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

[Ralph Muehleisen]

True indeed.

Except when your roofs are not covered with snow.

Here in Chicago, at least the past few years,  snow has come in bigger
chunks with more warmi and cold periods in between.  And rooftop snow has
been melting during the warm periods so  rooftops have been bare for more of
the winter.

It would be an interesting study (maybe I can find an undergrad or
interested masters student) to actually look at a typical flat roof small
commercial building and see if the increased summer efficiency of cooling
equipment from a cool roof offsets the increased winter heating 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 Wed, Dec 2, 2009 at 5:47 AM, Christopher Schaffner <
chris at greenengineer.com> wrote:

>  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™
> 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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