A.Steve Andrews responds: Yes,
radiant barriers in vented attics can cut cooling bills by
about 10%. They achieve this by reducing heat flow down through
the ceiling by 40% or more. But the better question is: Are
radiant barriers cost-effective? The answer is also yes, but
only in the right climates, only when properly installed, and
only at the right price (see map).
Here’s a summary of the latest information:
Climate.
The NAHB Research Center’s study
(funded by Eagle Shield) promised to yield good information on
radiant barrier performance in cold climates, but it has hit
contractual snags. So climate-related advice hasn’t
changed: Radiant barriers in attics make the most sense in
locations where there are 2,000 cooling degree days or more
(see map at right). They may also make sense in more moderate
climates where annual cooling bills still exceed annual heating
bills (excluding mild sections of the West Coast). In cold
climates, radiant barriers can make sense in a crawlspace, but
this remains unproven for attics.
Properly
installed. For attics, staple
the barrier beneath the rafters, drape it over the roof trusses
before the decking goes on, or staple it directly to the bottom
of the roof decking — shiny side down in all cases (see
illustration at left). Make sure you vent the attic, since this
is the best way to cool this space. Research shows that laying
a radiant barrier flat over horizontal attic insulation can cut
radiant barrier performance by up to 50% after five years due
to dust buildup.
Cost.
If you buy material directly from a
manufacturer, expect to pay between 7¢ and 15¢ per
square foot, depending on the quality of paper backing and
fiber mesh reinforcement. These two features can dramatically
reduce tearing during installation; test this by ripping up
samples before you buy. According to the Florida Solar Energy
Center, if a homeowner pays about 20¢ per square foot for
the installed barrier, the simple payback will be as little as
five years in a cooling climate (based on electricity costs of
8¢ per Kwh). The greater the cooling load and utility
costs, the faster the payback will be.
Steve Andrews is a residential
energy consultant and freelance writer in Denver,
Colo.