Quote:
Originally Posted by ctgottapee
The foil face doesn't work like reflecting like light does on a mirror facing you, although it will reflect light.
It prevents radiant energy from 'radiating' off of it, or away from its face.
With the foam installed under the floor, foil radiant barrier facing down, the foam insulation is taking on the heat from the floor above, slowing its transfer due to its Rvalue. That energy will try to transfer through the foam to the colder air below by radiating off the outside face. A radiant barrier prevents that energy from radiating off the face.
This is why a radiant barrier facing down installed in the attic can cut cooling costs in typically warmer southern climates. It stops the heat from radiating down off the hot roof. Of course heat will still travel via convection and conduction.
The radiant barrier's generic Rvalue is that under test conditions which always have heat flowing in the test condition. It has little use facing up in a crawl space as the crawl space is never going to be warmer than the floor above. An accurate "installed" Rvalue would show that.
Both real world heat flow and proper install methods is why the govt got all over the case of the radiant barrier promoters. Unless you live on a volcano, there is no effective Rvalue for the barrier facing up in a crawl space.
Another example: They wrap the guy who just finished the marathon in a thin radiant sheet - the radiant layer on the outside prevents his/her generated heat from radiating away keeping their heat close to them [their sweaty clothing and cool weather at that point are sucking the heat away]
The "Reflectix" sales site is a pretty good example of effective Rvalues in various situations [if the exact build conditions are followed for the typical climate zone]. They were thoroughly sued so their case examples are legit, although they don't explain them thoroughly - in some examples the other insulation used provides most the installed Rvalue, you can figure it out with comparison and a little math. They even removed the attic radiant Rvalues due to the US climate variance. In the winter, a radiant barrier facing into a cold attic does no good unless you live in the tropics basically.
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This is all very interesting.
Do you have sources that elaborate, validate, verify, explain or experimentally prove what you have just written?
Best,
-AC