Do Radiant Barriers Save Cooling or Heating Energy? InspectAPedia® -
Radiant barriers can save on cooling cost and may save a little on heating costs as well
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This article discusses the effectiveness and installation options of radiant barriers as building energy savers. Sketch at page top and the text are reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.
Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.
Article on Radiant Barriers as Energy Saver in Attics or Building Walls
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 part 2, PDF form, second half of the radiant barriers article from Solar Age.
"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.
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.
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 uninsulated 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:
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.
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Additional technical contributors & reference sources for this article are listed below.
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Solar Age Magazine was the official publication of the American Solar Energy Society. The contemporary solar energy magazine associated with the Society is Solar Today. "Established in 1954, the nonprofit American Solar Energy Society (ASES) is the nation's leading association of solar professionals & advocates. Our mission is to inspire an era of energy innovation and speed the transition to a sustainable energy economy. We advance education, research and policy. Leading for more than 50 years.
ASES leads national efforts to increase the use of solar energy, energy efficiency and other sustainable technologies in the U.S. We publish the award-winning SOLAR TODAY magazine, organize and present the ASES National Solar Conference and lead the ASES National Solar Tour – the largest grassroots solar event in the world."
Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education.
Excerpts from his recent book, Best Practices Guide to Residential Construction, Wiley (November 18, 2005) ISBN-10: 0471648361, ISBN-13: 978-0471648369, appear throughout this website, with permission and courtesy of Wiley & Sons. Best Practices Guide is available from the publisher, J. Wiley & Sons, and also at Amazon.com.
Excerpts with updates and annotations expanding the original Best Practices Guide text can be found in the online review and book summary at BEST CONSTRUCTION PRACTICES GUIDE and also at DECK & PORCH CONSTRUCTION, at INDOOR AIR QUALITY IMPROVEMENT GUIDE, and in other articles found at InspectAPedia.com such as HOUSEWRAP AIR & VAPOR BARRIERS, SOUND CONTROL in BUILDINGS, and other topics.
Carson Dunlop, Associates, 120 Carlton Street Suite 407, Toronto ON M5A 4K2Toronto. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. Thanks to Alan Carson and Bob Dunlop, for permission to use illustrations from their publication, The Illustrated Home which illustrates construction details and building components. Carson Dunlop provides home inspection education including the ASHI-adopted Home Inspection Training Program (home study course), publications such as the Home Reference Book, report writing materials including the Horizon report writer, and home inspection services. Alan Carson is a past president of ASHI, the American Society of Home Inspectors.
Passive Solar Design Handbook Volume I, the Passive Solar Handbook Introduction to Passive Solar Concepts, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v1.pdf
Passive Solar Design Handbook Volume II, the Passive Solar Handbook Comprehensive Planning Guide, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v2.pdf [This is a large PDF file that can take a while to load]
Passive Solar Handbook Volume III, the Passive Solar Handbook Programming Guide, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v3.pdf
"Passive Solar Home Design", U.S. Department of Energy, describes using a home's windows, walls, and floors to collect and store solar energy for winter heating and also rejecting solar heat in warm weather.
"Radiant Barrier Attic Fact Sheet, Sources of Government Information", U.S. Department of Energy, - PDF file. Click in the top area of this document to return to this website. Spells out the full text names of AFUE, ASTM, BTU, CCF, COP, DOE, FSEC,KWH, MIMA, NAHB, NFPA, ORNL, RB, RBS, RIMA, SEER, therm, TVA, and UBC - this gives the names, not extensive definitions of these acronyms.
"Radiant Barrier Attic Fact Sheet, Technical Panel who Developed this Fact Sheet", U.S. Department of Energy, - PDF file. Click in the top area of this document to return to this website. Spells out the full text names of AFUE, ASTM, BTU, CCF, COP, DOE, FSEC,KWH, MIMA, NAHB, NFPA, ORNL, RB, RBS, RIMA, SEER, therm, TVA, and UBC - this gives the names, not extensive definitions of these acronyms.
"Solar Water Heaters", U.S. Department of Energy article on solar domestic water heaters to generate domestic hot water in buildings, explains how solar water heaters work. Solar heat for swimming pools is also discussed.
"Heat-Transfer Fluids for Solar Water Heating Systems", U.S. DOE, describes the types of fluids selected to transfer heat between the solar collector and the hot water in storage tanks in a building. These include air, water, water with glycol antifreeze mixtures (needed when using solar hot water systems in freezing climates), hydrocarbon oils, and refrigerants or silicones for heat transfer.
"Solar Water Heating System Freeze Protection", U.S. DOE,using antifreeze mixture in solar water heaters (or other freeze-resistant heat transfer fluids), as well as piping to permit draining the solar collector and piping system.
"Solar Air Heating" U.S. DOE also referred to as "Ventilation Preheating" in which solar systems use air for absorbing and transferring solar energy or heat to a building
"Solar Liquid Heating" U.S. DOE, systems using liquid (typically water) in flat plate solar collectors to collect solar energy in the form of heat for transfer into a building for space heating or hot water heating. The term "solar liquid" is used for accuracy, rather than "solar water" because the water may contain an antifreeze or other chemicals.
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