Has anyone had any experience with radiant barrier paints that are applied under the roof deck to reduce radient heat transmission? Examples include Radiance and Thermalite. I was wondering if it is effective and more cost effective than the foil backed products.
http://www.advancedsiding.com/radiance_insulation.htm
http://www.thermilate.com/index2.shtml
thanks
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I've never seen any convincing evidence that they do much good.
Do they make a difference? Enough of one to substantially effect energy costs? Enough to pay for the additional costs? Long term?
Hard to say. There are sites that make great claims of savings but most of these are based on evidence or assertions that rests on research conducted or at east paid for by the manufacturers. Sometimes they don't have anything more convincing than a 'Mr Wizard' type experiment. The few academic and third-party studies are much less optimistic and vary between faintly positive to neutral. From the literature I have seen it is hard to tell.
I can say from personal experience, admittedly both subjective and anecdotal, that at least in houses and additions that incorporated these features there was a definite benefit in terms of attic temperature. The effect was noticeable to me. Of course seeing that I'm talking about Florida in late summer the effect might be less discernable in cooler climates. I would say there was at least a ten or twenty degree difference in the late afternoon temperature of the attic.
Hard to say if this difference is enough to save money or improve comfort. The two types I have directly experienced are foil stapled onto the bottom of the rafter cords and two types of radiant barriers applied to the underside of the roof deck, foil factory applied to the sheathing and a proprietary silver colored spray paint applied after the deck was installed.
Seemed to me that the foil setup was a bit more effective, cooler, but this could be an artifact of my finding the combination of a radiant barrier under an air space attractive intellectually. I can say all three cases were noticeably cooler than similar situations. In one case, an addition, I had to crawl through the attic and transition between sections of the attic sharing the same orientation, shingles and lack of shade. I could literally hold my hand below the old decking lacking the foil and the new sheathing that had it. The old deck felt like a broiler with heat radiating strongly while the new deck was much more comfortable to be under. Touching the decks they seemed roughly similar, painfully hot.
Does this mean anything more than making any electricians who crawl through your attic a little more comfortable? Is it enough to make a difference in energy costs? Will the effect last long enough to pay for the extra cost?
I can give you a firm: maybe.
When P.T. Barnam was asked if the nickel it cost to get into the sideshow was worth it he is reported to have answered: 'You pay your money. You take your chances'.
It is BS. The "barrier" applied directly to the underside of the roof sheathing with do nothing more than conduct the incident radiant heat into the space, as well as the roofing and sheathing presently does. Radiant barriers have to be exposed to the radiant source such that they can reflect the radiant energy away. As shown by the siding company, completely incorrect. As shown by the paint company, their product is used on the exterior surfaces and may or may not help.
If you want to reduce the heat gain from the sun into your home, there are a few factors to consider. Color, R-value, exposure and shading. A light colored (white, cream, light tan, light gray) surface ill absorb 40% of the incident radiation that a matte black surface will, all other factors being equal. Medium colors like brick red, green, will absorb 70%. A roof or wall that has twice the insulation value will transmit half the incident energy (again, with all other factros being equal) in a steady state situation.
" 58390.4 in reply to 58390.1 It is BS. The "barrier" applied directly to the underside of the roof sheathing with do nothing more than conduct the incident radiant heat into the space, as well as the roofing and sheathing presently does. Radiant barriers have to be exposed to the radiant source such that they can reflect the radiant energy away. "No true.http://www.southface.org/web/resources%26services/publications/factsheets/14radiantbarriers.pdf&e=10141"
Radiant barrier products are usually rated by their reflectivity
(how much radiant heat they reflect)and their emissivity (how
much heat they radiate or emit from their surface).For opaque
products like aluminum foil,the sum of the reflectivity and
emissivity ratings equals one.Most radiant barrier products
have an emissivity rating of 0.05 or less,and conversely ####reflectivity rating of 0.95 or greater.This ability to both reflect radiant heat and to not emit it from
its surface when its temperature increases,is how radiant
barriers can help lower your utility bills.If the foil is laid on top
of the ceiling insulation,it will reflect the radiant heat from the
roof that strikes its surface.Likewise,if the foil is stapled to the
underside of the roof or along the rafters,with the shiny side
facing down,it will not emit radiant heat from its surface."http://www.fsec.ucf.edu/bldg/pubs/cr670/Report on affect of colors and materials reflective roof systems.FSEC also has out a number of reports on radiant barriers and also one on whole roof systems that included the roof, radiant barriers and insulation.I think that Southface also has more information.
Per your own reference Bill, "One critical point in installing a radiant barrier for any location is that the shiny surface must face an air space."
This is a very simple and direct statement that confirms exaclty what I said, albiet, in different words.
Additionally, in your quoted example "This ability to both reflect radiant heat and to not emit it from its surface when its temperature increases,is how radiantbarriers can help lower your utility bills..." The key word here for you to undesrtand (I agree with the statement) is both. The paint, in direct contact to the underside of the surface, does not face an air space and therefore, does not have the ability to reflect the solar energy away from the occupied space. It does reflect heat back toward the occupied space. The benefit of this quality is limited. at best.
You refer to exellent information sources (one does refer to the other). To be of any benefit to this discussion, it would help for you to both read and undestand your references. While everything that you quoted is true, the information, by itself, is incomplete and invalid.
"The paint, in direct contact to the underside of the surface, does not face an air space and therefore, does not have the ability to reflect the solar energy away from the occupied space."You are right. It does not have the avbility to reflect the solar energy.HOWEVER, it does not emit it into the attic either. Same results.http://www.fsec.ucf.edu/Pubs/EnergyNotes/EN-15.HTM"Aluminum foil across the attic airspace reflects heat radiated by the roof. Even if the radiant barrier material has only one aluminum foil side and that side faces down, it still stops downward heat transfer because the foil's low emissivity will not allow it to radiate the roof's heat to the insulation below it.""Q: My material has only one foil side; should the foil face the roof?No. In attics, single-sided radiant barrier material should be installed with the foil side facing down. Even if you use a double-sided radiant barrier material, it is best to install it at the rafter level so that the bottom side faces the attic airspace and will not collect dust. This may run counter to our intuitive feel for "how things work," but it does work, and work well.To understand how it works, remember the two properties of aluminum foil from our Thanksgiving turkey and light bulb analogies; foil reflects radiant energy very well but does not radiate heat well. It does not emit heat to the cooler surfaces around it.If you install a single-sided radiant barrier with the foil side facing up, the aluminum will (for a time) reflect the thermal energy radiated by the hot roof.If you install a single-sided radiant barrier with the foil side facing down, the aluminum simply will not radiate the heat it gains from the roof to the cooler insulation it faces. "http://www.fsec.ucf.edu/Ed/bpm/lessons/lesson03/radiation.htm"The plain metal plate has the highest rate of temperature increase and highest end temperatures for both the plate and air temperature measurementsThe plate temperature of the radiant barrier facing back away from the lamp ends up much higher than the plate temperature of the radiant barrier facing the lamp. This result is because the radiant barrier facing the lamp reflects radiation back to the lamp keeping the plate surface cool (so there is little conduction through the plate), while in the case where the radiant barrier faces away from the lamp the plate is heated by the lamp and the heat conducts through the plate to the radiant barrier and then to the temperature sensor taped to it.The air temperatures behind the plates for both the radiant barrier facing the lamp and the radiant barrier facing away from the lamp are very similar. This result may be a surprise since the plate temperature of the radiant barrier facing away from the lamp is much higher than the case where the radiant barrier faces the lamp, but a radiant barrier reflects radiated heat and also is a poor radiation emitter even when the material itself heats up, so again little heat is re-radiated or emitted to the temperature sensor ¾" behind the plate."And look at figure 5.http://www.fsec.ucf.edu/bldg/pubs/rbs/"The FPC Monitoring project has evaluated radiant barrier systems (RBS) as a new potential DSM program. The objective was to examine how the retrofit of attic radiant barriers can be expected to alter FPC residential space conditioning loads. An RBS consists of a layer of aluminum foil fastened to roof decking or roof trusses to block radiant heat transfer between the hot roof surface and the attic below. The radiant barrier can significantly lower summer heat transfer to the attic insulation and to the cooling duct system. Both of these mechanisms have strong potential impacts on cooling energy use as illustrated in Figures E-1 and E-2."Shows E-2 and E-4 show the SHINNY SIDE DOWN.
The information developed by the Florida Solar Energy Commision is interesting and extensive, however, some of the data presented in their study is valid, some is not.
I will partially agree that "... it does not emit it into the attic either." is true, but the statement "Same results." is not. If the only mechanism for transfering heat from outside of the space to inside of the space were radiation, this would be true. That only happens in a theoretical vacuum. Heat transfer in our atmosphere is effected by radiation, conduction and convection.
If, as the sole source you quote would have us believe, that the surface heats up but emits no radiation (a questionable "result" at best, there are no perfect reflectors and there are no 0 emittance substances in or out of the lab), the material will continue to transmit heat into the space via conduction and convection. Not the same result. The real result is that the heat gets in.
By their own admission, "...these mechanisms have strong potential impacts on cooling energy use..." , this is preliminary and unproven. They are focusing on a single method of heat transfer. What they will discover (and most likely fail to publish so prominently) is that the "potential" is never going to be fully realized. However, they also might be on to something that will have a great impact. Bottom line here is that they (and you and I) do not know that as a poven and verifed fact.
If you want to look at the greatest body of tested and fully validated works on heat transfer, as it applies to actual occupies structures, look into the ASHRAE Fundamentals Handbook. Hundreds of references and repeatedly verified tests, studies and applications available to show how things really work. Also, see the list below.
I maintain that painting the underside of a roof structure with anything that does not siginficantly increase the resistence to heat conduction is, at this point in development, not going to affect any signicant reduction in heat transfer into the space.
Some real good information on the subject in general can be found in the following:
HUD, "Alternatives to Lumber and Plywood in Home Construction," prepared by NAHB Research Center, Upper Marlboro, Maryland, April 1993.
Nisson, Ned, "Research Center Seeking Ways to Fix Thermal Problems in Steel Framing" Energy Design Update , Cutter Information Corp. Vol. 14, No. 3, March 1994.
Dennis, William F., "The Resurgence of Steel," ASTM Standardization News , Volume 23, Number 2, pp 36-41, February 1995.<.li>
ASHRAE Handbook of Fundamentals , American Society of Heating and Refrigerating, Air Conditioning Engineers, Atlanta, Georgia, 1993.
Kosny, J. and Desjarlais, A. O., "Influence of Architectural Details on the Overall Thermal Performance of Residential Wall Systems," Journal of Thermal Insulation and Building Envelopes , Vol. 18, July 1994.
Desjarlais, André, Kyle, D. M., Childs, P. W., and Christian, J. E., "Laboratory Measurements of the Drying Rates of Low-Slope Roofing Systems," Proceedings of the Low-Slope Reroofing Workshop, Oak Ridge National Laboratory CONF 9405206, Sept. 1994.
Childs, K. W., "HEATING 7.2 Manual," Oak Ridge National Laboratory Report ORNL/TM-12262, Feb. 1993.
Kosny, J. and Christian, J. E., "Thermal Evaluation of Several Configurations of Insulation and Structural Materials for Some Metal Stud Walls," Energy and Buildings , Summer 1995 to be published).
Kosny, J., "Wooden Concrete - High Thermal Efficiency Using Waste Wood," Proceedings of the American Council for an Energy-Efficient Economy 1994 Summer Study on Energy Efficiency in Buildings, Berkeley, CA.
Christian, J. E., "Thermal Mass Credits Relating to Building Envelope Energy Standards," American Society of Heating and Refrigerating and Air-conditioning Engineers, Transactions 1991, Vol. 97, Pt. 2.
Model Energy Code, Council of American Building Officials, Falls Church, Virginia, 1995 Edition.
ASHRAE Standard Energy-Efficient Design of New Low-Rise Residential Buildings, American Society of Heating, Refrigeration and Air-Conditioning Engineers, Inc., Atlanta GA. ASHRAE 90.2-1993.
ASTM, 1989 Annual Book of ASTM Standards Section 4 Construction, Volume 04.06 Thermal Insulation; Environmental Acoustics, ASTM, Philadelphia, PA, 1989.
ASTM, 1995 Annual Book of ASTM Standards Section 4 Construction, Volume 04.07 Building Seals and Sealants; Fire Standards; Building Constructions, ASTM, Philadelphia, PA, 1995.
North American Insulation Manufacturers Association, "The Effect of Insulation on Air Infiltration," Roofing/ Siding/ Insulation, Volume 71, No. 9, September 1994.
Kosny, Jan, Christian, Jeffrey E., "Reducing the Uncertainties associated with Using the ASHRAE ZONE Method for R-value Calculations of Metal Frame Walls," ASHRAE Transactions 1995, V. 101, Pt. 2.
AISI (American Iron and Steel Institute) Residential Steel Framing Manual for Architects, Engineers and Builders, Low-Rise Residential Construction Details RG-934 American Iron and Steel Institute, 1101 17th St. N.W. Suite 1300, Washington D.C., June 1993.
USG - United States Gypsum Company "Drywall Steel Framed Systems," System Folder SA-923-1992 Edition, 092501/USG-3.
Hoke, Jr. J. R. "Architectural Graphic Standards," The American Institute of Architects, John Wiley & Sons, ISBN 0-471-81148-3.
DOE, Office of Energy Efficiency and Renewable Energy, "Voluntary Home Energy Rating System Guidelines," Federal Register , Vol. 60, No. 142 July 25, 1995.
Christian, J.E., and Kosny, J., "Toward a National Opaque Wall Rating Label" Proceedings Thermal Performance of the Exterior Envelopes VI , ASHRAE ISBN 1-883413-29-X, December 1995.
Edited 5/26/2005 9:23 am ET by Timbo
.... I totally agree, though I cannot offer a library of documentation to back up my understanding of how roof structures work.In simple terms, radiant paints have their place - in space. I worked on a satellite cooler for a brief period in my life and can see where these high-performance paints can do a lot of good - in space.Back on earth, you're dealing with heat transfer across several mechanisms. Radiant transfer is but one of them. Therefore, a paint or Al layer won't do much other than empty your pocketbook. Instead, I'd spend the money on a closed-cell Polyurethane applied directly to the roof deck. Once the attic is sealed and insulated, the HX into the space will be greatly reduced.
Constantin-
I disagree with you on this. Radiant heat transfer is a very powerful mechanism in an attic in the summer. Reducing the amount of heat radiated from the bottom of the roof deck will reduce the amount of conducted heat that the insulation will have to cope with. In a vented attic, it will allow the convective heat losses from the ventilation to be more effective. Why do you disagree with this?
"Radiant heat transfer is a very powerful mechanism in an attic in the summer." This partially true.
"Reducing the amount of heat radiated from the bottom of the roof deck will reduce the amount of conducted heat that the insulation will have to cope with. In a vented attic, it will allow the convective heat losses from the ventilation to be more effective." - This is similar to another's post on the subject. The statement by itself is true. In the context of the subject at hand is immaterial, because it doesn't apply.
The point is, that reflective radiant barriers work. Painting the underside of a roof structure in the hopes of eliminating or reducing outside to inside heat flow is futile.
In the case in your roof structure the difference in temperature that you are measuring is simple conduction through a temperature gradient. This is how heat transfer occurs. The air space is acting as insulation, not as a radiant barrier. All surfaces are somewhat reflective and heat transfer is resisted at a change in surface and/or change in material. If you were to calculate an equivalent resistance of your roof (an approximated. overall R-value) you would include: the outside air surface (at an average wind speed of say 15 mph), the roofing, the air space, the decking, any insulation that might be present and the inside surface.
Sealed and foamed attics are nice too. In my unvented attic in central TX I used a 3/4" airspace under an RBS above the sealed roof deck to help mitigate the heat transfer from my dark green composition shingle roof. My shingles typically reach 175 degrees but the top of the insulation has never reached 132 degrees according to my datalogger. The temps also stay hot a little longer due to the airspace not being vented. The cost and looks of venting was something I judged to be un necessary and undesireable. I am thrilled with the overall performance.
In reference to my last post; I have no data on RB paints and am unable to find any. Aluminum is superior but I don't know by how much. The RB paint may not be as justifiable in a cost analysis as the foil.
First, thank you all for your input. I was not optimistic about the thermal paint solution. However, while I am unable to quote numerous publications to discuss the benefits of the aluminum foil backed (inside the attic) materials, I can tell you that numerous people have noted that virtually identical homes in track subdivisions where one home is using a foil backed roof decking enjoyed a cooler attic space.
Ray, I am here in Austin TX (actually live in Ceder Park / Jonestown area) and plan on building in Oct of this year. If you happen to have any good references for quality subs or good suppliers for material I would appreciate your insight. I am going with the unvented attic and open cell spray foam insulation. In fact, I plan on having a shed dormer in the attic for an observation deck. I hope it works out well in our hot and humid climate.
If you are a builder and would consider your market to be the Jonestown area, I would be happy to refer neighbors to you when they look for a custom home builder. There will be a lot of 3000+ sq ft homes built in this area over the next few years.
I have made a career out of Mechanical Enginnering. Much of that has been in the analysis and design of building heating and cooling systems. I have read (and I am fairly confident, understood) all of those references, and many more. I DO have an extensive library on the subject and follow new developments as much as possible. I like to discuss these issues and welcome any credible challenge to what I accept as "known". Information presented in other posts does not yet meet the "credible" criteria or at least is less than compelling.
A simple energy analysis of the non-reflective barrier is enough to dismiss its effectiveness for reduction of heat transfer. Energy, via radiation, strikes the non-reflective surface. Since the energy is not refelected (nor is it destroyed) it is absorbed. According to the information presented, the barrier emits no radiant energy. So then, we are to believe that this thin paper/foil sheet absorbs endless amounts of radiant energy and passes none. Reflective barriers are effective means to reduce the heat transmitted into a space due to solar radiation. Once that radiation is incident on a surface, absorbed and conducted through structural elements into an intermediate space (i.e. through the roof deck, into the attic) the radiant barrier is of no use. The energy (heat) is already in. The best option at that point is to remove the heat before it is transmitted into the conditioned space.
One of the simplest and most effective combinations for pitched roof structures is the use of a reflective (shiny side up) insulating barrier used in combination with adequate ventilation. The heat, not reflected from the roof outer surface, gets another chance to be reflected, absorbed and convected out of the space.
Edited 5/27/2005 7:25 am ET by Timbo
Interesting.... I still prefer unvented roof systems that are constructed with the local conditions in mind. For example, I doubt that this far north (Boston) we'll ever have an issue with an unvented roof and Duraslate. If I were living in Texas or NM, I'd opt for a material that can stand up to the local conditions, like Terracotta Tile, Cement Tiles, etc. instead of putting those vent channels under my roof.
> According to the information presented, the barrier emits no radiant
> energy.Ah, there's the rub! Everything emits radient energy (as a function of its temperature), and a dark surface actually emits more than a reflective one, in most cases. A surface's propensity to absorb energy is exactly the same as it's propensity to radiate energy -- it's just a question of whether the surroundings of the surface are radiating more or less (basically hotter or cooler) that determines whether the surface under consideration will be a net absorber or a net radiator.
You are right in your belief that the heat will continue to build in a roof with a radiant barrier on the bottom surface. Of course it is absurd to believe that the foil can absorb an infinite amount of this heat. The deck with a foil face to the interior will indeed get hotter by several degrees than a deck without the foil. The roofing manufacturers' concern of decreased shingle life is directly related to this phenomena. The missing part of this picture is the fact that not all the heat in the deck must pass through to the interior.
In a roof deck, most of the heat is already emitted back to space. The foil skin on the bottom of the roof deck greatly reduces the emittance from the bottom surface of the roof deck. More of the heat energy is emitted to space in this case.
There are of course other ways in which heat is transported into the attic. The rafters for instance are emitting heat that they gain from direct contact with the foil roof deck surface. I'm going to let this lie for awhile.
This I'll buy: The foil is a half-a**ed decent approximation of a perfect "white body" -- something that had a very low rate of energy absorption. For this reason, it also has a low rate of energy radiation. So heat that gets into, say, foil-faced foam insulation against a roof will be "trapped" there and will not tend to radiate into the attic space as much as if the foam were, say, painted black. The roof will get hotter and more of the energy will be reradiated on the outside surface or transferred to air outside.Of course, the foil surface will be at about the temp of the insulation, so heat transfer from the foil to the attic air will occur pretty much the same as if the insulation were black (only a hair more so, since the insulation is hotter, due to the lack of radiation losses). In the particular case of an attic, the air against the top surface will already be the warmest in the space, so the amount of heat transfer will be lower than if the insulation were in a wall.How significant the difference in radiation is is hard to say. Probably could amount to a few degrees.Note that the important point here is that the foil (shiny, non-radiative) surface is exposed. If the foil were simply sandwiched between two pieces of foam, it would have no significant effect. Similarly, foil inside an insulated wall will have essentially no effect.
True, the foil must be clean and must have an air space of at least 3/8" to be effective. Have you ever been in an attic in the south that has a foil facing toward the attic? The difference in temperature is around 30 degrees in the afternoon. It is simply amazing the first time someone experiences it. This is a very common practice here in Texas as a result of it's effectiveness. I have no experience with the paint though.
I would think that to be reasonably effective you'd need a lot more than 3/8 inch. A foot or so, minimum.
Please test it. Use two pieces of comp shingle or two scraps of plywood painted dark on top. Adhere foil, shiny side down to the bottom of one of your samples. Place them in the sun on a hot day, supported above the ground. Place a thermometer 3/8" below the bottom surface of each. This phenomena is very well documented. You will see a large DeltaT. If you have a teenage child, this will make a good science fair project for them. The numbers I posted are not guesses but are accurate numbers based on extensive research.
One of the links that I posted way back in the thread from Florida Solar Institute is a class room project doing just that.
The problem is that when you have a closed space things change enormously, since virtually all radiation from the hot surface is captured by other surfaces that will tend to re-radiate it. If there's no way for the heat to escape then equilibrium will occur at about the same temperature, regardless of the colors involved.
Very true except that the roof structure doesn't only radiate, conduct or convect into the attic space. The roof deck has two sides. Assuming the material has any tendency to slow the radiation of heat into the attic it could be assumed that the deck and shingles would, given that the deck receives the same amount of solar radiation, tend to be somewhat hotter. Being hotter it would radiate into space, conduct into the atmosphere at a somewhat faster rate.
Timbo and all...interesting subject. I'd be interested in your opinion of a product I ran across some time ago. It's essentially radiant "chips" blown in on top of insulation. They make some interesting claims that seem plausible, but I'm no expert.
Any experience with this stuff?
http://www.rbschips.com/id5.html
http://www.rbschips.com/catalog/i1.htmlPJ
Everything will be okay in the end. If it's not okay, it's not the end.
They might help a little, but it's a lie that they would be unaffected by dust. Any radiant barrier placed on the attic "floor" is going to be compromised by dust.
If you read any of the stuff from FSI, southface, etc you will find that they say that an up face radiant barrier will work, for a short time, untill it get dusty.