In article , krw wrote:
In article ,
says...
krw wrote:

... The reflective barrier will not keep heat in; zero R value.
It will *reflect* IR radiation and is useful in areas with
lots of sun, but it adds zero to the R value.

Wrong.

Not wrong. Foil does nothing to "resist" the conduction of heat
therefor has no "R" value. It will REFLECT radiated heat, but do
ZERO for conducted heat.

With an air gap, the foil adds a real R-value.

Nonsense. The air gap adds R-value. The foil adds nothing to the
heat conduction. Foils is METAL, which is a CONDUCTOR.

Regardless of the technicalities of wording, reflective barriers
are extremely important when the R value is low.

I experimented with some 3/16 inch Polyethelene foam with reflective
barrier on both sides. In my basement on the cinderblock, which is R 3.5,
I laid down a sheet against the block. I added another layer on the 3 inch
wood framing members. Measuring the temperature differentials was very
remarkable. Better than I could have imagined. It compared very favorable
with the 3 inches of fiberglass on other areas. I was thinking, and may still do,
add preforated foil on top the insulation in the attic.

greg