[BLDG-SIM] Cool Roof simulation

Jon McHugh mchugh at h-m-g.com
Tue Oct 17 14:34:47 PDT 2006


Jon,

No, it is just the opposite. Cool roofs make most difference with  attic
deck insulation and unvented attics.

The delta T across the insulation is much increased with an unvented
attic (185 vs. 85 to interior of attic= 100 F delta T) With a vented
attic (140 attic to 75 F interior) = 65 degree delta T.

So cool roofs actually will have greatest impact when the battle is all
to be fought right under the roof.

Duct heat transfer is important, but is affected by sizing and a host of
factors (getting the attic heat balance correct).

Simulating heat transfer in attics is complex. Our residential
simulation, EnergyGauge USA does a good job. Here is more than you want
to know, but that's what it takes to get it right.

http://www.fsec.ucf.edu/bldg/pubs/techsupport_cec/index.htm

The software can be downloaded for a 15-day free trial:
http://energygauge.com/usares/default.htm

Danny

Jon McHugh wrote:

> Hi Danny,
>
>Do you want to weigh in on this?
>
>I am assuming that primary additional benefit from cool roofs is 
>reduced heat transfer to ducts in attics.  Savings is much reduced when

>considering spaces with insulation at roof deck.  Is this correct?
>
>Jon McHugh 

-----Original Message-----
From: BLDG-SIM at GARD.COM [mailto:BLDG-SIM at GARD.COM] On Behalf Of Graham &
Megan
Sent: Tuesday, October 17, 2006 1:41 PM
To: BLDG-SIM at GARD.COM
Subject: [BLDG-SIM] Cool Roof simulation

We have looked into low absorptance, high emissivity roof coatings on
several occassions.  Your results aren't too far off those we have come
up with using EQuest and Apache.

Judging by the mesaured data available from SkyCool, it seems there is a
property / physical mechanism that is not represented by conventional
absoprtance and emissivity.  There is speculation that the coatings have
an extended surface area from which they emit.  This extended area could
be a result of surface roughness or the material emitting from multiple
layers within the coating ... If this is in fact what is happening, the
sums will show a greater benefit though no program to my knowledge would
allow you to account for this.  In simple terms it would be represented
as an emissivity of greater than 1 across the projectd area of the roof
...

Anyone else getting similar advice from other coating suppliers?

Regards,
Graham

----- Original Message -----
From: "Aulbach, John" <jaulbach at nexant.com>
To: <BLDG-SIM at GARD.COM>
Sent: Tuesday, October 17, 2006 6:24 AM
Subject: [BLDG-SIM] Cool Roof simulation



Hello All:

I am looking for a fairly straight forward to model a "Cool Roof" in
eQuest.

My general understanding would be to do a Parametric Run, changing the
outside emissivity from, say 0.75 to 0.3. But I am getting VERY little
energy benefit.

My building is a 140,000 sf office, 2-story (70,000 sf roof). It has a
CW VAV system with HW reheat, and return air is through ceiling plenums.

I am only seeing about a 50,000 kWh saving out of 11,000,000 kWh HVAC
usage.

Any experience, suggestions?

John R. Aulbach, PE, CEM
Project Manager
Nexant, Inc.
100 North Barranca, Suite 820
West Covina, CA 91791 USA
Phone: 626-430-9054
Fax: 626-430-9060
email: jaulbach at nexant.com


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