For some extra reading if anyone is interested on the topic: buildingscience.com/documents/building-science-insights/bsi-136-piltdown-man-does-thermal-resistance www.greenbuildingadvisor.com/article/radiant-barriers-a-solution-in-search-of-a-problem
The Stefan-Boltzmann law is the law describing the emission of black-body radiation. However, the explication the web site you put up gives is poor in detail. I think the field deserves the benefit of better, clearer, explanations of the physics underlying these claims. Not everyone can absorb all of the details, but it is certainly within the ability of architects to master. And we need to have a more robust collaboration between engineers who have finely honed skills in these areas, and the designers and builders who need to apply them. Matt Risinger has done a fantastic job of getting ideas out there. But the crucial next step is to train the next generation of professionals who have a firm enough grasp of the science to model and apply the techniques that become available. I think you guys are great as well. I love hearing from you. I’m advocating for this because the country needs better energy efficiency, as does the world, and it will likely be the builders who provide it. Like everyone else who follows your contributions, I’m very excited by all the great things that are happening.
Just had the EnergyQ radiant barrier installed under my new shingles in Oklahoma and I regret it. I had collected attic temp/humidity readings for two months prior and one month after and there has been NO CHANGE in the delta temperature between the attic and outside. This product claimed to have a built in air gap but I now think that was BS and is the reason this didn’t work. I’m also now worried about the reduced vapor permeability of my roof assembly, especially if I ever decide to move to an encapsulated attic with spray foam.
UK perspective is that spay foam is a potential disaster because it's not possible to inspect the hidden contact surfaces where moisture condenses and rots the structure. Very similar to the air gap problem. It's very dependant on local climate type as to which aspect is most 'destructive/problematic' !
Radiant barriers still have value when facing inward. This follows from the relationship that for an opaque object, Emissivity + Reflectivity = 1. Radiant barriers have a very high reflectivity and therefore a very low emissivity. So when the radiant barrier facing an air gap is on the "cool side" (facing outward), its high reflectivity means it doesn't absorb much of the blackbody radiation coming across the air gap from the "hot side". Conversely, when the radiant barrier is on the hot side (facing inwards), its low emissivity means it just doesn't emit much blackbody radiation in the first place. Either way, radiant heat transfer across the air gap is reduced.
@@jimurrata6785 I agree, low emissivity on the inward surface or outward surface will have the same effect on radiative heat transfer. However, putting the low emissivity surface on the inside does mean the roof decking will be hotter, which (aside from more thermal expansion and contraction) means more thermal mass is heated and thus more opportunity for conductive and convective heat transfer with the inside air. But yes, from a radiative perspective, either side will work. Edit: I should add a condition here - that my comment above assumes that the roof deck conducts heat fairly well such that the backside of the roof decking is comparable temperature to the front. Obviously if the roof deck was 6" of foam, the underside would not re-radiate as much since it is at a lower temperature.
@@jamesroscoe7555 Absolutely. And while the deck doesn't have nearly the mass of something like a couple of layers of asphalt shingle it does act to conduct heat to the inside of the building envelope. One of the reasons for reflective on the underside is because it stays relatively clean. (a filthy mirror is no mirror at all) I'm really surprised that all these insulation baffles, for soffit to ridge venting, are always black plastic instead of laminated with reflective mylar or something.... Wait until you try to explain that insulation only slows the transfer of heat, never stops it (except for absolute vacuum, which still doesn't stop radiation)
@@jimurrata6785 I agree fully. And yes, the dirty upwards facing reflective layer is definitely an issue. Probably better than nothing at all, but I guess it depends on just how much dust and debris accumulates. Upward facing horizontal radiant barriers are pretty common, eg in staple up radiant. I think I have seen reflective insulation baffles for attic spaces, but you are right, most are not (and produced as cheaply as possible). It might make more sense to cut and cobble from reflective faced foam in that situation (while considering vapour permeability of the assembly).
Radiant barriers do work. I have installed them, no sponsor. But, the air gap is absolutely needed. Any foil face is the face you need to the side that is the heat you want sent away. If it touches anything on that side, it does not work as a barrier. Air is the "insulation". You could have loose batting on the foil side, if you want better insulation, but you need an air flow somewhere in that cavity.
Where I'm at in California, a radiant barrier is required on roofs, but it's almost always deployed as a radiant barrier bonded to OSB with the radiant barrier facing inwards. It has never sat right with me that that would be effective. You are also now starting to see not only the ceilings in homes being insulated, but the rafters as well. So the insulation gets pushed right up against the radiant barrier.
It works very well. The attic space when radiant barrier is glued to bottom of the roof membrane makes all the difference in the world. I built homes for over fifty years and I know from experience that it works.
@@thomaswayneward If I am remembering correctly, one manufacturer specified the OSB-bonded barrier face into the attic; the explanation is it is too slippery to be on top, creating an unacceptable safety hazard. And as you say, it still makes a significant difference.
@@teraxiel ...and the article, and the laws of thermodynamics. Despite being an unverifiable strange, his three sentence anecdote was enough to convince me that physics don't matter when someone who learned to build homes in the 70's is speaking from experience. It''s obviously not the insulation part that having an effect, it's definitely the radiant barrier that's sandwiched in the middle, acting as a thermal conduction bridge, doing all the work. /s
Not just for hot climates. I put one facing inward on a walk out basement door with a wood stove on the opposite side of the basement. It helps keep the heat where I want it in the cold season. 90% of the heat that would escape through that door is now reflected back into the house and rises. While still allowing the door itself to be an entryway for fresh air to start a draft.
We need more help! We are sometimes required in California to add a roof radiant barrier in Zone 2, or a marine climate. How is this helpful in California when this is also a vapor barrier?
One of the reasons why California has the foil facing inward on the roof is for winter heat to reflect back into the home reducing winter heating costs and reducing emissions.
Possibly if youre trying to keep radiant heat inwards inside the enclosure. But not sure how effective that would be. Its most likely those products are designed incorrectly or its just some marketing BS
Actually, in a mainly cold climate, it makes sense that inward facing radiant barriers can help keep the interior heat on the inside longer. That would make sense if your largest energy bill is for heat, and little is spent for cooling in a given year.
@@smersh007 Only in very poorly insulated attics. Ton of articles and reports out there conducted in the 80s and 90s that once you exceed R-30 they have little to no effect.
It works very well if used on the inside of the roof sheathing and you can stand to be in your attic during the summer. From a Texas builder with fifty years experience.
@@thomaswayneward Can you make sense of your statement? I cannot. Works well for what purpose, cooling the house? What does being in the attic have to do with it?
I would argue there's another big advantage if done right. The surface is clean, can be cleaned, can be selectively sealed, and serves as a barrier to toxic dust and volatiles from the underlying insulation, glues, etc. Particularly if you have a lot of air exchange going on with the rest of the building.
I live in southern Ohio. I have a garage attached to my house. The garage attic is not insulated and heats up like an oven in the summer. Would it make sense to attach radiant barrier 2 inches below the roof deck?
@@chuckhall5347 As long as there's a gap and you're venting it, it would help a bit. Just keep in mind that you're not going to be able to dry through it.
Apply cut foam, unsheathed, between the rafters. That will help a lot. They don't have to be too thick, just two inches is enough. From my fifty years of experience building homes.
I'm under a budget restraint, and it's looking like in regards to insulation and energy efficiency, I can ether have a great roof or great walls, but not both. In a hot humid climate, where would you spend the premium money?
Go nuts insulating the roof; air seal the walls and get code minimum insulation there. Run around with a foam gun and flashing tape yourself if you have to, to make sure the air-tight plane is truly air-tight. Make sure your roof design includes good overhangs, or at least get outdoor awnings or shades, to protect your windows and walls from the overhead noonday sun. And get as much of your HVAC equipment and ducting as possible out of the attic. The roof will absorb the lion's share of the incoming solar heat pretty much all day, and even with halfway decent ventilation, the attic temperatures can still get much hotter than the outside. You really want to protect your ceiling plane from that, so insulate away in the roof / attic zone. Walls will get hot only intermittently depending on sun angle and time of day and roof shading and landscaping choices. Preventing humid air infiltration is the key goal for your exterior walls. Before they put in the ceiling insulation, make sure the ceiling plane is air-tight. Foam gun in around all penetrations and make sure your recessed disk lights have good gaskets.
@@keithwingo514 Yeah - with a roof plane close in against the conditioned space, that's going to be the biggest thermal challenge. The underside of a plywood or OSB roof deck in a southern summer can easily exceed 140 F surface temperature and keep that up for many hours during the day (the shingles can go 160+). I've measured that many times myself with an infrared thermometer gun with my head poking up through the attic hatch into the sweltering attic space and aiming at the underside of the deck. I don't know what code minimums apply in your area, but you might consider subtracting R-5 or so from your total, installing fiberglass batts in the rafter bays to that lower total (R-25 or R-33 or whatever), and then putting some kind of rigid foam board between the rafters and the ceiling drywall to thermally break the contacts between the rafters (which are directly touching the roof deck) and the interior finish and give you your last R-5. Technically, a continuous R-5 foam board breaking contact with the rafters should give more insulating effect than an additional R-5 worth of fiberglass in the bays, so if your code minimum is R-30 then off the shelf R-23 batts in the bays plus off the shelf R-5 continuous one inch foam boards should give you superior performance even though it only adds up to R-28. In theory you ought to be able to do the math and show the building department and they'll bless your variant approach, but YMMV. Some point out that if the full rafter bay isn't going to be full of insulation then the batts are better off being hard against the foam board on the bottom than pushed up against the roof deck on top. And I'm not going into the decision as to whether to vent the deck by deliberately keeping the top few inches of the rafter bays clear and installing soffit and ridge vents, or whether you should go all-in on a hot unventilated roof deck.
@@Noam-w1l No vents or airspace. Rural area with aggressive critters and insects. If my understanding is correct, it's better to have something in contact with the underside of the roof deck to fight condensation too.
I have a question for you. Is all of the electromagnetic energy emitted by the sun and falling on the roof, actually close to entirely absorbed by the roof, heating it? I ask that because that would mean that the solar radiation is not what the radiant barrier is actually reflecting. If all of the sun’s energy is absorbed in the roof deck heating it, then the roof deck will emit “black body” radiation the intensity and frequency distribution of which, is dependent on the temperature, and only on the temperature of the roof deck. The radiant barrier would then reflect back half of that radiation, I think. I’ve never seen this question addressed and I’d appreciate any better explanation you can give me. You might want to ask a heat transfer engineer. Thanks.
@@DitDitDitDahDahDahDitDitDit Yes. The roof will re-radiate as a black body. However some of the radiant energy from the sun is conducted inwards and re radiated inwards if there is an interior air gap. The emissivity (and temperature) of each surface then matters. No, not all the energy from the sun is absorbed - some is reflected back, depending on the reflectivity/emissivity of the roof surface (why a white roof is cooler). Also note, that the roof (thankfully) has a much lower temperature than the sun, so the blackbody radiation from the roof is less intense and red shifted. How much the radiant barrier reflects back (or does not re-emit) depends on the construction and properties of each surface.
what about a radiant barrier on the interior of a window facing outwards? will it reflect the UV rays out or is it worthless because it is touching the window?
I have seen debate about radiant barriers continuously resurfacing. A few items that come to mind: Not building with an adjacent air gap doesn't mean that there is no air gap. Construction variations appear to frequently leave gaps where materials are not uniformly in contact. This means there may be still some benefit from the radiant barrier. This second item is similar, in the sense that there may still be a net energy bill reduction from a radiant barrier facing in the wrong direction. Any substance with heat energy will either feel warm or hot, or even very hot. It is also emitting radiant energy in all directions, until that radiation bounces off of a reflective surface. If the barrier is facing to the interior, radiant energy from the sun that has transmitted through the roof deck and shingles will reach the backside of the barrier and to some extent should then reflect back into the deck and shingles. This would probably mean that asphalt shingles will degrade sooner, but the interior home comfort should see some kind of benefit. However, I have no idea in the long run how these different expenses would compare. It seems obvious that the best strategy is a ventilated metal roof system.
It IS important that the air-gap is immediately adjacent to the reflective surface as it is the combination of air (insulation properties) and the reflector (radiation stop properties) that make the combination so effective (when done right). There are also considerations of vapour venting and/or venting of the heated air (chimney effect) to be included. In the modern well insulated home (no huddling round a small fire in the centre of the space with open vent for smoke) that we need the careful details around the critical inside-outside barrier. Historically it hasn't been a consideration in the construction industry...
There are radiant barriers that allow vapor transmission, like RadiantGuard which is what I put in my attic. It has pores every cm or so to allow vapor to pass through, so something like that would not be a vapor barrier.
@@ASIRIDesigns I'm not sure, but they claim 7.8 perms (I still installed it with an air gap on both sides, though, as I was installing under the rafters)
@@ASIRIDesignsThe product I will be, Attic Foil, has a perm rating of 14. It appears they recently released a Superperm version, although I'm not sure if the perm rating.
UV is INDEED blocked by metal - a “radiant barrier”. But, almost anything else that’s opaque also blocks UV. Wood, in any form, or asphalt, or metal, all block UV energy. And actually, glass itself blocks UV fairly well. But glass’s UV blockage can be enhanced with coatings if long term UV exposure needs to be avoided - to avoid fading of pigments, for instance.
And polycarbonate is opaque to UV. Yup, that's right a 99c pair of safety glasses will 100% protect your eyes from welding or snow blindness. Not that the extremely bright light in the lower part of the spectrum won't bother you, but you're not going to feel like glass has been rubbed in your eyes as you later try to sleep
You are incorect: Radiant barriers protect both inside and out, they both reflect radiant heat away AND prevent radiant heat from leaving the surface; a LOT of heat can be shed by hot objects via infrared, and since this rate is proportional to the temp difference to the FOURTH POWER, hot objects emit way more heal than cooler ones. A black surface emits this heat well, a shiny one does not
In general I find your videos tremendously informative, helpful, and correct. So I'm surprised to hear in this video at 1:59 that radiant barriers "will not work if they are facing inward," while showing the image of roof sheathing with a radiant barrier on the underside. Radiant barriers CAN work facing inward. I took Heat Transfer and Passive Solar Design in college; just to make sure Joe Lstiburek's quoted source was in agreement, I read 95% of the referenced article. The Building Science article says radiant barriers may not be EFFECTIVE when your attic is insulated to R-30 or more. Nowhere does it say they wont work if they face inward. (There are a bunch of other conditions at play in cases of failures.) Foil-faced OSB such as Tech-Shield installed as roof sheathing with the foil facing down will reduce heat transfer into or out of the attic. The same magic that makes foil reflect heat back at a heat source also keeps the heat from radiating downward from hot, foil-covered OSB roof sheathing into your attic. The cited studies indicate that it may not be worth it if you insulate your attic well. And radiant barriers facing UPWARDS are useless as soon as they get covered with dust and the surface is no longer shiny.
@@ASIRIDesigns I got the "if you are building in a hot climate" intro. As I said in my initial comment, radiant barriers still work facing inward in this case. They may not (probably won't!) work as well as manufacturers want you to believe. You still get heat conduction into the air contacting the barrier, and things are affected by the temperature of the surfaces. But per the physics, heat transfer is impeded by a radiant barrier whether it is radiating from your hot attic floor to the cool foil surface on the underside of the sheathing---think warm house on a cold winter night--or the opposite direction, as expected in hot climates during the day. This is probably irrelevant in most cases if the attic is over R-30, as mentioned in the Building Science article (and in the video, I now notice 🙂). And there are many ways to install radiant barriers that will spell disaster for the building envelope--so I'm not trying to defend them, just trying to put my education to use
Re: 2:00 - Radiant barriers aren't reflective insulation, and they *do* work to cut radiant heat transmission even when applied to the "back" side of a hot assembly (think: underside of a roof deck in the south in the summer, facing the attic with nothing touching it). Low emissivity means what it says on the tin - low tendency to emit radiant energy, even when hot. So when your looking at a hot low-e object, less heat is radiated onto you compared to a similar object with high e. The wood decking with the attached foil just gets hotter than it would without the foil, until it reaches a new equilibrium where it can shed the incoming solar heat conductively and convectively back into the shingles above and the attic air below, but without the use of the radiative heat channel. If you have HVAC equipment on the attic floor, and/or you store stuff there, and/or have scanty attic floor insulation, you'll benefit in a hot climate from a radiant barrier built into the underside of the roof deck, esp. if the attic has halfway decent ventilation. But if your HVAC is in your conditioned envelope and your attic has nothing but an R-30+ pile of fiberglass on the floor, the radiant barrier won't much matter becaise that huge insulation blanket will shrug off the extra incident heat that you get without a radiant barrier. A dirty little secret is that a not-very-low e shading layer can also cut radiant thermal transmission too, just not as much as low e. If you put up a layer of cardboard halfway up your ventilated attic, it will catch the heat radiation from the roof deck, get hot in turn, dissipate some of that heat into the attic ventilation air by conduction, and then re-radiate heat -- from both its faces. So half the heat re-radiating from the cardboard layer will return upwards and only half will radiate down to the attic floor. Cutting the radiant incidence by 50% isn't the same as cutting by 90%, but it ain't nothing. (There are some second order effects to confining heat in the upper layer of your attic, but we're hand waving them away by assuming halfway decent attic ventilation and halfway decent roof shingle quality that can withstand the shingles getting a few degrees hotter because the shingle and deck assembly can't shed heat as effectively into your attic.)
I'm gonna contradict you on this one. I had my attic above the garage being at 160 in the TX summer. I put radiant barrier on the interior with a gap for the air to flow behind the roof and the radiant barrier and the temperature dropped to 130 under the same conditions
Folks this information will get you into trouble with your home, it will create more problems that it solves. Just build a normal home with insulation and wrapped in plywood or OSB. Fifty years of building homes has taught me not to use anything on the exterior of your home that has not been done for at least twenty years and proved itself successful. It takes that long for problems to show up, even though the damage has been going on for the entire twenty years.
Everything that has been used for the past twenty years had a beginning. It had a first day, first year,etc. In other words, someone has to be the genie pig and start their project with whatever the new material and practice might be.
@@MrItalianfighter1 That's right, you said it; some one has to be the sucker that tests if something new, actually works. In this case it would be thousands of suckers. Take my advice for your own home, use the tried and true and you will not be sorry. In my years I have seen so many "new ideas" come and go in failure. Take sips for example, they were going to be the new money saving thing, but not many use them any more. I will let you figure out why.
This is wrong when it says the radiant barrier must face outwards to reflect heat. The LOW EMISSIVITY keeps the barrier from emitting heat inward too--heat absorbed by shingles and conducted through to the sheathing does NOT get emitted to the attic (typical construction). So radiant barrier works either way--the one requirement is that it must face the air gap.
For some extra reading if anyone is interested on the topic: buildingscience.com/documents/building-science-insights/bsi-136-piltdown-man-does-thermal-resistance
www.greenbuildingadvisor.com/article/radiant-barriers-a-solution-in-search-of-a-problem
The Stefan-Boltzmann law is the law describing the emission of black-body radiation. However, the explication the web site you put up gives is poor in detail. I think the field deserves the benefit of better, clearer, explanations of the physics underlying these claims. Not everyone can absorb all of the details, but it is certainly within the ability of architects to master. And we need to have a more robust collaboration between engineers who have finely honed skills in these areas, and the designers and builders who need to apply them. Matt Risinger has done a fantastic job of getting ideas out there. But the crucial next step is to train the next generation of professionals who have a firm enough grasp of the science to model and apply the techniques that become available. I think you guys are great as well. I love hearing from you. I’m advocating for this because the country needs better energy efficiency, as does the world, and it will likely be the builders who provide it. Like everyone else who follows your contributions, I’m very excited by all the great things that are happening.
Just had the EnergyQ radiant barrier installed under my new shingles in Oklahoma and I regret it. I had collected attic temp/humidity readings for two months prior and one month after and there has been NO CHANGE in the delta temperature between the attic and outside. This product claimed to have a built in air gap but I now think that was BS and is the reason this didn’t work. I’m also now worried about the reduced vapor permeability of my roof assembly, especially if I ever decide to move to an encapsulated attic with spray foam.
UK perspective is that spay foam is a potential disaster because it's not possible to inspect the hidden contact surfaces where moisture condenses and rots the structure. Very similar to the air gap problem.
It's very dependant on local climate type as to which aspect is most 'destructive/problematic' !
Got a link the product? Just curious as to the specs
Radiant barriers still have value when facing inward. This follows from the relationship that for an opaque object, Emissivity + Reflectivity = 1. Radiant barriers have a very high reflectivity and therefore a very low emissivity. So when the radiant barrier facing an air gap is on the "cool side" (facing outward), its high reflectivity means it doesn't absorb much of the blackbody radiation coming across the air gap from the "hot side". Conversely, when the radiant barrier is on the hot side (facing inwards), its low emissivity means it just doesn't emit much blackbody radiation in the first place. Either way, radiant heat transfer across the air gap is reduced.
Yup, doesn't matter which side it's on, as long as there's a gap to prevent conduction.
@@jimurrata6785 I agree, low emissivity on the inward surface or outward surface will have the same effect on radiative heat transfer. However, putting the low emissivity surface on the inside does mean the roof decking will be hotter, which (aside from more thermal expansion and contraction) means more thermal mass is heated and thus more opportunity for conductive and convective heat transfer with the inside air.
But yes, from a radiative perspective, either side will work.
Edit: I should add a condition here - that my comment above assumes that the roof deck conducts heat fairly well such that the backside of the roof decking is comparable temperature to the front. Obviously if the roof deck was 6" of foam, the underside would not re-radiate as much since it is at a lower temperature.
@maths9085 Builders aren’t physicists 😉 He needs to visit Wiki at least.
@@jamesroscoe7555 Absolutely. And while the deck doesn't have nearly the mass of something like a couple of layers of asphalt shingle it does act to conduct heat to the inside of the building envelope.
One of the reasons for reflective on the underside is because it stays relatively clean. (a filthy mirror is no mirror at all)
I'm really surprised that all these insulation baffles, for soffit to ridge venting, are always black plastic instead of laminated with reflective mylar or something....
Wait until you try to explain that insulation only slows the transfer of heat, never stops it (except for absolute vacuum, which still doesn't stop radiation)
@@jimurrata6785 I agree fully. And yes, the dirty upwards facing reflective layer is definitely an issue. Probably better than nothing at all, but I guess it depends on just how much dust and debris accumulates. Upward facing horizontal radiant barriers are pretty common, eg in staple up radiant.
I think I have seen reflective insulation baffles for attic spaces, but you are right, most are not (and produced as cheaply as possible). It might make more sense to cut and cobble from reflective faced foam in that situation (while considering vapour permeability of the assembly).
Radiant barriers do work. I have installed them, no sponsor. But, the air gap is absolutely needed. Any foil face is the face you need to the side that is the heat you want sent away. If it touches anything on that side, it does not work as a barrier. Air is the "insulation". You could have loose batting on the foil side, if you want better insulation, but you need an air flow somewhere in that cavity.
Where I'm at in California, a radiant barrier is required on roofs, but it's almost always deployed as a radiant barrier bonded to OSB with the radiant barrier facing inwards. It has never sat right with me that that would be effective. You are also now starting to see not only the ceilings in homes being insulated, but the rafters as well. So the insulation gets pushed right up against the radiant barrier.
It works very well. The attic space when radiant barrier is glued to bottom of the roof membrane makes all the difference in the world. I built homes for over fifty years and I know from experience that it works.
@@thomaswayneward If I am remembering correctly, one manufacturer specified the OSB-bonded barrier face into the attic; the explanation is it is too slippery to be on top, creating an unacceptable safety hazard. And as you say, it still makes a significant difference.
@@thomaswaynewardIf the membrane is adhered to the radiant barrier, you're no longer preventing radiant heat transfer... Rewatch the video.
@@thomaswayneward I mean, you're kinda saying the guy in the video is wrong...
@@teraxiel ...and the article, and the laws of thermodynamics. Despite being an unverifiable strange, his three sentence anecdote was enough to convince me that physics don't matter when someone who learned to build homes in the 70's is speaking from experience. It''s obviously not the insulation part that having an effect, it's definitely the radiant barrier that's sandwiched in the middle, acting as a thermal conduction bridge, doing all the work. /s
Not just for hot climates.
I put one facing inward on a walk out basement door with a wood stove on the opposite side of the basement. It helps keep the heat where I want it in the cold season. 90% of the heat that would escape through that door is now reflected back into the house and rises. While still allowing the door itself to be an entryway for fresh air to start a draft.
We need more help! We are sometimes required in California to add a roof radiant barrier in Zone 2, or a marine climate. How is this helpful in California when this is also a vapor barrier?
Many come with perforation, allowing the passage of vapor. We use these types here in Florida
One of the reasons why California has the foil facing inward on the roof is for winter heat to reflect back into the home reducing winter heating costs and reducing emissions.
Is there any benefit to inward facing radiate barriers? There are a lot of products out there.
Possibly if youre trying to keep radiant heat inwards inside the enclosure. But not sure how effective that would be. Its most likely those products are designed incorrectly or its just some marketing BS
Actually, in a mainly cold climate, it makes sense that inward facing radiant barriers can help keep the interior heat on the inside longer. That would make sense if your largest energy bill is for heat, and little is spent for cooling in a given year.
@@smersh007 Only in very poorly insulated attics. Ton of articles and reports out there conducted in the 80s and 90s that once you exceed R-30 they have little to no effect.
It works very well if used on the inside of the roof sheathing and you can stand to be in your attic during the summer. From a Texas builder with fifty years experience.
@@thomaswayneward Can you make sense of your statement? I cannot. Works well for what purpose, cooling the house? What does being in the attic have to do with it?
I would argue there's another big advantage if done right. The surface is clean, can be cleaned, can be selectively sealed, and serves as a barrier to toxic dust and volatiles from the underlying insulation, glues, etc. Particularly if you have a lot of air exchange going on with the rest of the building.
I live in southern Ohio. I have a garage attached to my house. The garage attic is not insulated and heats up like an oven in the summer. Would it make sense to attach radiant barrier 2 inches below the roof deck?
@@chuckhall5347 As long as there's a gap and you're venting it, it would help a bit. Just keep in mind that you're not going to be able to dry through it.
Apply cut foam, unsheathed, between the rafters. That will help a lot. They don't have to be too thick, just two inches is enough. From my fifty years of experience building homes.
I'm under a budget restraint, and it's looking like in regards to insulation and energy efficiency, I can ether have a great roof or great walls, but not both. In a hot humid climate, where would you spend the premium money?
Go nuts insulating the roof; air seal the walls and get code minimum insulation there. Run around with a foam gun and flashing tape yourself if you have to, to make sure the air-tight plane is truly air-tight. Make sure your roof design includes good overhangs, or at least get outdoor awnings or shades, to protect your windows and walls from the overhead noonday sun. And get as much of your HVAC equipment and ducting as possible out of the attic.
The roof will absorb the lion's share of the incoming solar heat pretty much all day, and even with halfway decent ventilation, the attic temperatures can still get much hotter than the outside. You really want to protect your ceiling plane from that, so insulate away in the roof / attic zone. Walls will get hot only intermittently depending on sun angle and time of day and roof shading and landscaping choices. Preventing humid air infiltration is the key goal for your exterior walls.
Before they put in the ceiling insulation, make sure the ceiling plane is air-tight. Foam gun in around all penetrations and make sure your recessed disk lights have good gaskets.
@@Noam-w1l Thanks for the info. No attic, free air all the way up to roof underside. Mini-splits and window ACs. I'm doing all the work myself.
@@keithwingo514 Yeah - with a roof plane close in against the conditioned space, that's going to be the biggest thermal challenge. The underside of a plywood or OSB roof deck in a southern summer can easily exceed 140 F surface temperature and keep that up for many hours during the day (the shingles can go 160+). I've measured that many times myself with an infrared thermometer gun with my head poking up through the attic hatch into the sweltering attic space and aiming at the underside of the deck.
I don't know what code minimums apply in your area, but you might consider subtracting R-5 or so from your total, installing fiberglass batts in the rafter bays to that lower total (R-25 or R-33 or whatever), and then putting some kind of rigid foam board between the rafters and the ceiling drywall to thermally break the contacts between the rafters (which are directly touching the roof deck) and the interior finish and give you your last R-5.
Technically, a continuous R-5 foam board breaking contact with the rafters should give more insulating effect than an additional R-5 worth of fiberglass in the bays, so if your code minimum is R-30 then off the shelf R-23 batts in the bays plus off the shelf R-5 continuous one inch foam boards should give you superior performance even though it only adds up to R-28. In theory you ought to be able to do the math and show the building department and they'll bless your variant approach, but YMMV.
Some point out that if the full rafter bay isn't going to be full of insulation then the batts are better off being hard against the foam board on the bottom than pushed up against the roof deck on top. And I'm not going into the decision as to whether to vent the deck by deliberately keeping the top few inches of the rafter bays clear and installing soffit and ridge vents, or whether you should go all-in on a hot unventilated roof deck.
@@Noam-w1l No vents or airspace. Rural area with aggressive critters and insects. If my understanding is correct, it's better to have something in contact with the underside of the roof deck to fight condensation too.
I have a question for you. Is all of the electromagnetic energy emitted by the sun and falling on the roof, actually close to entirely absorbed by the roof, heating it? I ask that because that would mean that the solar radiation is not what the radiant barrier is actually reflecting. If all of the sun’s energy is absorbed in the roof deck heating it, then the roof deck will emit “black body” radiation the intensity and frequency distribution of which, is dependent on the temperature, and only on the temperature of the roof deck. The radiant barrier would then reflect back half of that radiation, I think. I’ve never seen this question addressed and I’d appreciate any better explanation you can give me. You might want to ask a heat transfer engineer. Thanks.
@@DitDitDitDahDahDahDitDitDit Yes. The roof will re-radiate as a black body. However some of the radiant energy from the sun is conducted inwards and re radiated inwards if there is an interior air gap. The emissivity (and temperature) of each surface then matters. No, not all the energy from the sun is absorbed - some is reflected back, depending on the reflectivity/emissivity of the roof surface (why a white roof is cooler).
Also note, that the roof (thankfully) has a much lower temperature than the sun, so the blackbody radiation from the roof is less intense and red shifted. How much the radiant barrier reflects back (or does not re-emit) depends on the construction and properties of each surface.
what about a radiant barrier on the interior of a window facing outwards? will it reflect the UV rays out or is it worthless because it is touching the window?
@@dovahkindragonborn9827 Radiant barriers deal with infrared radiation, not the UV spectrum unless it's been specifically designed for it.
I have seen debate about radiant barriers continuously resurfacing. A few items that come to mind: Not building with an adjacent air gap doesn't mean that there is no air gap. Construction variations appear to frequently leave gaps where materials are not uniformly in contact. This means there may be still some benefit from the radiant barrier. This second item is similar, in the sense that there may still be a net energy bill reduction from a radiant barrier facing in the wrong direction. Any substance with heat energy will either feel warm or hot, or even very hot. It is also emitting radiant energy in all directions, until that radiation bounces off of a reflective surface. If the barrier is facing to the interior, radiant energy from the sun that has transmitted through the roof deck and shingles will reach the backside of the barrier and to some extent should then reflect back into the deck and shingles. This would probably mean that asphalt shingles will degrade sooner, but the interior home comfort should see some kind of benefit. However, I have no idea in the long run how these different expenses would compare. It seems obvious that the best strategy is a ventilated metal roof system.
It IS important that the air-gap is immediately adjacent to the reflective surface as it is the combination of air (insulation properties) and the reflector (radiation stop properties) that make the combination so effective (when done right).
There are also considerations of vapour venting and/or venting of the heated air (chimney effect) to be included.
In the modern well insulated home (no huddling round a small fire in the centre of the space with open vent for smoke) that we need the careful details around the critical inside-outside barrier.
Historically it hasn't been a consideration in the construction industry...
There are radiant barriers that allow vapor transmission, like RadiantGuard which is what I put in my attic. It has pores every cm or so to allow vapor to pass through, so something like that would not be a vapor barrier.
@@RedMike-ym8hd What percentage of the surface are holes?
@@ASIRIDesigns I'm not sure, but they claim 7.8 perms (I still installed it with an air gap on both sides, though, as I was installing under the rafters)
@@ASIRIDesignsThe product I will be, Attic Foil, has a perm rating of 14. It appears they recently released a Superperm version, although I'm not sure if the perm rating.
UV is INDEED blocked by metal - a “radiant barrier”. But, almost anything else that’s opaque also blocks UV. Wood, in any form, or asphalt, or metal, all block UV energy. And actually, glass itself blocks UV fairly well. But glass’s UV blockage can be enhanced with coatings if long term UV exposure needs to be avoided - to avoid fading of pigments, for instance.
And polycarbonate is opaque to UV.
Yup, that's right a 99c pair of safety glasses will 100% protect your eyes from welding or snow blindness.
Not that the extremely bright light in the lower part of the spectrum won't bother you, but you're not going to feel like glass has been rubbed in your eyes as you later try to sleep
Right, but not all radiant barriers are suitable for UV protection, just like not all materials designed for UV resistance are radiant barriers.
@@ASIRIDesigns Show me a reflective surface that is transparent to UV.
@@jimurrata6785 Many Many Many windows
New strat: unroll $20 of aluminum foil on your attic floor to reflect the summer heat back toward the roof.
I legitimately think that could be a good idea. Then vent the roof space to let the heat escape
You are incorect: Radiant barriers protect both inside and out, they both reflect radiant heat away AND prevent radiant heat from leaving the surface; a LOT of heat can be shed by hot objects via infrared, and since this rate is proportional to the temp difference to the FOURTH POWER, hot objects emit way more heal than cooler ones. A black surface emits this heat well, a shiny one does not
In general I find your videos tremendously informative, helpful, and correct. So I'm surprised to hear in this video at 1:59 that radiant barriers "will not work if they are facing inward," while showing the image of roof sheathing with a radiant barrier on the underside. Radiant barriers CAN work facing inward. I took Heat Transfer and Passive Solar Design in college; just to make sure Joe Lstiburek's quoted source was in agreement, I read 95% of the referenced article. The Building Science article says radiant barriers may not be EFFECTIVE when your attic is insulated to R-30 or more. Nowhere does it say they wont work if they face inward. (There are a bunch of other conditions at play in cases of failures.)
Foil-faced OSB such as Tech-Shield installed as roof sheathing with the foil facing down will reduce heat transfer into or out of the attic. The same magic that makes foil reflect heat back at a heat source also keeps the heat from radiating downward from hot, foil-covered OSB roof sheathing into your attic. The cited studies indicate that it may not be worth it if you insulate your attic well.
And radiant barriers facing UPWARDS are useless as soon as they get covered with dust and the surface is no longer shiny.
Hi there, thanks for watching. I think you missed the intial context of that statement - it was in reference to hot climates.
@@ASIRIDesigns I got the "if you are building in a hot climate" intro. As I said in my initial comment, radiant barriers still work facing inward in this case. They may not (probably won't!) work as well as manufacturers want you to believe. You still get heat conduction into the air contacting the barrier, and things are affected by the temperature of the surfaces. But per the physics, heat transfer is impeded by a radiant barrier whether it is radiating from your hot attic floor to the cool foil surface on the underside of the sheathing---think warm house on a cold winter night--or the opposite direction, as expected in hot climates during the day.
This is probably irrelevant in most cases if the attic is over R-30, as mentioned in the Building Science article (and in the video, I now notice 🙂). And there are many ways to install radiant barriers that will spell disaster for the building envelope--so I'm not trying to defend them, just trying to put my education to use
Re: 2:00 - Radiant barriers aren't reflective insulation, and they *do* work to cut radiant heat transmission even when applied to the "back" side of a hot assembly (think: underside of a roof deck in the south in the summer, facing the attic with nothing touching it).
Low emissivity means what it says on the tin - low tendency to emit radiant energy, even when hot. So when your looking at a hot low-e object, less heat is radiated onto you compared to a similar object with high e. The wood decking with the attached foil just gets hotter than it would without the foil, until it reaches a new equilibrium where it can shed the incoming solar heat conductively and convectively back into the shingles above and the attic air below, but without the use of the radiative heat channel.
If you have HVAC equipment on the attic floor, and/or you store stuff there, and/or have scanty attic floor insulation, you'll benefit in a hot climate from a radiant barrier built into the underside of the roof deck, esp. if the attic has halfway decent ventilation. But if your HVAC is in your conditioned envelope and your attic has nothing but an R-30+ pile of fiberglass on the floor, the radiant barrier won't much matter becaise that huge insulation blanket will shrug off the extra incident heat that you get without a radiant barrier.
A dirty little secret is that a not-very-low e shading layer can also cut radiant thermal transmission too, just not as much as low e. If you put up a layer of cardboard halfway up your ventilated attic, it will catch the heat radiation from the roof deck, get hot in turn, dissipate some of that heat into the attic ventilation air by conduction, and then re-radiate heat -- from both its faces. So half the heat re-radiating from the cardboard layer will return upwards and only half will radiate down to the attic floor. Cutting the radiant incidence by 50% isn't the same as cutting by 90%, but it ain't nothing. (There are some second order effects to confining heat in the upper layer of your attic, but we're hand waving them away by assuming halfway decent attic ventilation and halfway decent roof shingle quality that can withstand the shingles getting a few degrees hotter because the shingle and deck assembly can't shed heat as effectively into your attic.)
I'm gonna contradict you on this one. I had my attic above the garage being at 160 in the TX summer. I put radiant barrier on the interior with a gap for the air to flow behind the roof and the radiant barrier and the temperature dropped to 130 under the same conditions
sorry; DISAGREE= radiant barriers absolutely DO work in WELL insulated assemblies
@@someguydino6770 Got any measurements you'd be able to share with us?
Folks this information will get you into trouble with your home, it will create more problems that it solves. Just build a normal home with insulation and wrapped in plywood or OSB. Fifty years of building homes has taught me not to use anything on the exterior of your home that has not been done for at least twenty years and proved itself successful. It takes that long for problems to show up, even though the damage has been going on for the entire twenty years.
Everything that has been used for the past twenty years had a beginning. It had a first day, first year,etc. In other words, someone has to be the genie pig and start their project with whatever the new material and practice might be.
@@MrItalianfighter1 That's right, you said it; some one has to be the sucker that tests if something new, actually works. In this case it would be thousands of suckers. Take my advice for your own home, use the tried and true and you will not be sorry. In my years I have seen so many "new ideas" come and go in failure. Take sips for example, they were going to be the new money saving thing, but not many use them any more. I will let you figure out why.
This is wrong when it says the radiant barrier must face outwards to reflect heat. The LOW EMISSIVITY keeps the barrier from emitting heat inward too--heat absorbed by shingles and conducted through to the sheathing does NOT get emitted to the attic (typical construction). So radiant barrier works either way--the one requirement is that it must face the air gap.