I bought a house last fall. It has a cathedral ceiling in the living room and there's a lot of heat gain through that ceiling. I knew it would be a problem when I bought it. I suspect it has pretty minimal insulation. I don't want to tear off the roof/roof deck or the interior wallboard, nor do I particularly want to deck over the existing roof or under the existing ceiling. At least for now. And blowing in insulation is out due to the need to retain an airspace between roof sheathing and insulation, ditto (at least in this climate) for spray foam application. So, I've been reading up on low emissivity paint, aka radiant barrier paint, along with radiant barrier materials in general. It's taken me a while to get my mind around it -- you can take a tin foil barrier, install it with the shiny side facing into the room, and ... it will block radiant transfer of radiant heat from the outside to the inside, via "low emissivity", which is a low ability to radiate energy from the shiny surface. Seems counterintuitive to me but reliable sources say that's true. They make low-emissivity paints, interior and exterior. By all accounts they are not as efficient as a tinfoil radiant barrier, but they do apparently block some radiation. Of course there appears to be a wide variation in quality. I can think of doing one or both of two things. My commensense notion of how this works says I should paint my asphalt shingles with low-e paint, on the back side of the roof where my problem is greatest (no shade trees). Still trying to weight the ugly factor on that one, so put the issues about how it will look aside. From a purely practical standpoint, Behr makes paint specifically for painting your asphalt shingle roof, so somebody must do it (though I've never seen it). Several manufacturers sell the additives to make the paint high reflectance/low emissivity in the infrared part of the spectrum. The other option is to paint the ceiling with low-e paint. Although I still can't quite get my brain around it, that too is supposed to work to block heat radiation into the house. Anybody here have any experience with low-e/radiant barrier paint? Or have a better understanding of the engineering/physics to offer guidance? Or experience painting an asphalt shingled roof? My gut tells me that, practical and looks issues aside, topcoating the roof surface should offer a better advantage because the infrared radiation should be stronger there. But that's a guess. Any advice would be welcome.
Your thinking is correct from the standpoint of heat transfer. I can't vouch for how well it works to paint an asphalt roof, as I have never tried it, but your logic is correct. Let's look at the physics: 1) Assume we use the same low-emissivity barrier on the inside and outside for two separate trials. Assume also that we are dealing with direct sunshine and that the barrier has good reflective characteristics. 2) Assume that the reflective barrier has a minimal R factor. In other words, the barrier is good at preventing radiation, but not good at preventing conduction of heat. Case 1 - Outside: The barrier will absorb heat from the surrounding air by conduction. This heat will also be transferred to the roof by conduction. Radiant heat transfer will be minimized by the low-emissivity characteristics of the barrier. Solar heating will be minimized by the reflective nature of the barrier. Case 2 - Inside: The outside roof will absorb heat by conduction, radiation, and solar radiation. This heat will be transferred to the barrier by conduction, but the low-emissivity of the barrier will minimize transfer by radiation. How do these two cases differ in practice? Radiant heating of the roof in case 2 will cause the roof temperature to rise well above the ambient temperature outside. Much of this heat will transfer to the barrier by direct conduction. The barrier will help by not radiating as much heat into your living space, but significant amounts of heat will transfer by conduction. With the barrier outside you minimize the amount of heat getting into the roof. With the barrier inside you only minimize the amount of heat radiated into the living space. Tom
qbee42, thanks, that clarified my thinking enormously. I'm kind of fuzzy-thinking my way through this. The puzzler to me was that it is acceptable practice to install the radiant barrier on the inside of the attic. They even make foil faced roof decking for this purpose for new construction, to be installed foil face down (facing into the attic). I read your response and it finally clicked -- inside the attic, but above the insulation. That makes sense. Inside the house but below the insulation, I would expect less benefit, possibly minimal benefit. Ah, yeah, it's all coming together now. That's why the impact of an attic insulation is symmetric (stops heat in summer, stops cold in winter) but the impact of attic radiant barrier is asymmetric (stops heat in summer, has negligible effect on heat transfer in winter). It's because the temperature differential between roof deck bottom and top of insulation is large in summer, but negligible in winter. It's been a long time since I've taken a physics class, but I get the feeling that understanding the potential effectiveness of a radiant barrier on the inside of a home is as simple as measuring the temperature differential between the two sides of the barrier. If they are at almost the same temperature, the net IR radiation flows between the two sides have to be close to nil, and the benefit of the barrier is therefore close to nil. So I need to measure the temp of my interior wallboard for starters. Clearly my next step is to find a source that breaks down the total heat transfer through a poorly-insulated ceiling into into conduction and radiation components. I can't seem to find one -- plenty of sources for windows, none easily found for walls. Tells me that this isn't typically a big issue, I think. Yeah, I'm going to measure my interior wallboard calculation and do some more digging, but basically, I think I need to paint my shingles. Yikes. And thanks.
Finished my research and I figured I would summarize that here before I forget it. An asphalt-shingle roof, even one with white shingles, reflects only about a 25% of the sunlight energy that hits it, and so will easily heat up 50 or 60 degrees above the ambient temp. Black shingles only reflect about 5% of the light, and will heat up about 80 degrees: Asphalt Shingles There are numerous low-tech off-the-shelf white roof paints, typically just regular paint with a lot of white titanium dioxide pigment, that will raise the reflectivity to about 80% and reduce the heat rise to 20 degrees or less. Huge improvement, and there's nothing high-tech there, just plain old white pigment and lots of it. Roof Coatings The problem is that a lot of people, including the manufacturers of some of those coatings, say you are never supposed to use the coatings on shingles of any type. Apparently the issue is that water can get under the shingle edges and with the high reflectance, it doesn't heat up and evaporate, and you can saturate your roof. However, Behr paint actually sells a white paint that they advertise as OK for asphalt shingles. Several other manufacturers sell their paints as a method for prolonging the life of worn asphalt shingle roofs. So, ... it's not clear who is right, or under what circumstances you can or cannot paint a shingled roof. I'm still looking into that. I've certainly never noticed a painted roof here in Virginia. But of all the options available to me, painting the back side of my roof is the most appealing at present. Behr.com: Products - Roof Paint You can also get low-tech aluminum-based coatings, but based on the data above, white is better. The reason is that the aluminum coatings are low emittance in the infrared region, so that they are not as good as white a radiating heat away once it builds up on the roof. The high-tech paints for application to the building exterior come in two types. Some have (e.g.) aluminum flakes or similar to make the paint reflective (high reflectivity and low emissivity), plus some other aspect to alter the emissivity. I don't much see the advantage there over pure white paint, unless I've missed something (or unless you need to have the aluminum coloring). Some other exterior paints hold "ceramic microspheres" to .... I'm not sure what. I have yet to read a good explanation of why hollow ceramic spheres would change the optical properties of the paint. I understand that they would insulate, a bit. It's possible that they might form a diffraction surface in the long infrared region, if they lined up in an organized fashion. But I have yet to see any cogent discussion of what it is those ceramic microspheres do to alter the optical properties of the paint. And the little test data I did find suggested the paints were minimally helpful. A lot of the ones designed for interior use hedge their bets saying, you'll feel warmer in winter but you won't notice it on your bills. So until I see an explanation or some test data to convince me otherwise, I think "ceramic microspheres" are not useful. I think their effect, if any, comes from providing a thin layer of very good insulation (the paint goes on very thick). But that's essentially worthless. Beyond being a thin layer of insulation, they apparently make for a very smooth and durable paint finish. Any claims beyond that seem to be speculation to me, not backed by data. Ceramic Coat It is possible that the microspheres improve the infrared emissivity of the paint, because the ceramics have high infrared emissivity. That is, you'd get a somewhat better heat-shedding white paint with the microspheres than without. But given the test data above, I'd have to believe that effect would be fairly minimal at best. And, high emissivity is exactly what I don't want if I'm going to paint the inside of my ceiling and hope to keep out infrared radiation. Which leads me to treatments you can apply on the underside of the roof. On the underside of the roof surface, what you are looking for is a high-reflectance/low emissivity surface. You want something that, once the IR radiation strikes it, it won't pass (emit) that radiation forward. As far as I can tell, all materials that would do that are silver colored. They are either aluminum-based sheets or aluminum-based paints. At least one shiny surface must face a significant (1" to 2") air gap in order for that to work -- otherwise the aluminum just conducts the heat forward. In new construction, they now make roof decks with aluminum foil glued to the underside. Or place aluminized Tyvek down before putting the roof deck down. In retrofits, they either paint the underside of the attic deck with aluminized paint, or hang those aluminized sheets on the bottom of the rafters (or, not as effective, lay them on the existing insulation on the attic floor). The problem with laying them on the floor is that if they get dusty, they stop working. These radiant barriers are quite effective at reducing summertime heat load. There is no controversy about that. So unless I want to pay the ceiling in my living room silver, there is no option to lower the emissivity of the interior surface with paint, as far as I can tell. So for me it boils down to just a few options: 1) Forget high-tech. Paint my shingles white using either the Behr paint advertised for that or by using some other "elastomeric coating" type of paint that is not indicated for that use. And risk rotting the roof (or not) depending on who is right about painting asphalt shingles. Other than the risk of rotting the roof that would be guaranteed to work. The fact that it is never done around here, given the high summer heat loads, probably tells me that it's a bad idea. 2) Tear open the roof and redo the insulation to modern standards. I wouldn't be here if I wanted to do that. 3) Tear off the fascia boards and try to slip sheets of aluminized radiant barrier up the rafter bays. That doesn't seem outright impossible, but it is kind of a mickey mouse solution. I doubt I could get a contractor to take that on, so I'd have to do that myself for sure. But, on the bright side, I didn't even know that radiant barrier material existed or worked until I started looking into this. Addendum: Aha, I think I figured out the scoop on painting your shingles. Latex OK, elastomeric coating, no go. Latex breathes, the elastomeric coatings are supposed to provide new waterproof roof membrane. That's why the elastomeric coating manufacturer said never to use it on shingle roofs. If water gets under it (and with all those seams between shingles, it probably will) then it will stay there and rot the roof. Here's a seemingly informed discussion of the issue. I think I'm going to paint my roof. 11634. Painting or sealing asphalt shingles
Wow, thanks for doing all that research & sharing it! I have the same situation, and my A/C has been struggling to keep up in the past week.
I may just not be seeing this in your very good review and discussion: why not paint an aluminum-based radiant barrier on the underside of the roof (that is, onto the exposed bottom surface of the decking in the attic)? Is that space not accessible to you?
Correct, it's a cathedral ceiling -- the only way to get there is to tear off either wallboard or the roof sheathing. For the rest of the house, with a regular attic, I'll either use the radiant-barrier paint on the bottom of the roof deck or hang foil-type radiant barrier across the rafters. The foil is better (emissivity about 3% versus 25% for the paint) but a lot more work. As an aside, this white roof thing is a pretty big deal. Apparently your classic black tar/membrane roof is just a 20th century relic. Some states have building code requiring additional insulation in commercial buildings if the roof isn't white, others are giving tax credits for making the roof white. For commercial buildings only so far as I've found. Looks like a really good cost-benefit tradeoff. Certainly looks that way to me -- the paint will set me back maybe $100 to $200. Plus a little for nonskid to throw in the paint (Behr warns that painting the roof makes it slippery when wet). And it gets weirder still -- somebody calculated that if every building in the world had a white roof, it would raise the earth's albedo by nearly a full percent, resulting in about 1 degree centigrade of global cooling: White Roofs to Fight Global Warming : TreeHugger Not sure I buy that but I like the concept. And there's good agreement this will make your shingles last longer. Between covering them and keeping them 40 degrees cooler in summer, it certainly should. So, no couple-of-tons-of-shingle-debris every 20 years off the roof of your house, that's a plus, at the cost of repainting your roof every N years. I'm going to check the local building codes and see if there's anything against it, then proceed. The only drawback at this point is the ugly factor. It's definitely going to be different, but will it be perceived as ugly? From what I read, even a light tinting basically destroys the reflectivity, but I'm going to look into that further. I can't believe a very light grey painted roof could have that much lower reflectivity than a snow-white one. And even then it would likely be better than the raw white shingle. But how pretty is a roof supposed to be, anyways? I'll think of it as avant-garde. Maybe 20 years from now everybody will have a white roof. Aha, gain, final final addendum: LRV, light reflectance value, it's stated on most paint samples, and it tells you reflectance in the visible spectrum. So I don't even have to do research, I just have to go read the labels down to the Home Depot to see the effect of tinting on reflection. Unless I can find whole-spectrum (visible and IR reflectance), I'll rely on that.
You might consider the solution I'm adopting. I bought 1000 sq ft of aluminum foil (radiantguard.com). Then I bought some 1x2x8 wooden strips. I nail the strips together to form a rectangular frame. 2 ft x 8ft big. Then I staple a similar sized piece of the foil to the frame and slide this frame on to my roof. For protection from the wind I nail 4 such frames to 2x4's at each end. See attached picture of frame. I haven't yet covered enough area to notice the difference. The thing I like about this plan is that I can take these frames off in the fall and slide them into the attic to preserve the heat in winter. The foil has high reflectivity and emmisivity. so it helps both ways. It should be kept a few inches off the roof. This is done by the frame and the 2x4's. I'm in LA and it should help a lot with the 100+ days coming up in July/August. I aim to put these on most of the roofs hidden from the neighbours. If I need to put this on exposed roofs then I will cover them with patio sun shade fabric in case it is unsightly. As I said I only aim to do this for July/August/Sept.
I painted the living room, dining room, and a bedroom with a ceramic paint additive - ceilings and walls . . . didn't notice a thing. If you have proper space and location, have you considered planing deciduous shade trees? Summer shade, Winter solar gain. Yes, they take some time to grow, but they also add value to the property.
Same here. I think it's voodoo science just like the water/fuel additives. But it got a new coat of paint on the exterior walls and it made spouse feel better. Photovoltaic panels. Hurricanes. Ouch. We've covered our south roof with photovoltaic panels and water-heating panels. That works well, as you would expect it to. Our west roof, however, bears the full brunt of the sunshine onto an uninsulated cathedral ceiling. I'm intrigued with the idea of painting the asphalt shingles but I'm concerned how spouse and the homeowner's association would see it. One more physics question: what about painting the existing roof shingles with a reflective paint or covering them with reflective foil insulation, and then tacking another layer of matching shingles above that? It keeps the same appearance while hypothetically breaking up some of the thermal conduction. And when the hurricane comes, no problem-- just repair & recover.
Nope, that shouldn't work. The reflective surface has to be the top coat. And any low-emissivity surface has to face an air gap. All of these paints and materials conduct heat pretty well. They aren't insulators in the sense of stopping conduction. What they do is interfere with the transfer of infrared radiation. Everything I read says you need a (roughly) 1.5" airspace on at least one side of the radiant barrier for it to be effective. Otherwise, they just conduct the heat.
I bought a roll of that radiantguard material and have started using it where I can. The first thing I used it for was shades for the skylights, where it makes an amazing amount of difference in terms of reducing heat gain. Next task will be window quilts for some very large windows, with that as the backer. For those of you who have never seen it, it's aluminum-coated Tyvek with pinholes punched through it so it'll breath. It has a consistency somewhere between a thick sheet of paper and a thin but stiff piece of canvas. It's class A/class 1 fire rated, which I'm pretty sure means that the curtains will burn before this material will (a significant consideration if you're going to hang large sheets of it in and around your home). Anyway, that's a pretty slick solution, but I'm looking for something that's a low-key as possible. I'm pretty sure I'm going with white paint. At this point, I think we're going to paint the roof of our shed first, maybe let that weather a year, see how it looks, then paint the house.
I just did a quick measurement of my aluminum foil on the roof solution. Just under the foil (which is held up about 2 inches off the roof) I measured 92 F, while on the exposed roof I measured 114 F. This was after only about 15 minutes of leaving the thermometer out there. Its possible it would stabilize to a higher difference if I leave it out longer (I don't know how good the thermometer body is at picking up the exact roof temperature since the actual thermometer is held inside this plastic body).
Three ideas in no particular order. Plant trees. Lombardy poplars will grow 6-10 year, others slower. Cover your exposed roof with PV solar or cheaper, Solar hot water heat for domestic and space heat. Either will have the effect of taking some of the heat away. Both will shade the roof below so not as much heat can radiate into the house. Instead of ripping off the asphalt shingles, consider running 2x4 strapping on top of the shingles 24"O.C. Then put polyisoboard between the 2x4s (or commercial blue or pink styrofoam ~R 6 per inch if I remember. Then reroof with standing seam metal roofing. Not cheap, but would have the added benefit of reducing your winter heating load. Icarus
Ah! Failure of imagination on my part: we have a cathedral also but there is sufficient space between the ceiling and the roof to permit access. The small space in your situation makes a radiant barrier less effective. It might be better and cheaper to just blow a buncha insulation into that space. When we bought a new roof we used foil-lined decking, so we are sympathetic to the concept.
