Hi,
I've been struggling with this issue for a while now. What is a good way
to come up with an acceptable U-value for slab-on-grade floors based on
the F-values listed in ASHRAE Standard 90.1?

A while back, I received an Excel sheet from someone at HAP support.
It's supposed to help you arrive at a U-value, but there seems to be too
much freedom to come up with a wide range of U-values as you can change
several variables. One thing in particular I'm curious about is defining
the Effective Width of Slab. The directions on the Excel sheet say to
use the distance from the exterior wall of a given space to the opposite
interior wall if you are finding a U-value for just the space. However,
it also says you can use the distance from the perimeter to the center
of the building if you are finding the U-value for the entire building
slab. Two things: (10) This is a lot of work for a building with 200
and some spaces and doesn't seem worth the effort. (2) if the building
isn't a square, how do you define the center of the slab (aside from
using calculus)?

Sincerely,

N. E. Fisher & Associates
Matt Scott
e: mscott at nefisher.com
p: 610-369-3190
f: 610-369-3194