How to Model a Slab on Grade F-Factor

Yure · February 9, 2023, 7:25pm

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.

chris · March 10, 2023, 3:28pm

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:

github.com/ladybug-tools/honeybee-energy

Add support for C-Factor and F-Factor boundary conditions

opened 12:18AM - 30 Apr 21 UTC

chriswmackey

new development

I know that C-Factors and F-Factors are technically constructions but I think th…ey would probably be better represented in the schema as their own type of boundary condition that gets added by honeybee-energy to the core set of boundary conditions. This way, there's still a "real" construction to fall back on if someone changes the boundary condition. I am thinking that we can have a method that loops through the faces to find any ground boundary conditions and then tries to convert them to C or F factors by analyzing the geometry. So we would just expose this in Grasshopper as a component that takes a list of rooms or a model and converts "Ground to C/F-Factors". More info here: https://discourse.ladybug.tools/t/ashrae-90-1-f-factors-for-floors/14267/3

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.

Yure · March 10, 2023, 7:15pm

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?

edpmay · May 3, 2023, 6:36am

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

best,
@edpmay

Yure · May 9, 2023, 11:55pm

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.

edpmay · May 10, 2023, 6:52am

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.

best,
@edpmay