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

[Ralph Muehleisen]

Brad,

The emissivity and absorptivity of a passive material at any given
wavelength are equal, as stipulated by Kirchhoff over 100 years ago.
The only materials that violate this law are phase change materials
(which only violate it in a time average sense) or materials that have
some funky quantum mechanical mechanisms for heat removal.

Most common building materials have a similar absorpitivity (and
emissivity) for both short wavelengths (visible light i.e. solar
energy) and long wavelengths (infrared).  It is quite possible however
to design materials with a different absorptivity=emissivity for short
wavelengths and long wavelengths.  This is what makes a so-called
"cool" roof.

If you write a simple spreadsheet to compute the temperature on a roof
using an energy balance you can show that for a standard roof (i.e.
absorptivity=emissivity about the same at all wavelengths) the lowest
roof temperature and therefore lowest heat gain to the building will
be at a roof for a low absorptivity=low emissivity.

As important (or perhaps more important) in the overall roof design is
controlling the roof surface temperatures.  High temperatures mean
that the rooftop will undergo large temperature swings which means
large expansion/contraction and more thermal stress on the roof.  High
roof temperatures also mean locally high air temperatures which means
any rooftop condensers will have decreased efficiency.

A high absorptivity black roof can truly get hot enough to boil water
or fry eggs (210+ degrees F) on a hot summer day without wind
throughout much of the US.  An aluminum roof on the other hand might
get up to 160 F.

I've got an excel spreadsheet that I use in my building enclosure
design class that does the heat balance calculation and will show you
the heat transfer from solar insolation, long wavelength radiation
exchange to the sky, convection to the sky and conduction to the
interior.
Anyone who wants a copy can email me and I'll email it back to them.

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 Sun, Nov 29, 2009 at 2:57 PM, Brad Painting <bradpainting at gmail.com> wrote:
> It seems to me that a roof that emits more radiation will have a greater
> warming effect on the building. Some houses in warm climates have radiant
> barriers because the aluminum has a low emissivity, thus blocking the
> infrared radiation. But both LEED and Energy Star suggest high emissivity
> for warm climates. Does this make sense?
>
> From LEED NC Reference Guide v. 2.2:
>
> "To maximize energy savings and minimize heat island effects, materials must
> exhibit a high reflectivity and a high emissivity over the life of the
> product."
>
> >From Energy Star website:
>
> "In warm and sunny climates highly emissive roof products can help reduce
> the cooling load on the building by releasing the remaining heat absorbed
> from the sun."
>
> Thanks,
>
> Brad
>
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