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.
