Underfloor Patchwork

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This is a question that I've wondered about for a while. When you come
out of the wizard, an UF material is created for each zone(like below),
with different properties for each zone - with an amazing variation in
properties across the board. Does anyone know why? Is it reasonable to
assign one material to all of them (assuming they are the same
construction and sitting on the same soil), and if so, how do I know
which material to pick out of the 20+ construction assemblies created?

Vikram Sami, LEED AP

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Vikram - I was always under the impression that eQuest assigns a u-value
to respective zone-slabs based on their area and proximity to the
perimeter. I've never tested this theory, but I would assume that heat
loss coefficients would be calculated to reflect the variation of losses
as a zone relates to the whole "slab."

Matthew Higgins, LEED AP + ASHRAE-HBDP

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Long time back there was a write up on how to calculate the impact of
soil - and generate fictitious layer for walls/floors in contact with
soil - I think it was in the building simulation user news, with respect
to DOE-2.1 e. Depending on the perimeter area, the equivalent R-value of
the fictitious material is different. Probably equest uses the same
procedure.

-Rohini

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Vikram,

There have been some thoughtgul answers regarding U-values unique to
zones. As I recall from ASHRAE practices, slab heat transfer is related
more closely to some "F" factor (relating to the perimeter) than to a
U-factor.
Look at your dark blue cross pattern in the left foreground. This seems
to correspond to a zone (a hallway?), but not to proximity or distance to
the edge-of-slab. Therefore, I am guessing (and this is only a guess) that
eQuest is not trying to establish different U-values for these different
color regions, but that eQuest is merely assigning a floor segment to each
zone for calculation purposes. In other words, this does not seem
consistent with giving emphasis to the slab's perimeter for heat-transfer
calculations.
However, please tell me this: If my guess is correct, should I have
been assigning external walls and roofs to each individual zone's external
surfaces rather than to a floor's external surfaces?

Lars Fetzek, EI

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Vikram,

I?ve had the same question, and I would love to hear other people sound off?

I believe eQUEST is trying to account for additional heat loss in perimeter zones, where the slab meets the foundation wall. I?ve noticed that all the construction layers are the same except for the outer-most layer. eQUEST usually creates a zone-specific layer like ?EL1 UFMat (G.S1.U2.M1)? of type ?R-value?. The resistance of this material is what varies; core zones usually get something like R-100, and perimeter zones get something in the R-1 to R-20 range depending on whether foundation insulation is specified in the wizard. If you choose the default core/perimeter zoning scheme, what eQUEST is doing becomes a little clearer. The perimeter zones will all have a unique UF construction, but the layers will have identical properties. Naturally, it gets more complicated with custom zoning when a perimeter zone extends fairly far into the core of the building.

To test my hypothesis I?d have to sit down with the ASHRAE Handbook, determine a specific construction?s F-value based on the location?s heating degree days, and then use design temps and the linear feet of perimeter exposure to backtrack into an equivalent U-value that I can compare to eQUEST. However, the answer will still be an estimate at best.

To answer your question, I usually simplify the post-wizard defaults down to one core zone construction and one perimeter zone construction. For the perimeter zones, I usually make a perimeter construction with a U-value equal to the average of the perimeter zones? post-wizard UF constructions. eQUEST does not account for the fact that the soil?s thermal properties fluctuate as it gains and loses moisture, so I wouldn?t lose too much sleep over it.

Thanks,

DAKOTA KELLEY

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