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

Ralph Muehleisen muehleisen at gmail.com
Tue Dec 1 21:26:23 PST 2009

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

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 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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