Radiant
barriers are used more in hot climates. Some authorities
do not allow them in cold climates, especially if they
are made of multiple layers of aluminum foil, separated
by an air space. In some tests, these systems used in
walls show a negative R-value! Heat loss was actually
greater with these systems in place.
The
foil can be:
The
foil needs to remain clean and shiny to do its job. It
also needs a minimum of half an inch of air space in
front of it. Burying the foil in insulation will dramatically
reduce its effectiveness. The air space has to be still
for the system to be effective. Convective loops eliminate
the insulating value and may pump heat out of the house.
This is the suspected mechanism that resulted in the
negative R-value in tests.
Radiant
barriers may be used in walls and if so, you won’t
usually find them. You may see them attached to the underside
of roof sheathing or rafters in attics.
These
systems may be more effective by helping to cool homes
in the summer rather than helping to control winter heat
loss. There is no downside to their presence, although
there may not be an upside either, especially if the
foil is dust-covered or if there isn’t any dead
air space.
Radiant
barriers can be a problem if they are installed on the
cold side of insulation. There they may act as a vapor
barrier on the wrong side.
