[BLDG-SIM] effect of window blinds on solar heat gain

[D. Charlie Curcija]

The attached report contains number of inaccuracies, which need to be
addressed for anyone that wants to make use of it.  First of all, "Shade
Coefficient" is non-existing expression.  There used to be something called
"Shading Coefficient", which was the ratio of transmitted solar gain through
a glazing in question and solar gain through the double strength single
glass.  This quantity is long abandoned, because it was true only for normal
incidences, it was somewhat OK for of normal angles of mostly clear glazing
and is grossly inaccurate for spectrally selective low-e coated glass, which
is starting to dominate market.  Concept of Shading Coefficient was so
problematic that ASHRAE decided to drop it completely almost 10 years ago.
I was the chair of handbook subcommittee which was in charge of fenestration
chapter when this happened.  As a transition, we provided some crude
conversion from SC to SHGC, with the caveat that is only a CRUDE
approximation for off normal angels and is to be used for measured SC data
that existed earlier.  
 
Then, report says that "Shade Coefficient" is calculated using NFRC and
ASHRAE approach.  Not only that NFRC nor ASHRAE do not use Shade Coefficient
or Shading Coefficient, but there is no approved procedure at NFRC to
calculate performance of windows with shading devices.  WINDOW program,
which is used by NFRC to rate windows, in its approved version does not deal
with shading devices either.  There is research version 6.0 (downloadable
from windows.lbl.gov), which can model SHGC of shading devices, including
venetian blinds and woven shades, but that is not approved by NFRC yet (and
it wont for another year or so, until it is tested and validated fully).
 
Finally, report shows how darker shades increase peak cooling load without
ever mentioning what is the SHGC of the window assembly, how is that
calculated, or what are the solar optical properties of the shade itself, so
it is really difficult to corroborate why would cooling load be higher.
 
Is there any technical documentation of the Carrier HAP program that would
detail assumptions and algorithms used?  One would expect for reputable
company like Carrier to be more careful with its "facts" and claims.
 
Charlie Curcija
 
-------------------------------------------------------------------
D. Charlie Curcija
DesignBuilder Software
18 Tanglewood Rd.
Amherst, MA 01002
 
Tel: (413) 256-4647
Fax: (413) 256-4823
cell: (413) 575-3487
email:  <mailto:curcija at designbuildersoftware.com>
curcija at designbuildersoftware.com
web: http://www.designbuildersoftware.com
<http://www.designbuildersoftware.com/> 
 
Support open document format as the best way to assure full compatibility
and interoperability! 
 
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From: BLDG-SIM at gard.com [mailto:BLDG-SIM at gard.com] On Behalf Of Stirling
Walkes
Sent: Thursday, September 28, 2006 12:36 PM
To: BLDG-SIM at gard.com
Subject: [BLDG-SIM] effect of window blinds on solar heat gain


Alec,
 
 
Attached is a link to a Carrier HAP document regarding this issue.
 
http://www.xpedio.carrier.com/idc/groups/public/documents/marketing/hap_ehel
p_009.pdf?SMSESSION=NO
 
Regards,
 
 
 
V. Stirling Walkes, P.Eng., LEED AP
SMS Engineering Ltd.
770 Bradford Street 
Winnipeg MB R3H 0N3 Canada
phone: 204.775.0291
fax: 204.772.2153
Swalkes at smseng.com

>>> "Alec Stevens" <astevens at dmiinc.com> 9/27/2006 2:01 PM >>>


Hi,

Does anyone have any source that documents the effect of (interior) window
blinds on solar heat gain?

We are working on a project with motorized blinds that would respond to a
photocell.  The blinds are a fabric mesh that allows 40% visible
transmittance.

 

Has anyone used a reliable factor to account for any reduction in solar heat
gain from this type of window blind?

 

Thanks for any info that you may have!

 

Sincerely,

Alec Stevens, PE, LEED AP

DMI

450 Lexington St., Newton, MA 02466

p: 617-527-1525 x105 f: 617-527-6606

e: astevens at dmiinc.com

 

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