We are in the middle of remodeling our home, and are having
difficulties sourcing the following material on short notice:
1/2" plywood roof sheathing with radiant barrier applied
Are there any sources in the SF bay area (we are in Palo Alto) that
might have this in stock or available in a couple of days? Note that
this a plywood product is required -> several places have an OSB
product available, but the OSB does not meet the structural
requirements of the design.
Any thoughts would be appreciated.
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.
Hope this helps
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.
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.
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.)
The key idea is that if an object is in equilibrium with its
surroundings, then the emissivity e = 1 - reflectivity r. So a highly
reflective surface has low emissivity. This allows the radiant
barrier plywood roof deck to heat up from the sun without radiating as
much heat into the attic as normal plywood. A side effect of this is
that the radiant barrier plywood will be hotter than normal plywood
would be under the same conditions.
If I understand you, a shiny hot surface will be a poor emitter of
heat, even if it is much hotter than its surrounding air? And a flat
black pipe will be a much better heater/emitter than will a shiny
I've got a lot of general background stuff, but I never picked up on
that one. It's intuitively obvious that a flat black surface would
absorb heat, but no so obvious that it would also be a better
emitter. I got some readin' to do...
How much a surface emits (radiates) is independent of the temperature
of the surrounding air, it just depends on the surface temperature.
At a given surface temperature, a more reflective material will emit
energy at a lower rate than a black surface would.
On Aug 10, 9:50 pm, email@example.com wrote:
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
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.
Dennis, you are correct but studies, especially by FSEC show that dust
collection and the resulting radiant barrier degradation is when a
foil is placed on top of attic floor insulation. System deterioration
is nil or minimal when the foil is placed on the bottom of the
rafters, between the rafters or next to the roof decking.
I am repeating a lot of what I have heard from a local home inprovment show
down in the houston area, but the guy is heard all over the state, google
Tom Tynan, and see what you can find, but what the barrier does is reflect
the sun radaiant heat back, it realy dont reflect that much back into the
attic, just keeps it from getting so hot, we put that bubble type stuff up
in my folks house, and where it was as compared to where it was not, you can
feel a difference, best way to figure out where to put it, is too look at
where the sun hits the house, mostly on the south side, as that tends to
collect most sunshine.
Hope this helps
Craig, unless there is shade on the roof "all day long," one should
consider radiant barrier on all areas of the roof, including attached
garages. The entire goal of the a radiant barrier system (including
ventilation) is to reduce the temperature of the attic so there is not
as much heat the attic insulation has to "resist" ("R"-value) and
thereby migrate into the living spaces of which requires the HVAC unit
to run longer. All roof areas will contribute to heating the attic
even if they face the East and North. Granted, the roof decking
surface temperature may be approximately 10 degrees F lower on a North
or East facing roof (compared to the South and West roof lines), but
when the surface temperature is 120+ degreees, those roof areas will
still be cooking the attic.
On Wed, 08 Aug 2007 17:01:45 -0700, firstname.lastname@example.org wrote:
Thanks for the kind offer.
Quickly perused your site and followed a couple of links.
What do you recommend for new construction ?
And, maybe it's counter-intuitive but why is the installation on a
retrofit foil side down ? Why not foil side up ?
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