Floor Slab F-factor

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Could someone please help me with how to input or figure out what the
F-factor is for slab-on-grade floors? I've just input floor areas through
the wizard and selected the type of foundation insulation I have from the
drop-down lists. I have received a comment back from the LEED reviewers to
explain what I have and how my baseline case uses the unheated floor slab
F-factor of 0.73. Several of my thermal zones are interior spaces which have
a concrete floor with Earth contact but no exposed perimeter, but I do have
Earth contact floors in all of my zones so there is supposedly heat loss
from all of them. Any help with this would be appreciated.

Thanks,

Rick Routh

Routh Consulting Engineers's picture
Joined: 2011-09-30
Reputation: 0

Attached is a doc describing how eQUEST converts the slab edge insulation
f-values into equivalent underslab insulation (and why). The wizard does
this automatically giving you custom insulation levels under each zone
based on the perimeter length, area and insulation selected.

Hope this helps.

Brian

bfountain's picture
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Joined: 2011-09-30
Reputation: 201

Brian:

Thanks for your reply. The attachment you mentioned didn't come through with
your message. Could you please resend?

Thanks again,
Rick

Routh Consulting Engineers's picture
Joined: 2011-09-30
Reputation: 0

Rick,

If you'll flip your copy of 90.1 to Appendix A, you'll find there's a
section for "slab on grade floors" (not just "floors"), and
corresponding table that will describe the exact construction layers to
achieve a target F-Factor. Matching these slab-on-grade construction
layers/insulation will mean you're achieving the target F-factor for
your perimeter zone(s).

corresponds to an un-insulated 6" slab, but you should review that
section/table in its entirety (it's small) to confirm.

There are resources outside 90.1 that will lead you to similar F-factor
information, including a paper that explains how DOE2 handles perimeter
insulation that pops up every so often on this list (sorry not sure
where it's found online), but your Appendix A bullets out what you need
pretty well for this case. I'd suggest verifying what construction
layers Appendix A is suggesting you use, make any construction
modifications that may be necessary in your baseline model, and respond
to your reviewer with your findings and any changes made in response,
with updated results if changes were necessary. Be sure to cite
appropriate 90.1 sections where applicable in your written response.

If the reviewer (not you) is bringing up perimeter/core slab heat loss
under discussion, take care! She/he may be either testing you or
otherwise may not fully understand what F-factors are supposed to
represent: a perimeter heat loss factor. Carefully word your response
in a way that demonstrates you understand this distinction, but don't
turn it into a lecture as an uninformed reviewer may take offense and
unfairly turn the reviewing process against you and your project.

Best of luck!

NICK CATON, E.I.T.

Nick-Caton's picture
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Joined: 2011-09-30
Reputation: 605

I received a reply for help on a previous post on this subject indicating
there was a document somewhere that explains how eQUEST converts an F-factor
for perimeter slabs into an equivalent U-value for an earth contact floor.
The respondent noted an attachment to his message, but it didn't come
through. I've tried replying directly to have him resend but haven't heard
back as of yet. Is there anyone else who is familiar with this document that
could send it to me or direct me to where I might find it?

Thanks,

Rick Routh

Routh Consulting Engineers's picture
Joined: 2011-09-30
Reputation: 0