How to Model a Slab on Grade F-Factor

Hello -

I was looking at Title 24 and it shows that a slab on grade for residential should have an F-Factor of 0.73 W/m-K. After reading online, it seems that Energy Plus allows you to model this. I wonder if this is something I could model in honeybee? If so, how would you suggest me to do that?

Thanks in advance,

Tagging @chris and @mostapha for visibility.

Hey @Yure ,

Sorry for the late response. You are right that EnergyPlus has an object for F-Factors and we’ve had an open issue to eventually support them in honeybee:

But it hasn’t been prioritized since F-Factors are kinda a simplification of what we currently do for ground constructions and changing your model to use F-factors isn’t likely to change the energy use much if you’re using a ground construction with an R-value that’s aligned with your F-factor.

If you badly needed R-factors now, you can write a Python component that post-processes the honeybee-generated OSM using the OpenStudio SDK like what you see here. But we’ll get to this eventually and you can probably just use an appropriate R-value for now without worrying about the energy results changing much.

Hi @chris -

is there any simple way to compare F factor and R value?
I know the F-Factor I want to have: F-Factor: 0.073 W/m-K. How do you know what is that in R value?

Hi @Yure

I was digging into this a little bit for another project I was working on, and came on these descriptions of the F-Factors in case they are helpful to you:

ASHRAE 90.1, Appendix A

Manual J:

And FWIW: I was able to add F-Factors to en E+ model using the process shown here: ASHRAE 90.1, F-Factors for Floors? - #5 by edpmay


Hello @edpmay -

Thanks for your response. I tested your script and I have a few questions:
1- in principle, I understand that your #1 python script is the one that has a base case that then is used by #2 to apply to the area of the slab that meets the criteria, and script #3 assigns it to the code. right?
so, in theory, my new energy results include this new parameter?
I just changed the Ffactor on your script to the Ffactor I need which is 0.073 W/m-K. would that be enough to make my slab compliant ?

Best, and thanks for sharing your script.

Hello @Yure ,

That is correct, that should be all you need to do (if I remember correctly). If you want to check that it was applied correctly, I would recommend opening up your file in OpenStudio, and see if you can find the surface? If it worked, it should show a “GroundFCfactorMethod” now, instead of a “Ground” as the Outside Boundary Condition.


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