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Posted by on August 14, 2007, 4:43 pm
On Aug 10, 9:50 pm, websu...@cox.net wrote:
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> > One of the major problems with any radiant system is that the dust that
> > forms on its surface will degrade it reflectivity. One of the biggest
> > problems with radiant barriers. Oak Ridge has a good deal of info on this,
> > one doc I see referenced from your website "Radiation Control Fact Sheet"
> > found on the Oak Ridge website. It discusses how these barriers defrade over
> > time. Even so, people who live in the sun belt, Arizona and the like, would
> > benefit greatly from adding a radiant control barrier to their homes.
>
>
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> > >> What I know about it, its a spray on, like paint, can be spryed with
> > >> airless
> > >> equipment, as to uniform, depends on how well you can spray, like paint,
> > >> down here in texas some insulators are also into spraying it, I want to
> > >> say
> > >> I have heard that Sherwin Williams stores have the paint, or check with
> > >> the
> > >> Florida Solar Energy Center, may can get more info from there.
>
>
> > >> > Hi Craig.
>
> > >> > I'm not failure with a spray-onradiantbarrier, do you know anything
> > >> about
> > >> > it?
> > >> > Wonder as to how uniform it would be?
>
> > >> > > One way I have seen it done is to have theradiantbarriersprayed in
> > >> the
> > >> > > attic, that is done a lot in retrofit, and seems to work pretty well,
> > >> the
> > >> > > plywood is still the way to go in new const though.- Hide quoted
> > >> > > text -
>
> > >> - Show quoted text -
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> > > All, spray on paints are, by definition of several sources, are "not"
> > > radiant barriers. They are actually Interior Radiation Control
> > > Coatings (IRCCs). A radiant barrier has to have a minimum of 10%
> > > emissivity and 90% or more reflectivity (California Title 24 code is
> > > even more stringent). There are several manufacturers of the
> > > reflective paints, all with different reflectivities. A foil is a
> > > "known quantitiy" by virture that the inherent properties of aluminum
> > > foil have a 3% emissivity and 97% reflectivity. Paints are almost
> > > always applied by a contractor just because most people do not want to
> > > rent a spray applicator , deal with the mixing of the paint, etc.,
> > > etc. Whereas, foils can be applied by different methodologies.
> > > Another consideration about paints is that if the decking ever has to
> > > be replaced due to damage, the paint goes away as well.
>
> > > Regarding the San Francisco situation, concur that if you can't find
> > > the OSB or plywood decking with the foil already adhered to it, there
> > > are foils and other foil products that can used as underlayment.
>
> > > I have a list of links to several unbiased, government and university
> > > technical sources on my website: www.reflectafoam.com/technical.html.
> > > If anybody needs further info or specific information, don't hesitate
> > > to contact me.
>
> > > Thanks, Donnie.
>
> Okay, I've been an energy nut for a while, and have known about
> radiant barrier decking for some years. But this is one I don't get.
>
> The radiant barrier is essentially a layer of aluminum foil on one
> side of the plywood. The aluminum foil reflects heat. In the attic,
> the barrier is on the bottom side of the plywood, so would reflect
> heat back into the attic, which is generally where I am trying to get
> the heat away from. Isn't this counterproductive????
>
> If someone were to put the foil on the top surface, it would be
> useless because the shingles would be in contact with the foil,
> killing it's effectiveness immediately.
>
> What'd I miss?
>
> (On the other hand, if I have a foil surface on the TOP of the
> decking, but under a curved tile roof, there would be a space for the
> foil to be effective, by reflecting the heat back up to the tiles.
> The dust problem remains.)- Hide quoted text -
>
> - Show quoted text -
The best way to comprehend the physics of a radiant barrier is to
think about the emissivity and not the reflectivity. By definition, a
radiant barrier must have a minimum of 10% or less emissivity and be
positioned so it is exposed to an air space, i.e., your attic. With
that said, emissivity and reflectivity are closely related in that
when you add them together, they must equal "1" (as Wayne accurately
states).
But because most people more readily understand "reflecitivity" and
not "emissivity," companies typically promote a material's
reflectivity. So when you position your roof decking with the foil
already adhered to it facing downward toward the attic, think that the
foil is "emitting" only 3% of the radiant heat from the other side.
It doesn't matter how the radiant heat on the other side impacted the
foil, i.e., radiation, conduction, convection, the foil "still only
emits 3% of that heat." As is with IRCC paints, they typically
promote a paint reflects 65%-75% of the radiant heat, but...they also
emit 25%-35% of the heat from the roof decking.
Thanks, Donnie.
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