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

Nick Caton ncaton at smithboucher.com
Mon Mar 28 10:35:23 PDT 2011


Alright, we are on the same page!

 

Main Question Answer in short:  Use the F-factor of a 6” slab as listed in the table.  It’s less work for you, it’s defensible to a reviewer, and this should not significantly impact any calculations relative to the conductivity of an otherwise equivalent 4” slab.

 

Fleshed out responses:

1.       I’ve observed the same.

2.       This “runaround”/dead-end you’re also pointing out is the first point of defense against any reviewer or local code officials who would have you do anything else.  90.1 explicitly does not permit any calculations/methodology outside of using that table.

3.       So that you can sleep easy at night, note that 90.1 also draws a line regarding what constitutes a  significant deviation by defining the term “adequately represented”:  

a.       A1.2.b.  reads (abbreviated): “An assembly is deemed to be adequately represented if … changes … to the base assembly do not increase or decrease the R-value by more than 2 from that indicated…”  

b.      This may seem somewhat arbitrary in discussion of F-factors, but you can be reasonably assured (and further defend the decision to use a 6” F-factor) that an additional 2 inches of concrete in your proposed slab construction, considering all possible concrete mixes, is very likely not going to swing the net R-value of the slab + footing by more than 2. 

c.       My materials charts reference (from 2001 fundamentals) lists only 2 exceptions to that statement – they’re the very least-dense options for “polystyrene aggregate” and “foam concretes*,” which stand apart with a shared listed density of 20pcf.  These two in the list have an “R-per-inch” value above 1.  All other values for concrete are around or less than 0.5 per inch.

d.      If you have to, I’m pretty certain you can quickly calculate the net difference from a 6” slab using materials in fundamentals per above (or better yet, whatever’s listed in Appendix A for concrete), and show any meticulous reviewers/officials that the net difference in terms of R-value is less than 2, referencing A1.2.b.

 

~Nick

 

* On a complete tangent, a friend of mine did a masters report on lightweight concretes in construction during my last semester, so I once got to play around once with a sample of foam-infused concrete block…  That stuff is a mind bender like clear Pepsi from the 90’s!  Their densities can be in the vicinity of a block of cork, for reference.  Much fun playing one-handed catch with a CMU =). 

 

 

 

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: Patrick J. O'Leary, Jr. [mailto:poleary1969 at gmail.com] 
Sent: Monday, March 28, 2011 6:22 AM
To: Nick Caton
Cc: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] f-factor & table a6.3 (90.1-2004 & 2007)

 

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:

The R-value of any insulation, if present

The configuration of said insulation, if present,

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:

What’s on top of the slab

Slab height relative to grade / footing exposure

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

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

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.

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

 



 

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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