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

Christopher Schaffner chris at greenengineer.com
Wed Dec 2 07:37:55 PST 2009


Another point to consider is that ambient temperature affects HVAC system
performance. If you¹ve got rooftop mounted condensers, they¹ll operate much
better on a ³cool² roof.

Not sure how you¹d model that effect.

--

Chris Schaffner, PE, LEED AP, LEED Faculty
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 7:38 AM, "Bishop, Bill" <wbishop at pathfinder-ea.com> wrote:

> I¹ll 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/
>  
> We¹ve 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® 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 ³warm² roof will really help much. Go with the cool
> roof. 
> 
> --
> 
> Chris Schaffner, PE, LEED AP, LEED Faculty
> 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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