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

Christopher Schaffner chris at greenengineer.com
Wed Dec 2 03:47:31 PST 2009


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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