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

[jyotirmay mathur]

The point related to decrease in reflectivity due to dust deposition (as
indicated by Charlie) is quite significant (especially in dusty areas,
places with high SPM in air, and having less rain) which may become one of
the major causes for deviation of the actual energy saving from simulation
results. One may have to exert little more while attempting to calibrate the
model with actual results if the dust is not cleaned periodically.

jyotirmay mathur

Dr.-Ing. Jyotirmay Mathur
Coordinator, Energy Engineering Program
Associate Professor, Mechanical Engineering Department
Malaviya National Institute of Technology
J.L.N. Marg, Jaipur (India) -302 017
Phone:
+91-141-2529109 (R)
+91-141-2529061 (o), 2713211 (direct)
+91-94142-50329 (mobile)
http://jyotirmaymathur.tripod.com/


On Tue, Dec 1, 2009 at 4:46 AM, Acker, Brad <backer at uidaho.edu> wrote:

>  This has been an interesting thread. What I am wondering is do you plan
> on getting a LEED point for a cool roof? If so and you are not going to use
> a "pre-approved" product how do you plan to show compliance with whatever
> emissivity or reflectance values are required? If pretty sure if you just
> painted your roof white you would need to show some test data per the
> correct procedures that your roof has the correct values. Anyone have
> experience with this?
>
>  Brad in Boise
> ------------------------------
> *From:* bldg-sim-bounces at lists.onebuilding.org on behalf of Abaza Hussein
> *Sent:* Mon 11/30/2009 3:07 PM
>
> *To:* bldg-sim at lists.onebuilding.org
> *Subject:* Re: [Bldg-sim] Why should roofs have high emissivity?
>
>  A key issue is the difference in values between the reflectance and the
> emissivity of a surface. For example, some white paint has higher emissivity
> than reflectance which makes it better than aluminum foil.
> Hussein Abaza
> ----- Original Message -----
> From: "Larry Degelman" <ldegelman at suddenlink.net>
> To: "Brad Painting" <bradpainting at gmail.com>,
> bldg-sim at lists.onebuilding.org
> Sent: Sunday, November 29, 2009 8:43:05 PM GMT -05:00 US/Canada Eastern
> Subject: Re: [Bldg-sim] Why should roofs have high emissivity?
>
> Yes, it makes sense.  High emissivity allows a roof to radiate its stored
> heat to the sky, thus aiding the cooling effect, so, yes, good for hot
> climates!
>
> Note also that ASHRAE 90.1 gives credit for (i.e., relaxes) the U-Factor
> requirement for such roofs in climate zones 1, 2 and 3 (the warmest zones).
> The ASHRAE Standard uses the term "High Albedo" for roofs with high
> reflectance AND high emissivity -- this is addressed in Section 5.5.3.1.1
> and Table 5.5.3.1 in the 2007 version.
>
> Regards,
>
> ===========================
> Larry O. Degelman, P.E.
> Professor Emeritus of Architecture
> Texas A&M University
> College Station, TX 77845
> ldegelman at suddenlink.net
> ===========================
>
>
> From: Brad Painting
> Sent: Sunday, November 29, 2009 2:57 PM
> To: bldg-sim at lists.onebuilding.org
> Subject: [Bldg-sim] Why should roofs have high emissivity?
>
>
> 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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