[Bldg-sim] Why should roofs have high emissivity?

[Bishop, Bill]

I'll add some general info to the discussion and do a little summing up.
 
LBL has studied emissivity and reflectance and has annual energy impact
results for hot and cold climates:
http://eetd.lbl.gov/HeatIsland/CoolRoofs/HeatTransfer/Heating.html
 
The Heat Island Group has a lot of information available pertinent to
the emissivity discussion:
http://eetd.lbl.gov/HeatIsland/
 
We've been focusing on roofs, but window design also has to consider the
same effects. For glazing, we add the property of transmissivity
(radiation passing through the surface) and we have to get more specific
about the types of radiation (solar, visible, UV, infrared) and the
direction (outside vs. inside, as well as angle of incidence).
http://www.commercialwindows.umn.edu/issues_energy1.php
 
For opaque building surfaces, such as roofs, walls and window frames, it
is understood that unless stated otherwise:
Reflectivity and absorptivity values are for solar radiation.
Emissivity values are for thermal/far-infrared radiation.
 
1 - reflectivity = absorptivity
 
So when ASHRAE 90.1 Table G3.1 requires you to model a roof reflectivity
of 0.30, you can use an absorptance of 0.70 (such as when modeling in
eQUEST).
 
While there are technical distinctions, the following terms are
generally used interchangeably:
Reflectance/reflectivity
Absorptance/absorptivity
Emittance/emissivity
 
~Bill
 
William Bishop, EIT, LEED(r) AP | Pathfinder Engineers & Architects LLP
Mechanical Engineer
 
134 South Fitzhugh Street
Rochester, NY 14608
T: (585) 325-6004 Ext. 114
F: (585) 325-6005
wbishop at pathfinder-ea.com
www.pathfinder-ea.com
P Please strive to live sustainably.
________________________________

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Christopher
Schaffner
Sent: Wednesday, December 02, 2009 6:48 AM
To: muehleisen at iit.edu; Chris Jones
Cc: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] Why should roofs have high emissivity?
 
Of course, if your roof is well insulated, it will be covered with snow.
I doubt very much that your "warm" roof will really help much. Go with
the cool roof. 

--

Chris Schaffner, PE, LEED AP, LEED Faculty(tm)
Founder and Principal

The Green Engineer, LLP
Sustainable Design Consulting
50 Beharrell Street
Concord, MA 01742
T: 978.369.8978
M:978.844.1464
chris at greenengineer.com
www.greenengineer.com


On 12/2/09 12:26 AM, "Ralph Muehleisen" <muehleisen at gmail.com> wrote:
Chris makes a good point to consider with cool roofs.

A cool roof will indeed reduce solar heat gain to the roof which reduces
its temp (good in summer) and reduces cooling requirements in summer but
will increase heating requirements in the winter.

Until someone develops a material where the emissivity changes with
temperature (and not just wavelength) a cool roof that is good in summer
will be bad in winter.

So, in colder climates, a cool roof can indeed increase the overall
energy use of a building.

Even so, some northern cities like Chicago, mandate cool roofs in
building code.  Why?  Because the cool roof will reduce the urban heat
island effect where the city has increased temperatures compared to the
surrounding areas.  

The thought is that the overall benefits of the  reduction in urban heat
island effect in summer is more important than the increased energy use
that comes from increased winter cooling load.


Ralph

Ralph Muehleisen, Ph.D., P.E., LEED AP, FASA
Assistant Professor and Director of the Miller Acoustics Lab
Civil, Architectural and Environmental Engineering
Illinois Institute of Technology
Chicago, IL 60616
muehleisen at iit.edu
tel: 312-567-3545  fax:312-567-3519


On Tue, Dec 1, 2009 at 6:28 PM, Chris Jones <cj at cr-jay.ca> wrote:
In some cases it may be counter productive to use a high emissivity
roof.  I have worked on uncooled warehouses where the team used an
approved roofing product to get that point but the heating energy
increased enough to lower the savings enough to lose an EAc1 point.  






Chris Jones
14 Oneida Avenue
Toronto, ON M5J 2E3.
Tel.  416-203-7465
Fax. 416-946-1005


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