Well, the law of diminishing returns certainly applies to any insulation
project. But you can certainly insulate with higher effective R values than
22. Don't see what you're getting at there. One foot thick fibreglass can
give you about R-38, irrespective of the coatings or any 'facing' such as
drywall or OSB over it.
Remember, a higher R-value *behind* the foil surface makes the foil surface
temperature closer to the ambient temperature. And that reduces radiant
losses as well.
As far as a foil barrier on the ambient side, it isn't really all that
valuable in most circumstances. Yes, it would certainly reduce the heat
gain from direct sunlight, and reduce radiant heat loss. But if you look at
how much heat is lost to the environment due to simple convection, you will
realize that even if radiant heat losses were cut to zero, it wouldn't
reduce the total heat losses by a significant percentage in most cases. You
won't get a lot of improvement for the 'buck'. And a lot of folks don't
like the idea of living in an aluminum foil sided house.
Radiant losses can be a big issue if the temperature difference is large
and/or you've already taken steps to reduce the other forms of heat loss
(conduction/convection). Or if your goal is to reduce absorption from the
You've just reinvented the "linearized radiation conductance" :-)
G = 4x0.1714x10^-8Tm^3 Btu/h-F-ft^2, where Tm is the mean Rankine temp.
But air spaces also transfer heat by convection and conduction...
That also depends on convection and conduction, which depend on the temp diff
and the direction of heatflow.
Our local college keeps liquid helium for their electron microscope's
superconducting magnet in a Dewar vacuum flask surrounded by liquid
nitrogen, with insulation around that.
H2 boils at 4.2 K. N2 boils at 77.3 K. So 2 mirrors with e = 0.03 would lose
5.67x10^-8x0.03x40.75^3 = 0.000115 W/m^2C by radiation, ie 0.00002027
Btu/h-F-ft^2, ie US R49335, vs an R20 house wall.
1/2" is just typical spacing between glazing on double-paned windows. It
doesn't factor into radiant transfer if the two surfaces are flat planes
whose area is >> spacing between them. Yes, the emissivity of many
materials are higher than 0.5. But the example (half of which has been
snipped here) compared the radiant heat loss with/without a low-e coating,
versus the conduction of having two films in direct contact. (remember Duane
had questioned why I said that emissivity is 'pretty much irrelavent' if
there is no air-gap).
The absolute temperature of most building materials is >> the delta
temperature between said materials.
The affects of convection and conduction were already discussed. Those
affects far outweigh the effects of radiant heat transfer in most building
applications. Only after reducing conduction with conventional insulation,
and controlling convection by properly designed air-gaps, does the use of
low-e coatings become a significant factor.
Interestingly, the spacing of glazing in double-paned windows is chosen to
minimize both convection currents and conduction. From a conduction
standpoint, a wider gap is better, but a wider gap allows stable currents to
set themselves up between the panes, bad from a convection standpoint.
Knowing the properties and pressure of the fill gas, one can choose a gap
that has the falling gas film on the cold pane interfere with the rising gas
film on the warm pane, and inhibit long-path (full height of pane)
I'm sure you mean the 'He' boils at 4.2K ;-)
This is a bit offtrack, but about attic insulation...
From your calculations of a typical EeffHr (radiation coefficient) of
.04555 this would give a maximum effective R value of 22 no matter how
thick the insulation. Thicker would get you closer to it but there are
Also, not knowing the Eeff of a paper barrier, it would certainly be
higher than .05
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