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[EnergyPlus_Support] F-factors

Dear Group,
e+ has a documented technique of converting the user inputs of F-Factor, Area and Perimeter into a Equivalent U-Value Contruction. It seams however that ASHRAE 90.1 and HOF has too little information on how the F-Factors in standard 90.1 were created (for example, which T_in and T_out were used, how were losses into the soil accounted for, which air film resistances were used, etc.) for me to trust this technique as a compliance tool.

I propose the following solution:
Baseline case...
In a nut shell, a curve fit for the "fully insulated" case of the F-factor table in 90.1...R-insulation is then a function of F-factor input. Thus an equivalent construction can be created that satisfies the Standard in terms of Rated R-insulation covering the entire slab, concrete thickness and soil conductivity as per the description in Appx A. As Dr Huang suggested, use a 0.3 (also noted for this thickness at the back of 90.1 appx A) at the soil conductivity perscribed by 90.1 to catch the thermal mass effects of the soil.

As a check, a construction defined in this way can be looked up for its F-factor and should give the right R-value using interpolation.

Proposed case...
1) look at the actual design
2) deturmin the F-factor from Table A6.3
3) Calculate equivalent slab construction as for Baseline case using the F-factor from step 2)

It should be said that if your slab does not meet the criteria for "slab-on-grade" as per 90.1, then the standard has no requirement of it and it should be modelled identically to that of the proposed case model (e.g. Basement slabs are not slab-on-grade)

Your comments please, after reading some of the rest of these corrispondances.

Kind regards,


Further reading...

I would like to draw your attention to how COMcheck software handles slab-on-grade F-factors.

Another interesting take is that of DOE2 http://simulationresearch.lbl.gov/dirpubs/un_articles02.pdf
The tabled F-factors here were deturmined by Huang using 2D parametric studies...the tabulated values don't match those in ASHRAE 90.1. See the onebuilding forum note by Joe Huang http://www.gard.com/ml/bldg-sim-archive/msg01047.html

Comments, please.

Kind regards,

Jean Marais

On 19 April 2013 16:59, Tianzhen Hong <thong@xxxxxxx> wrote:

When we developed this feature for EnergyPlus, we did quite some research on the topic and you also pointed out, ASHRAE did not have every details on F-factor. A few years back, I was developing EnvStd, a tool used for ASHRAE 90.1 envelope tradeoff, F-factor was used to calculate the equivalent heat transfer between the zone and outdoor, there is no such a term of "heat flow into soil environment".


On Fri, Apr 19, 2013 at 2:32 AM, Jean Marais <jeannieboef@xxxxxxxxx> wrote:
HOF 18.31 equation 41 with equation 42 for "at grade surfaces" can be combined as follows:
q = Pexp*F*delta_t ...unfortuanately delta_t is not defined here...judging from the previous section for slabs below grade, I must currently assume delta_t = (T_in - T_ground), though for perimeter losses delta_t = (T_in - T_ODA) may be more appropriate.

In context of slabs below grade, I see that the for heat to travel from T_in to T_ground, it must travers Rfilm_in, Rcon, Rinsulation (if any) and possibly a R_soil_to_slab (probably negligable). However, for slabs at grade the heat travels partly into the ground and partly into the ODA via the ground. i.e. I could assume that the F-factors were calculated somewhat as below:

qtot = heat flow into soil environment + heat flow into outdoor environment through perimeter effects
       = [Area * U_slabconstructionwithinteriorairfilmresistances * (T_in - T_ground)] + [Pexp * F * (T_in - T_ODA)]
       = [Area / (Rfilm_in+Rcon+Rins) * (T_in - T_ground)] + [Pexp / (Rfilm_in+Rcon+Rins+Rsoil_path_to_outdoors+Rvertical_insulation+Rfilm_out) * (T_in - T_ODA)]   ...there is a small overlap here, but should be negligable.

As you can see, if this is the case,it is very important to establish what Rsoil_path_to_outdoors was used for the F-factor ratings in 90.1. Alternatively,
1) soil conductivity used for the F-factor ratings in 90.1 = 1.30 W/m-K as per A6.1
2) soil heat flow path length for the F-factor ratings in 90.1...can be assumed the distance of the corrisponding F-factor insulation and type. E.g. F-0.90 corrisponds to R-1.8 and 600 mm vertical insulation, which approximates a soil travel path of 2x600 mm.

HOF does not mention any air film resistances in chapter 18.31, but 90.1 lists these for rating values in A9.4.1. Assuming an average for heat flow up and down is probably a good approximation for the interior air film resistances as the temperature of the soil cycles over the annual average for half the year and under the average for the other half (sinusoidal).

The values in HOF table 24 are in the same range as those in 90.1 table A6.3. Furthermore, HOF equation 42 defines q = heat loss through perimeter...this leads me to believe that the F-factor rating values in 90.1 express only heat losses of perimeter effects and does not include those to the soil.

In summary:

T_ground = anual_mean_ground_temperature used for the F-factor rating values is assumed at 0.3 m...0.3 from Winkelmann's slab-on-grade model...we assume the rating values of 90.1 included variation per climate zone and that this corrisponds to user input.
T_in = anual_mean_indoor_temperature used for the F-factor rating values is assumed to have been assumed at 20ºC for conditioned spaces 
T_ODA = annual_mean_outdoor_temperature ...it is assumed that the F-factor rating values had assumed the annual_mean_outdoor_temperature for calculation and that climate zone variation was taken into account. It is assumed this temperature will closely match the user input.

Rcon = 0.077 m2-K/W ...as in EngRef which co-insides closely to values in 90.1 Table A3.1B
Rfilm_in = 0.14 m2·K/W ...which is the average of the 0.11 m2·K/W for heat flow up and 0.17 m2·K/W for heat flow down as per 90.1 A9.4.1...this differs from EngRef!
Rfilm_out = 0.03 ... as per EngRef and 90.1 A9.4.1 for all exterior facing up or sideways surfaces...NB not for external floors
k_soil = 1.30 W/m-K ...as per A6.1

Insulation_configuration_best_fit_for_F-factor_from_Table_A6.3 (Perimeter_Insulation_is_Vertical, Ins_mm, R-value)

the rest

Someone may shed some more light on how the rating values were calculated so that we can back calculate the U-value of the slab construction.

Kind regards,


On 19 April 2013 08:50, Jean Marais <jeannieboef@xxxxxxxxx> wrote:
Thanks Mike, I will check the handbook and see if revers calculatiosn can bring some light. 

Funny thing that definition...the definition is aimed at slabs that lay exposed above the grade line and are exposed to the OD environment (that means that their is no requirement for basement slabs!, i.e. for basement slabs the baseline is modeled as proposed case), but includes "or is less than or equal to 600 mmj below the final elevation of the nearest exterior grade".

If I learn anything more, I'll post it.

Kind regards,


On 18 April 2013 17:46, Michael J Witte <mjwitte@xxxxxxxx> wrote:

Just to clarify, for "slab-on-grade" the slab edge is exposed to the air (directly or through insulation).  From Std. 90.1 Appendix A, section A6.1 "For the purpose of Section A1.2, the base assembly is a slab floor of 6 in. concrete poured directly on to the earth, the bottom of the slab is at grade line, . . ."

It seems there are several issues here.

1.  How to convert from F-factor to equivalent U-value.  This is what your users need to know, regardless of how F-factor has been implemented or documented in EnergyPlus.  There appears to be some guidance here, but I do not have access to my handbook at the moment to check this:


2.  The second issue is whether EnergyPlus has implemented and documented the f-factor model as intended.   I don't have the answer to that at the moment.  What I can tell you is that the source code matches the v8.0 documentation.  It creates two material layers (which you should be able to report in the eio output using Output:Constructions).  

Concrete layer (inside layer):
  ! Add the 6" heavy concrete for constructions defined with F or C factor method
  IF (TotFfactorConstructs + TotCfactorConstructs >=1 ) THEN
    MaterNum = MaterNum + 1

    Material(MaterNum)%Name = '~FC_Concrete'
    Material(MaterNum)%Thickness     = 0.15d0       ! m, 0.15m = 6 inches
    Material(MaterNum)%Conductivity  = 1.95d0       ! W/mK
    Material(MaterNum)%Density       = 2240.0d0     ! kg/m3
    Material(MaterNum)%SpecHeat      = 900.0d0      ! J/kgK
    Material(MaterNum)%Roughness = MediumRough
    Material(MaterNum)%AbsorpSolar = 0.7d0
    Material(MaterNum)%AbsorpThermal = 0.9d0
    Material(MaterNum)%AbsorpVisible = 0.7d0
    NominalR(MaterNum) = Material(MaterNum)%Thickness / Material(MaterNum)%Conductivity
    Material(MaterNum)%Resistance = NominalR(MaterNum)

  Rcon = Material(iFCConcreteLayer)%Resistance

The second layer is the insulation layer (outside layer) with Rfic:

      Reff = Area / (PerimeterExposed * Ffactor) - Rfilm_in - Rfilm_out
      Rfic = Reff - Rcon

   ! ASHRAE Handbook Fundamental 2005
   !Thermal resistance of the inside air film, m2.K/W. Average of 0.14 (heat flow up) and 0.11 (heat flow down)
  REAL(r64),PARAMETER :: Rfilm_in = 0.125d0
    !Thermal resistance of the outside air film used in calculating the Ffactor, m2.K/W. 0.17/5.678
  REAL(r64),PARAMETER  :: Rfilm_out = 0.03d0

Looking at example file 1ZoneUncontrolled_FCfactor_Slab_UGWall.idf
    slabconst,               !- Name
    0.12,                    !- F-Factor {W/m-K}
    232.26,                  !- Area {m2}
    61.0;                    !- PerimeterExposed {m}

! <Material CTF Summary>,Material Name,Thickness {m},Conductivity {w/m-K},Density {kg/m3},Specific Heat {J/kg-K},ThermalResistance {m2-K/w}
 Material CTF Summary,~FC_Insulation_1,  0.0000,         0.000,      0.000,        0.000,   31.50    
 Material CTF Summary,~FC_Concrete,  0.1500,         1.950,   2240.000,      900.000,  0.7692E-01

When the Ffactor surface heat balance is done, the inside convection coefficient is varying as it does for any surface in EnergyPlus.  This seems reasonable.

However, the outside surface temperature is set directly to the current value of  Site:GroundTemperature:FCfactorMethod, so there is no outside film coefficient resistance during the simulation.  This makes me question whether Rfilm_out should be dropped from this equation:

      Reff = Area / (PerimeterExposed * Ffactor) - Rfilm_in - Rfilm_out

And as you ask, where did the values for Rfilm_in and Rfilm_out come from?  Do they match the ASHRAE f-factor calculations.  

3.  Finally, is the DesignBuilder team planning to implement the F and C-factor option?


On 4/18/2013 1:08 AM, Jean Marais wrote:
Hi Linda,

Mr. Hong is not responding...

Sorry. This issue is not going away. As stated, yes, the documentation has changed in v8, but still refers to air on the outside of the slab and no soil is mentioned, i.e.
"Q = Area * Ueff * (Tair,out ? Tair,in) = (Tair,out ? Tair,in) * (Pexp * F-factor)", "Tair,out is the outside air temperature in °C", "The outside air film resistance Rfilm, out = 0.03 m2·K/W.", etc. 

If a slab is "on-grade" then I fail to see how air_outside can factor into the equation. Mr. Hong has insinuated that these issues have been corrected "The air film (either inside or outside) is not included in the calculation of effective U-factor. The soil layer is modeled as part of the 6" concrete and an insulation layer to come up with the equivalent U-factor.", but the documentation of the last v8 release does not reflect it. As I am supporting DesignBuilder, and DesignBuilder does not currently support the F-factor objects for e+, it is important for me to explain to clients how to calculate the Ueff to properly reflect the f-factors in 90.1. No one has been able to explain this to me. I need to know what R-soil ASHRAE was using for these f-factors AND which film factors (if any) are included in the f-ractor rating values.

This information would help many users who have been ignoring this issue for many years. I'm always supprised how long people have been "conforming" to ASHRAE 90.1 for simulations.

Kind regards,


On 17 April 2013 14:51, Linda Lawrie <linda@xxxxxxxxxxxxxx> wrote:
I presume he meant the current V8 release.  But I'm not sure.

At 11:21 PM 4/16/2013, jeannieboef@xxxxxxxxx wrote:
May I ask for your source of information? Do you mean the next release, as the v8 documentation includes the film resistances?

Kind regards,


Sent from my iPhone

On 16.04.2013, at 22:36, Tianzhen Hong <thong@xxxxxxx> wrote:


We clarified the documentation in the coming E+ 8.0 release.

The air film (either inside or outside) is not included in the calculation of effective U-factor. The soil layer is modeled as part of the 6" concrete and an insulation layer to come up with the equivalent U-factor.

To: linda@xxxxxxxxxxxxxx
Subject: [EnergyPlus !7333]: F-factor method engineeringreference documentation error?
Date: Wed, 10 Apr 2013 09:39:30 +0000
Jean Marais updated #7333

F-factor method engineeringreference documentation error?

Ticket ID: 7333
URL: http://energyplus.helpserve.com/staff/Tickets/Ticket/View/7333
Full Name: Jean Marais
Email: jeannieboef@xxxxxxxxx
Creator: User
Department: General
Staff (Owner): -- Unassigned --
Type: Issue
Status: Open
Priority: Medium
Template Group: Default
Created: 10 April 2013 09:39 AM
Updated: 10 April 2013 09:39 AM
Due: 11 April 2013 09:39 AM (1d 0h 0m)
Resolution Due: 12 April 2013 09:39 AM (2d 0h 0m)

Dear Team,

How can there be an air-resistance-film on the underside of the slab? I am comparing to DOE-2E
Surely a soil layer should be assumed here. And then which soil layer was used for the F-factors listed in ASHRAE 90.1?

Please advise.

Kind regards,



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