There is a company in town that will spray the inside of your attic with reflective white paint containing tiny glass beads. The idea is that the glass reflects the heat and keeps the attic from getting hot.
This doesn’t fit with my (limited) understanding of how heat transference works. Can reflective material on the *backside * of a roof reflect heat? If so, how?
IF it is exactly as advertised and reflects solar heat back into the roof top you will have problems with the roofing shingles sooner rather that later.
This paint is quite expensive. It was sold, locally, by a painting firm as a once for life application to the outside of homes to reduce summer A/C expense, etc. etc. They were sued for non-performance and run out of business.
In my opinion it is a get rich quick product scheme for the manufacturer of the micro ceramic spheres used in the paint and the painting contractor.
Help me remember my physics. Heat transfers in three or four ways. One is radiant which is how the sun works. The others are… what? Once the rays of the sun hit the roof, my understanding is that the heat is transfered to the attic by one of these other methods. Right? So any reflective material anywhere other than the outside of the roof itself would be ineffective.
Glass beads are used in light-reflective materials. That’s what lets you see road stripes when your headlights hit them. I’ve never heard of using them for heat reflection. It seems to me, if you want to reflect heat out of the attic, you’d spray that stuff on the roof, not inside. Painting the inside of a roof might give you a vapor barrier in the wrong place, giving you a rotting roof.
Heat is transferred by Conduction, Convection, and Radiation.
You are right. :smack: Radiation in this case would not be an over riding concern.
The inside surface of the roof is heated by conduction, the temperature dependant on the shingles, decking etc. It is usually very warm to quite hot to hold you hand on. In any event the product is a scam and does not perform as commonly advertised.
Caveat Emptor!
An attic should be as close to the outside temperature as possible. This is accomplished by ventilation. Generally, you don’t encourage the roofing structure to maintain a different temperature.
Besides the fact that this sounds like a scheme, you should probably limit your concerns to the best attic practices:
In winter, you want the attic cold, as this is vital to roof life. A warm attic in winter could lead to ice melting/refreezing, and this could force ice and water under shingles, etc.
I would disagree with your analysis.
Radiation is a major contributor to transferring heat from the undersde of a roof to the ceiling below. Coating the underside of the roof with a coating that resists radiant heat transfer will reduce the amount of heat that flows through the roof.
Heat mechanisms in a ventilated attic : The sun radiates heat onto the top surface of your shingles. How much heat is absorbed by the shingles depends on the surface characteristics of the shingles, matte black surfaces absorb radiant heat far more readily than shiny light colored surfaces. The heat is transferred by conduction through the roof materials until it gets to the underside of the roof. The heat is then lost from the underside of the roof in two ways, convection and radation. Convection is the energy lost to air flowing over the surface of the roof underside, heating up the air. Radiation is heat transferred directly to the attic floor the same way heat comes from the sun. The amount of heat lost from the underside of the roof to convection depends on the rate of flow of air and the temperature of the air in the attic, that lost by radiation depends on the temp of the underside of the roof and the attic floow and the radaitive property of both surfaces. Just as shiny surfaces resist taking in heat, they also resist giving it off.
So a coating on the underside of your roof can in principle reduce heat gain in your attic. Since the coating does not impact how much heat goes onto the roof from the sun, slowing the loss through the roof will as mentioned above increase the temperature of your shingles. And if your attic is not ventilated, the temperature of your shiny roof underside will rise and offset some of the improvement of the lowered emissivity of the surface, continuing to rain heat down upon your cceiling. If ventilated, the surface temp will still rise, but the higher temp will increase the convective heat transfer and pull more of the energy off the roof into the airstream to be exhausted out the top of the roof.
Of course, in real life the coating will get covered with spider webs and dirt, thus fouling it’s shiny surface and making it completely worthless.
Refer to previous post by BoringDad:
Radiation between two ‘black body’ surfaces is proportional to the diff. in the 4th. powers of the absolute temperatures. The under roof surface is usually ‘c or d’ grade plywood. This is a far cry from a black body radiator. The temperature is quite warm but not really hot as the shingles reflect most of the solar heat. Radiation 101
The Ceramic Micro Spheres are mixed in a water base paint vehicle. This is essentially a flat latex paint with little heat control properties in and of itself. The OP mentioned glass spheres which would have no insulsting value at all unless they were hollow so that the trapped air/space would provide the insulation. I suspect the salesman/contractor was ignorant of the details of his product.
As suggested in other posts, attics are best cooled by natural ventilation and in extreme cases by forced fan ventilation. 
You could, when laying a new roof put down heavy duty aluminum foil under the felt and reflect all the solar heat back into space. As a substitue cover the underside of the roof deck with foil. :rolleyes: (Then replace the roofing sooner rather than later!)
Think of it in terms of interfaces.
The problem is that the heat transfer into the paint layer is primarily by conduction. The paint layer then acts (more or less) like a black body radiator. If you’ve ver seen an infra-red picture of -well, anything- you know that the surface color or treatment generally doesn’t affect its IR radiation too much.
Okay, so you noticed the “generally”, and are asking yourself why this bead paint couldn’t be one of those few special surfaces. Well, without going into a lot of technical stuff, just remember that a photon of IR (radiated heat) starts at roughly 100 nm = 0.1 micrometers wavelength). To a photon, those glass beads are so huge that they are *not" part of the paint layer, but separate objects, like Jimmy Hoffa in a concrete foundation somewhere, or the interface between roof and air.
To the extent that reflected radiated IR (heat) matters at all, it will mostly reflect between the beads and the paint holding them. That won’t help you much. On the small scale of a photon, it’d be like embedding “reflective” silver balls in concrete. That wouldn’t make the cement appreciably more reflective, would it?
If you really want a reflective surface, you might consider one that is far more reflective – like aluminum foil. The atoms on the surface of the foil form an arrangement that is extremely good at reflecting both visible light and IR, as you probably know from cooking and grilling. Stapling aluminum foil between your rafters may look silly, but it’d be cheaper, work better, and you can do it yourself in a hour or two (heed the concerns others have offered, though: roofs aren’t just hats to keep the rain out; moisture/heat trapping are genuine concerns)
Aluminum foil, which is far more reflective to IR than any beaded paint, also offers an excellent test of the limits a thin reflective surface layer (like a layer of paint) to block conducted heat: a single sheet of foil in the air between your hand and hot coals will serve as excellent protection from radiated IR, but pick up a hot coal, using a single sheet of foil as “reflective insulation”, and you’ll burn your hand badly. (If you must do this experiment, use something safer than a coal, like a hot baked potato – or better yet, a cup of hot coffee)
I realize that I’m oversimplifying the science greatly, but I hope these examples will give you concrete confidence in the answers you’ve been getting, in a way that complex differential equations might not.
Yes the heat transfer is proportional to the diff. in the 4th. powers of the absolute temperatures. How is this relative to the general issue at hand?
Certainly plywood is not a black body radiator. But it does have a much higher emissivity than aluminum foil for instance.
Shingle reflect most of the solar heat?!?! No, they do not. That is why attics get so hot. Most asphalt shingles suck up heat like a sponge. The only thing that prevents the underside of the roof from getting too hot is ventilation, heat loss to the ceiling of the rooms below, and the tiny insulating value of the plywood roof.
I typed my reply over lunch, and so didn’t have time to investigate the product in question (or apparently to comment that I didn’t investigate the product.) However, even if the product mentioned does not work, it does not negate the logic of radiative barriers properly used.
Well, no, you couldn’t. If the reflective surface is in contact with anything, simple conduction will be the driving factor, thus completely negating the low emissivity of the lining.
Actually, covering the underside with foil IS a succesfull heat transfer barrier. There is in fact a commercial reflective insulation product (laminated to a bubblewrap carrier for toughness.) And shingle life is dependent upon area of the country. Houses built with structural insulated panels have no ventilation under the shingles, and are directly insulated by a foot or two of foam under the shingles.
The conclusion arrived at during a c 2002 SDMB thread is that this foil-faced radiant barrier (available at Home Depot, among others) wouldn’t quite work as advertised. First, fiberglass insulation particles would eventually settle on the foil, thus decreasing the barrier’s reflextivity and, two, trapping a superheated layer of heat between exterior shingles and interior radiant barrier would hasten the wear of shingles.
Naturally, no one provided the necessary calculations to back up their conclusions, but that was the consensus.
Yes, radiant barrier laid flat above ceiling would fail to be useful after it became covered with dust.
Radiant barrier under the roof would work and would not likely fail, but would raise the temp of the roofing. Whether this would cause the roofing to fail depends on the type of roofing and the climate. To simply discard this option out of hand based on an unstated set of assumptions is not tremendously helpful to those trying to determine if it will be helpful to them.
Other posters hashed out those “unstated set of assumptions” in an earlier thread. I’m not discounting any option. Merely repeating what other posters said. Contact the Department of Energy.
From assorted websites. Hope this helps.
What are radiant barriers? Are radiant barriers an effective way to upgrade my home’s insulation performance?
Radiant barriers recognized by energy codes come in three basic types: single layer film material, multilayer or bubble film material, and single films applied directly to the underside of roof (plywood/OSB) sheathings. Radiant barriers are not insulation, and by definition, have no R-value.
However, there are some radiant barrier products that have entrapped air spaces (bubble pack or multilayer films) where an R-value may be available for the product. In these situations, the product is operating as an insulation as well but the winter R-value (heat flow up) is very small. Often the manufacturer of these product types will list the summer R-value (heat flow down) because it’s so much higher than the winter time (heat flow up) value.
Testing has shown that it is more cost effective to add insulation than a radiant barrier. Additionally, the actual reduction in heat flow achieved through properly installed radiant barriers is substantially less than that claimed by some manufacturers. In cold weather, radiant barriers may reduce beneficial heat gains from the sun; because of this, they are mainly sold in areas with warmer climates.
Radiant barriers are a new technology with potential for reducing attic energy loss. They reflect heat radiating from the roof back to the roof. They can be placed on the rafters so that the attic becomes cooler. The disadvantage of this is that the attic is cooler in the winter as well as the summer (Oak Ridge National Laboratory, Report Brief #ORNL/CON-226.) It provides no help in lowering attic humidity. The more effective location is directly over the insulation-- where it traps moisture and collects dust. Moreover, as the R-Value of insulation increases from R-19 to R-30, the barrier’s effectiveness decreases to zero (Oak Ridge National Laboratories, Technology Applications Bulletin #587.) Any decision to install a radiant barrier should be balanced against the cost of increasing insulation levels to R-30.
Ventilation in attics helps evacuate heat and humidity build-up. Without adequate ventilation, the danger of mildew in the attic is great.
If you have an attic, consider installing our RBS Chips. Stapling RBS up to your rafters will not give you the performance you are looking for and may void the warranty on shingle roofs.