daestrom wrote:


"News" wrote in message
reenews.net...


"daestrom" wrote in message
...


snip


I always thought the shiny side reflects, so needs to be facing where
heat needs to be reflected back and there needs to be a 1" gap between
that and any other surface. Having it under floors facing down should
not be effective. Yet I have read that some makers say it does not
matter which way it goes, I find that hard to believe.


Let me see if I can clarify it a bit for you.

Radiant heat transfer involves two surfaces. The 'hotter' one radiants
infrared energy, the 'cooler' one absorbs it. How well a particular
surface emits infrared energy when heated is measured by its emissivity.

So to reduce radiant heat transfer, we can coat the cold surface with
something that reflects infrared energy so it doesn't absorb as much.
-OR-, we could coat the hot surface with something that doesn't
radiate/emit infrared energy as well. Either one will reduce the amount
of infrared energy that gets from the 'hot' surface to the cold surface.

Now, it just so happens, that with very few exceptions, surfaces that
are poor at absorbing infrared are poor at emitting infrared. And
surfaces that are good at emitting infrared are also good at absorbing
infrared. Polished metal and metal foils are very poor at absorbing and
emitting infrared. So radiant barriers have a metalized/foil surface.
There emissivity is quite low ( 0.2, some as low as 0.05). Non metalic
materials (wood, plaster, glass wool, etc...) are good
absorbers/emitters (emissivity 0.8, often 0.9).

So, in the case of under-floor radiant barriers, if we cover the 'hot'
surface with a material that is a poor absorber of infrared (and hence
is a poor emitter of infrared), we get about the same overall affect as
if we had covered the 'cold' surface with it. We could cover either one
and get about the same affect, at least in the short term.

I've been think about that as far as my staple up radiant goes. It
looks to me that I want to cover most of the area (perhaps all) between
joists with flashing thickness aluminum to spread the heat out. That's a
lower operating temperature for the working surface. I don't think the
emissivity is as low as foil, but probably around .08. It seems to me
that part of the joist should also be covered in a radiant barrier.

It looks like you can gain a good bit of insulation value just from
having a dead air space with an IR opaque boundary.

I see from my "Passive Solar Energy Book" That a horizontal foil
surface with heatflow down has an R value of 4.55. That would seem to
imply face nailing 1" polyiso foil covered, with the foil facing down
onto the joists. Perhaps R17 total. Does that sound about right?

Jeff































But once the poor absorber/emitter is covered with dust, the heat can
travel from the foil to the dust by conduction (a very good transfer
mechanism). And household dust has a very high emissivity, so it
absorbs/emitts infrared quite well. So the dust layer completely
circumvents any savings of the radiant barrier. So we *really* want to
keep the radiant barrier clean.

And by putting the poor absorber/emitter on the underside, we have it in
a position (facing downward) where dust and dirt are less likely to
settle on it.

daestrom