Where is the
best placement for a radiant barrier? Radiant barrier, cellulose, home, energy,
fiberglass, radiant barrier, attic insulation, energy conservation, green
building


Where Should Radiant Barrier Be Installed?
Fact: There is more bad
information about radiant barriers than there is good.
Fact: Many sites are outdated. (#1 hit site on
google was last updated June 27,
2001)
Fact: Radiant barriers are not rated using
the R factor
Fact: Radiant barrier was not invented by NASA, it
was invented by Sir James Dewar, a Scottish physicist who invented the
Thermos Bottle among other things at the turn of the century.
Fact: Lowering the emissivity of a surface
(which is one aspect of a radiant barrier), can have negative consequences, as
in making your home use more power and become more
uncomfortable if placed in the wrong area.
(Note to reader: The information on this page
only applies to buildings in hot climates).
Under the Roof Decking:
-
This application has absolutely no benefit during
the winter, despite how mild your winters are. Would it not make sense
to purchase and install a product to control heat gain and heat
loss? This way your investment pays off all year and increases comfort
all year.
-
Lowering the emissivity of a roof decking the
attic with silver paint, laminated foil, or stapling a radiant
barrier up under the roof decking raises the temperature of the
surface above it, that's just basic physics and common sense. If radiant
heat is restricted from emitting from a surface (as in the bottom of your roof
decking, a chrome car bumper or a chrome socket in your toolbox), heated
by the sun, where does this heat go? With lower emissivity it's
going to increase the roof temperature and have to radiate back out...one
problem: the sun is beating on it at the same time. Is that going make
your roof hotter? Of course it is. Is this going to extend the
life of your roofing system? (shingles or on tile roofs, the
underlayment?). I'm not going to leave myself open telling you my
experiences so let me tell you what the largest shingle manufacturer told me
during a recent phone call: (question) "Yes, I have a question.
Will installing a radiant barrier under the roof affect the warranty of your
product?" (answer) "Our 25 year warranty covers manufacturers
defects". When pressed for an answer regarding the installation of
a radiant barrier and elevated roof temperatures I was again told:
"Our warranty only covers manufacturers defects, that's all I'm going
to say, goodbye". It seems I'm not the only one afraid to touch on this
topic so that leaves the reader on his/her own to decide if increasing the
roof temperature is going to lessen the life of the roofing system by making
it hotter and drying it out. Really a problem with concrete tile roofs
that rely on a thin piece of bitumen based (tar), felt (paper), to keep the
ceiling dry. Think twice before spraying paint under
your roof, buying foil backed OSB or stapling a reflective membrane
up under the roof deck.
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If increasing the roof deck temperature
when lowering the emissivity under the roof increases roof
temperature, how is this going to affect the air in the attic that
is in direct contact with this hotter roof? Stands to reason if the roof
deck is hotter, the attic air is going to get hotter from the
increased convective air movement from the hotter roof deck. If the
air in the attic is getting hotter via convection from the hotter
roof, wont this in turn make the insulation hotter and the
ducting in the attic hotter? Hotter than a house
without RBS on the bottom of roof deck?
Basic physics and the laws of thermodynamics guarantees it
will and the graph below illustrates it. Note: This
is not true in flat roof homes, only in homes with a pitched roof. In
flat roof homes installing a radiant barrier under the deck is a
perfect application.
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Next question: If the attic
insulation gets hotter with the radiant barrier than an attic with bare
wood, what happens when the sun goes down and the roof cools off? What
is this superheated insulation now facing? A radiant barrier. A
radiant barrier which now effectively holds the heat in the attic for
hours after the sun goes down. (again, see the graph below).
We would rather see the RBS right above the insulation, keeping heat out
during the summer and heat in during the winter, with multiple layers to
insure a lifetime of performance. This also does not affect roof
temperature as the emissivity of the roof has stayed the
same. Covering the ducting in the attic with RBS is also
another significant energy conservation method. Click here to
see our duct wrap page. NOTE: Installing a radiant barrier in the attic
must also be accompanied with increasing the attic ventilation. The
combination of a radiant barrier and increased attic venting will have a
significant impact on energy use and home comfort.
Before you spend your
energy conservation dollars on anything, especially a radiant barrier
product, do some research, follow the logical path and ask a lot of
questions. Specifically, ask for a lifetime performance warranty and
compare samples.
RBS placement in the attic is very
important
In the summer of 1993 an Arizona
electric utility began testing the performance of three types of Radiant Barrier
Systems (RBS) in three different placements in four identical unoccupied homes.
One home was a control house with no RBS. Data collection included time,
temperature and measurement of power usage. All power was shut off in these
homes except for the heat pump. The results of this testing brought to light
some interesting data as shown below. Our product, RBS
Chips, is outlined in green

The graph above illustrates some interesting
points:
Depending on the climate your home is located, RBS placement is critical
not only to performance, but to prevent damaging condensation problems within
your walls and insulation.
UNDER THE ROOF DECKING
Placing a radiant barrier
directly under the roof decking as illustrated above raises the attic
temperature (and energy consumption), between 2pm and 8pm. The reason is
simple, when the inside roof deck surface
emissivity is lowered from .95 (bare wood), to a lower
value the result is a hotter substrate hence the logic behind the radiant
barrier. This is a perfect application for a flat or nearly flat
roof deck. The reason being, the increased temperature of the roof
deck will not create convective movement since the roof is not pitched.
The steeper the roof, the more convection there will be which in turn heats
up the attic insulation and once the sun goes down, the elevated attic
insulation is now facing the radiant barrier on the bottom of the
roof. Again, this only applies to pitched roofs.
Spraying a roof silver
underneath the roof deck or using a plywood decking product
with a radiant barrier laminated to it as can be seen
above is an improper installation, especially if the attic is vented to standard
venting code.
Solution: In hot climates our Thermal Control Membrane product is
best. RBS Chips are perfect for cold
climates where moisture egress is a consideration.
Thermal Control Membrane installed between the trusses in the
attic. Easy to install with simple tools, no staples.
RBS Chips, the most efficient attic
insulation in the world

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