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Radiant Barrier problem - Page 4

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Posted by Solar Guppy on October 10, 2006, 2:55 am
My understanding is that the IR passes thru most building materials, i.e.
its transparent, meaning that the IR will pass directly into the living
space, raising the cooling load there, not have any real effect on the attic
space ( it was never heating anything in there to begin with ). You should
compare your kWh usage for a general idea if the load has been reduced from
a month from last year to the same month this year to gauge how effective it

Adding vents to the attic with proper soffit vents is the way to keep the
attic temps down ( all passive , you don't need powered fans ) , this will
reduce the heat load as well on the occupied part of the home.

I built a 3000sf home ( near zero energy )  installed foil radiant barrier ,
continuous soffit vents with both ridge and roof vents ( all passive ) with
R40 attic insulation. Electric bill was about 50.00 month in Central Florida
in mid-summer. Radiant barriers do work well , no direct experience with the
paint version, but its technically sound.

Posted by daestrom on October 12, 2006, 10:46 pm

Sorry, no.  Just about any building material you can find is opaque to IR.
In fact there are very few things you're likely to find that are not opaque
(besides air itself).

IR will be absorbed by almost any building material (like the insulation in
the attic) and is warmed by it.  The heat can transfer through the material
via conduction, and then be radiated away again from the opposite surface.
How much 'gets through' depends on the R value of the material and its
thickness.  So the overall affect is whatever insulative effectiveness the
thickness and R value of the material.

Radiant barriers (and radiant paints, which it seems the OP has), have been
controversial for some time now.  Some manufacturers have made claims of R
value that just cannot be verified and don't stand up to testing.  Some have
used 'equivilent R value' in their sales literature to try and get around
this and been caught by various court cases.

Radiant barriers *can* work, but they are misunderstood by a lot of folks.
If *radiant* heat transfer is a significant portion of the total heat
transfer between two surfaces that are separated by air (i.e. no significant
convection/conduction); then putting a surface/coating on *either* the hot
or the cold surface that has a low emissivity/absorptivity will reduce heat
transfer via radiation.

Radiant barriers suffer from accumulation of simple dust on them, it raises
the emissivity/absorptivity of the surface and negates the affect.
Government studies have found it is best to put such barriers on the
underside of rafters, so that they 'face downward' and don't accumulate

The govt testing data for radiant barrier foils have yielded only fair
results.  It reduces radiant heat transfer, but in some climates, that's
like putting a bottle cork in a pipe that's 12" in diameter.  Most home
heating losses are by conduction/convection and the radiant transfer is only
a very small fraction.  Manufacturers like to spout how it "reduces radiant
heat loss by 50%".  What they neglect to say is that "radiant heat loss" is
only about 10% of your heating costs, so you really only save 50% of 10% or
just 5%.

I'm sorry to say that 'radiant barrier paints' have done even worse in gov't
testing.  The sales pitch about tiny 'flakes' of low radiant material just
seems like smoke and mirrors (pun intended).   Such paints *do* work in high
temperature applications, such as painting the insulation on
high-temperature (>600F) steam piping.  At those temperatures, the fraction
of heat loss by radiation is larger and can justify the practice, but at the
150F or less range of an attic, I doubt the difference is even measurable.

Some folks are also confused by the isocyanic foam board with a radiant
foil.  In order to get any benefit, the foil must *not* be touching the next
layer of construction material (OSB, or drywall), but must face towards a
'dead air' space.  Otherwise, you're just wasting your money paying for the
foil.  If your stuffing the wall cavity with insulation (fibreglass,
rockwool, whatever...), then the foil will be touching *that* and you
*still* don't get any benefit from the foil.

One place where radiant barriers *have* made in-roads is in windows.  With
double-pane glass, filled with argon gas, the modern 'fenestration' (fancy
word for window), has reduced conductive and internal convective heat losses
quite a bit.  But the inner pane, near room temperature, radiants heat to
the outer pane (outdoor temperature).  This radiant heat transfer is a
substantial fraction of the heat that is lost, so a 'low -e coating' is
applied to the inside surface of one of the panes.  This reduces radiant
transfer between the panes and improves  performance.  This is in addition
to windows that have a coating to reduce 'solar gain'.


Posted by Mike on October 20, 2006, 4:28 pm
 On Thu, 12 Oct 2006 22:46:08 GMT, "daestrom"

Argon filling makes next to no difference to the thermal performance
of windows.

For all practical cases substitute 'no difference' for 'next to no


Posted by nicksanspam on October 20, 2006, 4:42 pm

I seem to recall it reduces the heat loss by about 20%.


Posted by daestrom on October 21, 2006, 5:20 pm

Not true.  Argon (1% of standard 'air') is a monatomic gas, versus all the
other major gasses in air (nitrogen and oxygen are both diatomic).  And it
has a  higher atomic weight of an Ar (41amu) atom versus N2(28 amu).  This
results in Ar having a lower thermal conductivity, and lower specific heat
than 'air'.  These mean that both conduction through Ar from one pane to the
other is lower, but also that convective heat transfer is lower.

For 'practical cases', some experts say the difference is about 15 to 20%.
I think that's more than 'next to no difference'.


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