Do Radiant Barriers Save Cooling or Heating Energy?
Radiant barriers - reflective insulation - CONTENTS: can save on cooling cost and may save a little on heating costs as well. Where to Install a Radiant Heat Barrier, & Which Way to Face the Foil?What are the Energy Savings from Radiant Heat Barriers? Roof Ventilation and Radiant Barriers. Radiant barriers are not a substitute for insulation in cold climates. Radiant barrier researchers think roof venting is unnecessary - are they right?Solar Age Magazine Articles on Renewable Energy, Energy Savings, Construction Practices
Radiant Barriers as Energy Saver in Attics or Building Walls
Sketch at page top and the text are reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.
The two links below provide a photocopy of the original two page Solar Age article "Radiant Barriers, they cut cooling bills and help a little on heating too", on the performance of radiant barriers in buildings. Just below the links we provide radiant barrier information updates, radiant barrier product photographs, and we summarize information on radiant heat barriers.
RADIANT BARRIERS part 1, PDF form, how radiant barriers work, how radiant barriers save energy - use your browser's back button to return to this page
"Radiant barriers are highly cost-effective in hot climates because they sharply reduce cooling loads. They work as if by magic, but the magic is based soundly on principles of physics."
This article explains the science of radiant barriers (summer heat travels through attics primarily as radiant energy, so when the sun heats up the roof to 190 degF (black roof) or perhaps 160 degF (on a white roof surface) heat is conducted through the roof materials to the roof sheathing where it radiates heat to the area below. Also see radiant barrier details for cathedral ceilings discussed
at CATHEDRAL CEILING VENTILATION .
Where to Install a Radiant Heat Barrier, & Which Way to Face the Foil?
Our radiant barrier photos just above show the installation instructions printed on the kraft paper facing of ALFOL, an aluminum foil radiant barrier "insulation blanket" product sold and installed in many homes in the 1940's. The waterproof kraft paper covering of this radiant barrier "insulation" product faces down into the attic space - a design later research showed was not the best performer. Dead air trapped between the double layers of foil above the kraft paper provided a slight increase in the R-value of this product.
A radiant barrier interrupts the radiant heat transmission using a reflective barrier (see our page top photograph of a radiant heat shield installed in a 1950's home). Research by the Florida Solar Energy Center (FSEC) found that radiant barriers work best when installed with foil-face down (towards the building interior) and applied under the roof sheathing.
This roof heat transmission mechanism explains why air conditioning equipment or duct work located in a hot attic or roof space will have to work extra hard to cool the building unless it is adequately insulated from these hot conditions.
What are the Energy Savings from Radiant Heat Barriers?
... a radiant barrier will cut cooling costs by 10 to 15 percent when added to a house with R-19 ceiling insulation. Adding an R-11 batt to the R-19 batt would save only about 5 percent.
Remember, this advice was for Florida, primarily a cooling climate, not a heating climate. Cooling cost savings are a net advantage in the U.S. about as far north as Baltimore.
In the only northern city they modeled - Chicago - the extra insulation outperformed the radiant barrier on an annual basis,due to the much better winter performance of the insulation. In general, wherever the cooling load is large enough to justify installing central air conditioning, a radiant barrier is worth considering.
Roof Ventilation and Radiant Barriers
Our photo (left, contributed by a Georgia reader) illustrates placement of the radiant barrier on the attic floor beneath a roof with no venting except louvered gable ends (that leaked).
For greater savings
on cooling, consider adding a radiant barrier to the
underside of the roof sheathing or draped between the
This can reduce peak cooling loads by 14 to 15%
and seasonal loads by an average of 9%. By doubling the
roof ventilation from 1/300 to 1/150, the annual savings
from radiant barriers rises to 12%. These numbers assume
R-19 ceiling insulation and cooling ducts located in
the attic, which are typical in Florida. With R-30 ceiling
insulation, the cooling benefits of radiant barriers are less
Researchers at the Florida Solar
Energy Center (FSEC) have found that adequate attic ventilation
can modestly lower sheathing and shingle temperatures,
and reduce an average home’s cooling load by
Details about combining roof color, roof ventilation, and radiant barriers to reduce cooling cost are found at: ROOF COLOR RECOMMENDATIONS
Besides the radiant heat barrier article linked-to above, we need to issue a radiant barrier warning and to raise a question or two about this energy savings approach:
Radiant barriers are not a substitute for insulation in cold climates
In older homes built between 1950 and about 1965 we have found radiant barriers installed alone, with no building insulation. In cold climates such as New York and New England, an un insulated home may be cooler in summer with a radiant barrier installed, but this system provides only modest heat loss savings in cold weather.
In the our radiant barrier "insulation" photograph at left and home shown in our page top photo of a radiant heat barrier, the foil was sold as "insulation" and no other insulation was installed. Each was a costly house to heat in cold weather - the new owners would want to install insulating batts in the attic floor.
In the Solar Age article above, radiant heat barriers are installed under the roof on a building at which insulation has been installed in the attic floor as well as the building walls. That's the way to do it.
Installing radiant barriers as a substitute for actual building insulation performs poorly. Radiant barriers work best blocking radiation of heat downwards towards the occupied space. In cold climates the radiant barrier actually prevents desired heat gain in the home during daylight hours.
The Solar Age article above correctly concludes that while a radiant barrier may provide a small net heating cost savings (over no insulation at all) a better approach would be to add insulation to the building. "According to ASHRAE data, a radiant barrier at a 45 deg. slope with heat flow upward is worth about R-2.5."
The article adds that installing radiant barriers on building walls is less effective than under roofs. "A radiant barrier facing a 3/4" [air] space provides about R-3. If the barrier is included as part of an insulation system (such as foil faced insulation that includes this material as a vapor barrier) that's great. But don't use radiant barriers as a substitute for wall insulation.
Radiant barrier researchers think roof venting is unnecessary - are they right?
According to the Solar age article above, researchers think that venting the roof cavity above the radiant heat barrier is unnecessary because "... the radiant barrier does such a good job of blocking attic heat gain".
What is missing from that analysis is a more comprehensive consideration of how building work in all weather and moisture conditions, especially in cooling and humid climates, and also missing is a possible impact on roof shingle life when the roof is allowed to heat to the highest possible temperatures.
We prefer to provide under-roof ventilation in buildings, not just to permit hot attic air to escape in summer (drawing in cooler air from outside at the roof eaves), but also to permit un-wanted moisture to escape year-round. Failure to adequately ventilate attics has been shown to lead to condensation in cold weather and in some homes, severe mold contamination.
In our REFERENCES list below we provide links to the full set of US Department of Energy Documents giving extensive research data and installation advice for radiant barriers. Of those documents, we recommend in particular, the following:
"Radiant Barrier Attic Fact Sheet, DATA TABLES: Cost of Radiant Barriers in Locations, Savings of Radiant Barriers by U.S. City, Average Reductions in Ceiling Heat Flow due to Radiant Barrier, Table of Typical Heating Equipment Efficiencies, Table of Cooling Load Factors for Radiant Barriers by U.S. City, Table of Cooling Load Factors for Additional Insulation by U.S. City," [PDF] U.S. Department of Energy, - PDF file. Click in the top area of this document to return to this website.
Here we include solar energy, solar heating, solar hot water, and related building energy efficiency improvement articles reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.
Continue reading at REFLECTIVE INSULATION for Properties and details of single and double bubble designed aluminum foil insulating product or select a topic from the More Reading links shown below.
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