I think you're confusing the placement of the "vapor barrier" with the
placement of a "radiant barrier".
In construction, it is best to place the *vapor* barrier on the 'warm side'.
So in heating climates, the vapor barrier is place on the inside and in
cooling climates on the outside. The logic behind this is you want to block
the moisture that seeps through the wall *before* it cools down and has a
chance to condense.
But the foil of a *radiant* barrier can be on either the hot or cold surface
and the difference is not very much. The most important part about radiant
barriers is that a) they be positioned/installed so the foil remains clean,
b) the have an air gap facing the foil (direct contact with the next layer
makes the foil useless), c) they are only effective if air convection
against their surface is not an issue.
A 2" thick foam board with *no* radiant foil will add about R12 to the
walls. If the foamboard has a closed-cell surface coating, it can double as
a vapor/draft barrier as well. Sandwiching a foil layer between other
materials with no air gap is a complete waste of money.
A 2" thick foam board with a radiant barrier applied to the board with
the board sitting behind a brick or concrete block wall has the required
airspace in front of the radiant barrier. If HardiPlank or HardiPanel
siding products or stucco are applied over the foam board, then paying
for a radiant barrier is nonsense.
You assume that the cladding is applied directly to the foam board. Not
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
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.
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
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?
Su, what the fuck does all that gibberish mean? Oh, I know, it means Nick
can show off he knows a couple of equations and has a calculator. So,
rather than impress us with all of your learning, why not take the time to
explain if this is good or bad and translates to dollars (or Euros) saved.
Yes, that closed cell EPS will be a good insulator.
Gibberish is right, "Covering the ceiling with foil gives you R10"
Covering nick with foil and putting him in " Nicks Lightbulb Sauna"
[easybake oven] of previous fame, for a month would do it, or maybe he
just got out of his Easy Bake Oven [sauna].
Thankyou for the very good points
The basement is going to be used for office, so it will be heated all
I should have said I'm situated in Denmark (just north of Germany :-)
Perhaps then my number of 6 degrees gradient is overly optimistic due
to fringe effects near the outer walls. I want as little insulation as
possible since adding too much to the floor will limit the height of
Currently no mostiture film is used. I want to add a wooden floor with
a carpet on top of this. The basement is dry, but my intention was to
add ventilation holes in the wooden floor and use no platic film to
allow the construction to"breathe". But adding plastic film might be a
good idea since this also limits heat flow caused by travelling
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