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

[Patrick J. O'Leary, Jr.]

you're pretty much right on the same track.  i realized after i sent it 
the email only had the main question buried in the middle and i have a 
side question to go with it.

main question:  what is the f-factor of a 4" concrete slab?
reason for question:  most architects in southern arizona specify, and 
contractors build, an unheatd 4" concrete slab for standard 
office/retail.  6" concrete slab for warehouses and other spaces where 
heavier loads are common.

secondary question:  does a building with a 4" concrete slab meet the 
f-factor requirements of 90.1 per table A6.3?  (unheated or heated, 
unheated in my case though).
reason for question:  need to demonstrate 90.1-2004/2007 compliance for 
floor f-factor.  unclear as to answer.

1.  all of the research/other references to f-factors that i've found 
also reference the 6" slab, the same research, and 90.1, and from what 
i've seen/can tell they only tested at a 6" slab with 8" and 4" walls 
and varying insulations.
2.  the problem with calculating an f-factor, even if using ashrae 
fundamentals derived constructions, per 90.1, is that calculations are 
prohibited by Section A9 Determination of Alternate Assembly U-Factors, 
C-Factors, or F-Factors, or Heat Capacities, SubSection A9.2(e), 
"Slab-on-Grade Floors:  no testing or calculations allowed."
3.  which leads me back to section a6.3.2, which states in reference to 
Table A6.3, "These /F-factors/ are acceptable for all /slab-on-grade 
floors/." - but is it really?  how does one defend a building official 
or leed review comment for a 4" slab f-factor?   if section a.6.3.2 is 
literally correct then the f-factor for an unheated 4" slab is the same 
0.73 as an unheated 6" slab.  since the f-factor is based on linear feet 
and not square feet this could make sense if the thermal capacity of the 
thickness of the slab does not matter for f-factor.  note though that 
4", 6", and 8" lightweight concrete all have the same conductivity 
(3.7), density (80), and specific heat (0.22) (per the 2009 
fundamentals).  knowing the thickness of the slab may be insignificant 
to the f-factor calculation is one thing, proving/demonstrating it to a 
code official/leed reviewer is another.  i have asked my local code 
officials (who are pe's also) and they don't know the the answer(s) 
either ...




On 3/28/11 2:41 AM, Nick Caton wrote:
>
> Hm…
>
> 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?
>
> ~Nick
>
> cid:489575314 at 22072009-0ABB**
>
> * *
>
> *NICK CATON, E.I.T.***
>
> PROJECT ENGINEER
>
> Smith & Boucher Engineers
>
> 25501 west valley parkway
>
> olathe ks 66061
>
> direct 913 344.0036
>
> fax 913 345.0617
>
> www.smithboucher.com_ _
>
> *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.
> http://ftp.resource.org/codes.gov/wa_energy.pdf
>

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