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

[Brad Painting]

Thanks for all of your thorough explanations. The issue is much clearer now.
I'll work my way through the resources and see if any other questions pop
up. Thanks again.

On Mon, Nov 30, 2009 at 11:19 AM, David Bryan <dbryan at amerindian.com> wrote:

>  Correction -
>
> There a quite a few materials in that report that reflect both solar
> visible and infrared but there's none that have reflectance in the near
> infrared and low in the visible. I think that means they didn't find dark
> colors that could be very effective cool roofs.
>
> <http://www.azcoolroof.com/downloads/Resources/Article%20Laboratory%20Testing%20of%20the%20Reflectance%20Roofing%20Materi.pdf>
> Dave Bryan
> Third Level Design
>
>
> Brad Painting wrote:
>
> Thanks for the replies,
>
> I get it now. The "emittance" would take the form of infrared radiation,
> which cannot pass through opaque objects. So like Alex said, the the roof
> material would reject energy to the air but not "downwards" through the roof
> lining.
>
> What was really throwing me was reading that the most emissive material is
> purely black, while the least is purely reflective (
> http://en.wikipedia.org/wiki/Emissivity). So it seemed that if you went
> with a high solar reflectance it would have to have low emissivity. But
> Dave, are you saying that these properties can be split somewhere along the
> electromagnetic spectrum? If a material reflects a certain wavelength, can
> it not emit that wavelength?
>
> We skipped over the section on radiation in Heat Transfer :(
>
> Brad
>
>
> On Sun, Nov 29, 2009 at 9:34 PM, David Bryan <dbryan at amerindian.com>wrote:
>
>> Remember that emissivity and absorptivity are generally equal and often
>> vary with wavelength. And for opaque materials, emissivity generally equals
>> (1- reflectivity) at a given wavelength.
>>
>> So it would be possible to have a spectrally selective roof which
>> reflected the sun's visible and short wave infrared energy well (high
>> reflectance, low emittance) but also radiated energy well at the longer
>> infrared wavelengths emitted at its temperature rather than the sun's (low
>> reflectance, high emittance). This would be the ideal cool roof material.
>>
>> This roofing material probably exists.
>>
>> Dave Bryan
>> Third Level Design
>>
>>
>>
>> Brad Painting 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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