"Jeff Thies" wrote in message
nk.net...
wrote:
Duane C. Johnson wrote:
I understand there is an associated R value associated with air gaps.
Sure...
However, I would have assumed that the R value of conventional
insulation, say fiber glass, would have been better than that
of an air gap.
Add the air gap's R-value to the insulation's R-value.
As long as we're talking about insulation, I've been trying to figure out
the radiation from the insulation and am not getting far.
From a EffHr 0.0455 (for that .05 foil Eff) and:
Thermal resistance R = 1/C, where C = Hc + EeffHr.
It appears to me that the max effective R value would be 22, and that
for an infinitely thick blanket with a foil outer barrier. It would seem
that the radiation from the insulation would mean these high R blankets
would have diminishing returns.
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.
It also seems that a foil barrier on the ambient side of the insulation
would be very valuable.
But things are not done this way, what have I misunderstood?
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
sun.
daestrom