseline constructions.”  I do not believe the intent of Appendix A is to restrict/limit the design decisions that may be made outside of the presented materials/tables.


I’m on a bit of a limb here, but I’m speculating at what may have prompted your question… does this miss the mark?








Smith & Boucher Engineers

25501 west valley parkway

olathe ks 66061

direct 913 344.0036

fax 913 345.0617



From: bldg-sim-bounces at lists.onebuilding.org [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Patrick J. O'Leary, Jr.
Sent: Thursday, March 24, 2011 7:12 AM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] f-factor & table a6.3 (90.1-2004 & 2007)


knowing that 

1) appendix a, section 6.1, references that "For the purpose of Section A1.2, the base assembly is a slab floor of 6 in. concrete poured directly on to the earth ..." meets the requirement for an unheated slab f-factor of 0.73, and
2) section a6.3, f-factors for slab-on-grade floors, a.6.3.1 states:  "F-factors for slab-on-grade floors shall be taken from Table A6.3", and
3) section a6.3.2 states: "These F-factors are acceptable for all slab-on-grade floors."

does the "all slab-on-grade floors" really mean any thickness of slab-on-grade?  i.e. section 6.1 references a 6-inch uninsulated slab meeting the 0.73 f-factor requirement, but per sections a6.3.2 and table a6.3 a 4-inch uninsulated slab (or an 8-inch uninsulated slab) would also meet the 0.73 f-factor requirement.  so why would section a6.1 specify a 6-inch slab when any slab thickness will suffice?  the user's manuals (both years) just refer to table a6.3 for f-factor values.

the state of washington has a similar section in its energy code (see link below), page 41, table 4-2.  section 1003.2 (also page 41) lists "All on-grade slab floors as assumed to be 6 inch concrete poured directly onto the earth."    note that on page 40, table 10.1, the f-factors decrease the deeper the below grade the slab-on-grade floor is.

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