"CJT" wrote in message
...
News wrote:


wrote in message
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Jeff wrote:
snip


One thing though about radiant barriers. It's well settled that the
upper surface of horizontal installations will not retain its low
emissivity. Unless you fancy wiping and cleaning off the dust every
year
or so, it will accumulate and lose its effectiveness.

snip

In keeping heat ina house, fine for the walls, no good in the attic.

I disagree, but I'm willing to listen to your reasoning if you care to
present it.


As I said before. Government and independent testing has shown that radiant
barriers lose much of their effectiveness if they get a layer of dust over
the foil side of them. The dust raises the emissivity to that of other
non-metallic materials ( 0.85).

In places like attics, the 'usual' installation of radiant barriers is not
across the floor, but attached to the rafters overhead, facing downward.
This avoids the dust buildup issue. Thus the radiant surface is 'aimed'
downwards to the floor space of the attic. In climates that need a lot of
A/C, this can work quite well. The solar heat gained by the roofing heats
the sheathing and rafters, but the radiant barrier prevents it from
radiating to the attic floor (ceiling of the living space). Testing for
their efficacy in such installations has gone well. Radiant barriers in
this sort of situation can be an inexpensive, easy to install way to reduce
cooling energy needs.

Sadly, for climates needing a lot of heating, the situation doesn't work so
well. Installing the radiant barrier on the rafters does little to reduce
heat loss from the attic floor (living space ceiling). One reason for this
is that attics in cold climates are deliberately ventilated to keep the
attic cool. This prevents ice damage and ice dam formation on the eaves.
Another reason for poor performance in heating climates is that with the
heat flow upwards, natural convection of air from the attic floor to the
radiant barrier far outweighs the radiant heat transfer component, so
reducing the radiant heat transfer does little to reduce the overall heat
transfer (most upward heat flow still happens from convection currents).

So, bottom line. If the direction of heat flow is upward, radiant barriers
don't work well. Either convection outweighs the radiant component, or the
surface gets contaminated with dust and requires cleaning, or both.

For downward heat flow, they can add to the overall insulation if installed
correctly.

daestrom