I still get this question all the time.  “Can you explain why an air space is needed for a radiant barrier to work?” Watch this video for a complete explanation.

Basically it works this way.  Radiant heat is heat that is transferred across either an air space or a vacuum.  This is how the heat from the sun reaches the earth.  Radiant heat acts similar to a sound wave.  By definition, you MUST have either an air space or a void for radiant heat to even exist.

If you don’t have an air space then you basically have a solid.  Heat can essentially only move through a solid by conduction. This is by direct contact.  This is how an egg cooks on a hot skillet.

So, without an air space, you cannot have radiant heat. Without radiant heat there is no way to have a “Radiant Barrier”.  In fact, because of the conductive nature of pure aluminum, if you install radiant barrier foil WITHOUT the required air space it will actually work AGAINST you and INCREASE Heat flow.

The bottom line is you MUST have an air space on one side of a radiant barrier for it to work.

Looking for more videos on this topic? Check out my posts below.

  • The #1 Attic Ventilation Problem
  • Enerflex® Radiant Barrier at Home Depot compared to AtticFoil® Radiant Barrier
  • Radiant Barrier Installation Summary – Block the Heat
  • Green Energy Barrier (and other products) Compared To AtticFoil® Radiant Barrier Foil
  • Does Radiant Barrier Damage Roof Shingles?
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    42 Comments to “New Video=>Why Is An Air Gap Required For Radiant Barrier To Work?”

    1. Gerald says:

      Ed
      I live in California. I am accepting bids from roofing contractors to replace my old cedar shake roof with a stone coated metal roof system on battens. The existing roof has solid plywood sheathing. I’ve been studying radiant barriers and your explanation was excellent. I have several contractors that want to use a radiant barrier system you described with a 1/4″ insulation pad sandwiched between two sheets of aluminum foil.
      A couple of the contractors want to put the radiant barrier UNDERNEATH the underlayment on top of the plywood with NO air gap. Doesn’t work as a radiant barrier, right? Also,virtually no insulating properties, right? Is there any value to doing it this way. Can your product go over the underlayment with a 3/4 inch air gap above the the bottom of the metal roofing?
      Thanks So much.
      Where can I purchase your product?
      Gerald Freeman

      • Ed Fritz says:

        Gerald,

        You have two questions: 1) Does insulation (1/4″ foil/foam) under a metal roof do any good? and 2) Does foil without an airspace on one side have any value?

        Answers –
        1) Putting insulation at the roof line will ONLY help if the roof is a cathedral type ceiling. If there is ventilated air (an attic) below the insulation then it looses virtually all it’s value. Just like your front door. It has insulating value until it’s swung wide open. Putting insulation at the roof line for a vented attic helps some with heat transfer, but you really only need a radiant barrier to the outside (under the metal roof on top of the decking) and put the additional insulation in the attic on the attic floor.

        2) You are correct. You MUST have an air space on one side of the foil for a radiant barrier to exist. By DEFINITION radiant heat is heat transfer WITHOUT contact – or in a waveform ACROSS a void. If you “sandwich” foil between two other products, then all three products become ONE solid with different conductive qualities. (heat can only move through a solid by conduction) In fact, aluminum is the MOST conductive of the three. So, in reality, putting aluminum foil (radiant barrier) in the assembly would actually cause MORE heat flow then without. It would actually work against what you are trying to achieve.

        Yes, you can put radiant barrier foil under metal roofs. Here is the page for this with a video and pictures: http://www.atticfoil.com/radia.....ingles.htm

        You can order online or via phone: AtticFoil.com

    2. Paul A'Barge says:

      So, the current technology in Texas appears to be plywood/osb decking with one-side radiant barrier nailed to the roof joists, foil-side down. Then (possibly) roof felt directly onto the osb decking and then shingles or metal roof over that (directly screwed into the decking).

      In this case, the only “void” or space is the attic, underneath the radiant barrier sheet glued to the osb 4X8’s.

      Am I right that this means no radiant barrier? In other words, does the void/space have to be on top of the radiant barrier, i.e. foil?

      Based on your explanation of the physics, I would assume so because once the radiation/waves enter the attic below the shingle/felt/osb/foil sandwich, the chances of reflecting back the radiation are gone, no?

      And if this is correct, there are a large number of roofs being installed in Texas advertised as “radiant barrier” that are not barriers at all.

      Thoughts?

      • Ed Fritz says:

        Paul,

        Foil backed decking materials have the airspace facing the attic – so, yes they do work and have the required airspace. These product like TechShield and PolarPly actually work off the EMISSIVITY quality of the foil in the Summer to reduce heat gain. When the foil is on the “cool” side it keeps the decking from “emitting” as much heat – it is technically NOT reflecting the heat back in the Summer. In the Winter, it WOULD be technically a “Radiant Barrier” since the foil faces the “hot” side or heat coming from the home.

        Either way, foil backed decking is a good option if you have the opportunity to replace the deck. It’s not quiet as effective as stapling AtticFoil to the bottom of the roof rafters, but since there is no additional labor it’s a good “bang for the buck”.

    3. Don Jones says:

      I am about to construct a new home with Insulated Concrete Forms and foam sealed attic. The house is to have composition shingles. Am I correct that to get the air space I would need to put a layer of decking with a radiant barrier (with the foil side up), laths and another layer of decking, or is there another way? What would be your recommendation?

      • Ed says:

        This would work great – in fact I did a similar system to this on my own house. You can achieve this with TechShield, or with AtticFoil attached onto plywood. You can do it like this: foil faced plywood facing up, wooden furring strips, foil faced plywood with foil facing down (toward wood strips) then felt and then shingles. Send me some photos of the install if you can!

    4. Armando Martinez says:

      Ed,
      Is it really worth it, putting a radiant barrier under a metal roof with an air gap? If I understand correctly, a metal roof already reflects a good portion of the sun’s radiation away from the home by itself. Would there be an added benefit to having a fully-functioning radiant barrier underneath or would it just be redundant?

      I’m looking at putting a new metal roof over my old shingle roof and was wondering if adding a radiant barrier with an air gap would help my house (and my AC unit) withstand the hot Texas sun. Thanks for all the info.

      Armando

      • Ed Fritz says:

        Armondo,

        Thanks for the comment. You are assuming that ALL metal roofs are highly reflective. There are some, but most are not. Just because something is reflective does not necessarily mean it does not get hot and radiant heat downwards. Think about a chrome bumper on a car. It is VERY reflective, but you would not want to sit on it on a hot and sunny day.

        Putting a radiant barrier below ANY type of raised roof is the best thing you can do both from an energy perspective AND an economic perspective. There is nothing that can make a bigger impact as far as heat gain for less money. Radiant barrier under metal and tile roofs is the fastest growing part of our business. Both customers and roofers are realizing what a great opportunity it is to make a big impact.

        Call us at 800-595-8772 if you have any other questions. AtticFoil.com

    5. Harrison Weightman says:

      Hi Ed,
      I am renovating an RV that I will have to live in as I travel throughout some very hot states and its like an oven. I can feel the paneling on the roof and and walls and its quite hot. Its construction is metal on top facing the elements, then 1 1/2 foam board,.then 1/4 paneling all this is glued together.forming on panel. I put some foil roll (aluminum on both sides with a foam core that is 5 mm thick) and I stapled it directly to.the paneling. However it was still quite hot to.the touch. Is this because I lack an air gap? I had kind of..thought this method would be similar to an attic with the foil barrier installed under the roof decking so.the..foil faces the attic. In that scenario, would the attic be the air.gap? And does this not.work in the Motorhome because I’m in the “air gap” which is I guess my living space?
      Also I did not cover the foil barrier nor do I plan to for right now. Will that.affect my radiant benefit if I remove the foil and re-install it over a 1/2 air gap of plywood furring strips?

      • Ed says:

        Harrison,
        If the foil is adhered to a layer and open on the RV side, it SHOULD be hot to the touch because the moment you touch it, you have eliminated the air gap and the heat will flow via conduction right into your hand. This is why the air gap is critical. A better way to test it would be to hover your hand right BELOW the foil and you should not feel the heat radiating like you would if the foil was not there. Truthfully it’s probably best if the foil is closest to the outermost layer, but it will still work to prevent heat from emitting into the space below the foil. Keep in mind that the total assembly will probably be a little warmer since it does not have the ability to vent some of the heat and since you have traditional insulation in the roof already and it hold heat well. In this case, you’re just aiming to get the best results you can with what you have to work with. That far down into the RV, I don’t know that adding the furring strips is going to give you any different of a result. It will just change the way in which the foil blocks the heat, but the outcome (97% rejection) is the same.

    6. nick says:

      doing addition. radiant barrier osb already installed on new const. my existing house which is about 3,000 sq. feet has slats that were filled in with more wood slats when the comp roof was done 30 years ago. my dilema is, do i remove all those slats and sheath in 5/8 radiant barrier or just put regular osb over existing slats and save about 3,000? i am putting an exhaust fan and going to blow in new insulation in the joist areas after the project is done. My other question is that I am having a vaulted ceiling in my new den. it is a big room, about 19×30 and the walls start at 11′ and it vaults to about 14′ it has 2×10 rafters and sheathed in radiant barrier osb. When insulation is put in the rafter bays is it ok if it makes contact with the underside of the radiant roof sheathing or do i need to have a air gap? thanks

      • Ed Fritz says:

        Nick,

        First, $3000 additional for radiant barrier decking is crazy. The actual cost for radiant barrier decking is only about $2-3 per sheet more. You will need about 100 sheets for a 3000 sq. ft roof which is $200-300 in cost. There is NO additional labor to put in on. Even with a mark up I think you should get it for less than $700 additional compared to regular decking. There is labor cost to remove the slats, but it still seems high. A more cost effective method would be to spend a few hundred dollars on AtticFoil Radiant Barrier Foil and staple to the bottom of the rafters inside the attic. You will actually achieve better results than the foil decking. As for the vaulted ceiling, yes you MUST have an air space between the insulation and the foil. Without it, you will be conducting the heat directly from the deck into the insulation.

        • nick says:

          Thanks for the response. Well I guess the cost of the increased thickness in plywood and going from 7/16 over existing slats to 5/8 radiant barrier plywood is one of the increases. The cost of ripping off all the slats is about 800 bucks. So maybe it isnt 3000 but is the payoff for doing this worth it? How long until the payoff in utility costs for stripping down and doing it in the radiant plywood? I’m in so. california, summer more extreme than winters here. I decided against my wifes wishes to go with the grand sequoia weathered wood comp shingles instead of the black color for obvious reasons also. As far as the vaulted ceiling, im more confused than ever. Had a guy out and all they do is spray in foam. The stuff looks impressive like the walls of an ice chest. He says this is the way to go with four and a half inches against the bottom of my roof deck. I asked him about it contacting my radiant barrier plywood and he says they do it all the time and this method is being more recognised and accepted. it will be air tight and no condensation problems and it seals all the nooks. I would be a little nervous about it in contact with the foil back. This is a no-no to have any contact with it, which i know you know about. I still would have to run it by my inspector to have a non-ventilated ceiling. The cost is 2,400 just for this 28×19 room to do in foam. I called my guy who bid on doing my whole addition in conventional fiberglass for $1,700 and he says its a waste and i wont recoup the money. He suggested putting a high-density r-30 and venting the ridge instead of having a sealed envelope. The 4.5 inches of spray in is equal to r-30 so maybe its not worth the money, and the venting and roof ridge vent wouldnt be very costly. Unless he wants to spray 7″ and give me an r-45 value for the same money, maybe I should pass on this. What do you think? What would you do? I really appreciate your input. It is hard to get an unbiased opinion.

          • Ed Fritz says:

            Full foam encapsulation is the best way to go over the long haul. Increase energy efficiency, plus you can usually downsize the AC equipment and offset some of the expense there. It will not hurt to spray foam on the bottom of foil, you will just loose the radiant barrier benefit.

    7. nick says:

      Ed, thanks so much. I’m having the roof installed right now, and am doing conventional non-vented ridge on vaulted area. Went to the city and they dont have a problem with sealing and not venting and using spray foam. Now I will try to get the price down a little or have them spray a little thicker. thanks again

    8. Nero says:

      ed,
      i’m residing my house…replacing the old t-111, sheathing, and fiberglass roll insulation. given what i’ve read it would make sense that one could see a benefit of using foil-backed if installed as sheathing on the sides of a building as well, assuming it was installed foil-side facing the studs. if i’m correct, why don’t i see it mentioned anywhere online?
      thank you in advance for sharing your insight with us diy-ers.

    9. G DOWD says:

      ED,

      I HAVE A VAULTED ROOM INCLOUSER 4X6 RAFTERS WITH 1 7/8″ T&G SUBSTRIGHT, AND A COMP ROOF COLOR BLACK WHATS THE BEST WAY TO DEAL WITH THE TEMP AT THIS POINT, FROM THE INSIDE ,RADIANT BARRIER ,FOAM OR BAT STYLE INSULATION.

      THANKS
      GARY

    10. Steve says:

      Hi , I have 4 quotes for roofs for a small detached one story 350 Sq feet apartment. Live in Cal- mild summer and winter, summer can get hot 10 days 98 degrees remainder in 80’s . I no nothing about the materials . is it necessary to apply OSB or radiant barrier solid sheeting over existing roof deck of 1 by 8 boards, and if OSB will 30 lb roof paper help with insulation. or would you recommend going for the foil or solid radiant sheeting. We are trying to stay in budget and did not know what would be recomended. They are going to put in heating and eyebrow vents too.?

      • Ed says:

        Your best (most economical) option would be to use radiant barrier decking in the construction of the roof. This will be installed so the foil side is DOWN, facing the attic space. Then, you can build a normal roof above that. Felt paper is more for waterproofing and offers no real insulation value. If you go with the radiant barrier decking, then add your regular insulation on the attic floor, as much as you can get in there. If the apartment will not have an attic space, then you should use the Cathedral Ceiling Method to install the radiant barrier AtticFoil.

    11. Jackie says:

      I am having my home re-roofed (from wood shake to compostion tiles) and they put the radiant barrier face down on top on the rafters and remaining board – facing the attic. Is this adequate or should they have put some kind of spacers between the radiant board and the existing board/rafters??

      • Ed Fritz says:

        This is an OK method considering you are switching from wood shake to composition. You will loose the benefit where it is “sandwiched” between the existing cross lathing and the new roof deck. If no laths are removed, then you will get about 50% benefit since about half the space would still be open. I would recommend removing every other lath and then putting the foil down before the new deck. This will help with the structural strength and allow for more open space to provide a greater benefit.

        • Jackie says:

          Thank you Ed..I have talked with the roofing company and explained my concerns. I’m waiting to hear from the district manager. Unfortunately, I’m a lay person and I don’t know how serious they will take me. I’ve not only read your articles but read several others supporting the importance of the airspace underneath.

    12. Richie says:

      We live in Texas — Our garage is a two car type with a metal door .
      I intend to install 3/4 ” foam rigid panels that are aluminum foil on one side.
      Should there be at least a 3/4 air gap between the panel and the garage door . Which side of the panel should the aluminum foil be facing ? The metal door or facing the inside of the garage ?
      Is there a better alternative to improve the insulation
      thanks Richie

      • Ed says:

        Yes, the foil should have at least 3/4″ of an air gap, so if you put the foil facing the door itself, you’ll want to install the panels so they pop out a little and don’t make direct contact between the foil and the door. Complete install instructions and a video here: How to Install Radiant Barrier on a Garage Door
        Using the foil-faced foam board in the way the install page shows is a great way to insulate the door. For further improvement, consider covering the walls & the attic space above the garage with radiant barrier as well.

    13. Brian says:

      Ed,

      I have been looking at your product for a while now. I’m building my home and I’m trying to make an energy efficient home.

      I have been doing a lot of research and I am hearing mixed ways on how to properly installed a radiant barrier with a metal roof. Some people are saying that there should be an air gap in order for a radiant barrier to be affective. So if that is true that makes me think the OSB with the radiant barrier wouldn’t be an effective solution since the metal roofing would be right on top of the OSB.

      Here are the options that I have in mind.

      1 – Metal Roofing with 7/16″ Radiant barrier OSB decking with the foil side down then attic foil stapled to the bottom of the roof rafters.

      2 – Metal Roofing with regular 7/16″ OSB decking then attic foil stapled to the bottom side of the rafters.

      3 – Metal Roofing with 7/16″ Radiant barrier OSB decking and no attic foil on the rafters.

      I would like to know which one is the best option in your professional opinion.

      Thanks

    14. Keith says:

      Ed,
      In an older 1.5 story house would there be any benefit to install a radiant barrier in the upper room? If the sloped ceiling(roof deck) had already been insulated with closed cell spray foam? Even if an air gap was present between the insulation and drywall? Or would this create an even hotter roof?

      • Ed Fritz says:

        Keith, I don’t really see any big benefit to putting AtticFoil “inside” of the insulation. It’s kinda like putting foil INSIDE a refrigerator as apposed to wrapping it. In general you always want to make radiant barrier your 1st line of defense against radiant heat and “regular” insulation – foam included to be your 2nd line of defense against conductive heat. It would help SOME, but I don’t think the small additional benefit is worth the time/money effort.

    15. Travis says:

      Hello Ed,

      Thanks for the your articles. I have an older house In Austin and want to retrofit with both radiant barrier and spray foam to get a air tight seal. Obviously there are tons of conflicting articles on this. I think the way to do this is, under the sheathing go between my rafters with 1/2 inch ridged foam one side faced with radiant barrier. Radiant barrier faces up but there is a 1 inch air gap created with small 1×1 stand offs. Then I can spray foam directly under this as there would still be 4 inches of rafter space. I would block off about a foot or two before the eves and at the ridge allowing air to come in from the soffit and up and out through the ridge vent. I have read several articles that said if the radiant barrier faces up with the 1″ air gap the RADIANT heat wouldn’t be reflected but conducted through the ridged foam board and then through the foam insulation. To avoid the issue all together I thought about just using spray foam directly on the sheathing but read this voids your roof warranty as it doesn’t allow air flow under the sheathing overheating the roof assembly. One other option would be to put full 4×8 sheets of the ridged foam backed with Radiant barrier directly onto the bottom of the rafters leaving a couple of feet at the eves and ridge then just spray foaming the attic floor. A few different option here but what are your thoughts which would you do, or would you do something different? Thank you, Travis

      • Ed Fritz says:

        You may want to consider doing this method: Radiant Barrier In A Cathedral Ceiling But, instead of adding batt insulation and sheetrock, just spray foam directly to the back of the foil and fully encapsulate the rafters by at least a couple inches. This will provide a radiant barrier, ventilated channel, and minimize thermal bypass through the rafters. You will achieve the same as your first proposal, but the foil alone is a lot easier to work with than foam board.

    16. Travis says:

      Hello Ed,
      Thank you for all the advice you and your site provides! Hopefully this question post as I tried last weekend and don’t think I was able to. I am located in Austin Texas and want to use Radiant Barrier and Spray foam. The ideal goal is to have an air seal as well as a radiant barrier at the roof line. Is the below assembly a good solution in your opinion
      Here is my thought (please let me know if you agree, disagree or would tweak in some way). There seems to be may contradicting articles on the web about this but I have looked at the science and feel I have a decent grip on the best way to accomplish it. Step one, 1”x1”x5” standoffs nailed to the rafter against the bottom of the roof sheathing. Then attach ½ in ridge foam with radiant barrier cut to fit between the rafters to the standoffs creating a 1” air gap. The foil side of the 1/2” foam is pointed towards the sheathing/roof. The ridged foam stops short of the eves and goes down to meet the attic floor. This allows air to come in from the eves up through the air gap and out through the ridge vent. Next spray foam is applied to the underside of the ridged foam 4” thick or even with the bottom of the rafters. This would seal the entire attic (where my HVAC equipment is) and provide the needed radiant barrier for maximum benefit (I think). If this is done I would pump some/minimal conditioned air in this space as well.
      Method 2 – Since some argue the foil will conduct heat and then transfer the heat through the spray foam into my attic, they prefer the ½” ridged foam to be installed on the bottom of the rafters with the foil facing towards the attic not roof. In addition, they prefer not to seal the attic and just stop the ridged foam board with Radiant barrier a foot short of the eves and roof, allowing maximum air flow. If this is the preferred method then I could spray foam the attic floor, only negative here is the HVAC equipment isn’t in a conditioned space. Maybe the Radiant barrier reduces the heat enough make this a non-issue. This seem to use less labor but maybe less effective.
      If it wasn’t feasible for you to remove the roof and decking which of these methods would you prefer or would you use a different method? Thanks in advance for any feedback.

      • Ed Fritz says:

        Travis, did you see my article on combining spray foam with a radiant barrier? In Austin, TX I would definitely go non-vented full foam attic. NO Questions! I’d like to talk to you on the phone to help further. One of the challenges of foam is air conditioning. I did my whole home in Mitsubishi Multi Port inverter units. The results are simply amazing with the foam. You need a system that will operate at part-load and provide good dehumidification. I can also offer some good advice from first hand experience and from many customers and contractors on how to combine radiant barrier with a foam enclosed attic.

    17. i am building garage with upstairs room built into trusses. so there is tiny “attic” at top, then a cathedral like sloped section, then eaves. i am using radiant barrier osb as roof sheeting. question is regarding ventilation… i intend to use some kind of thin plastic baffle/standoff to prevent my mineral wool batts from contacting the foil underside. But moist air from inside building (there is a bathroom), could possible condense on the cold osb foil. But do I care? It won’t rot because the aluminum foil will prevent moisture contact with osb! Only other way I can think to protect osb is spray closed cell foam but this would eliminate needed air gap.

      PS An old engineer friend of mine told me that if i applied tar paper over aluminum foil facing sky on roof the foil would still reflect a lot of heat because without some “thermal grease” or gooey interface between tar paper and foil there would be very little actual contact, like 90% no contact or more. but tiny air gaps. plausible? does IR still launch if air gap is tiny or is there some wavelength inhibition?

      • Ed Fritz says:

        If foil worked under shingles, every large product manufacturer would make/sell it. There may be a “tiny” difference with/without foil but it’s not worth the extra cost. We have done testing and the temps are the same with or without foil products “sandwiched” between the shingles and the deck. Trust me, no one wishes it worked more than me. I’d sell a lot of foil.

    18. Sujit Agrawal says:

      Quick question. My contractor is installing the Radiant Barrier ply on the existing roof slats which has minimum gap between them. Also we are installing solar reflective shingles.

      Is it advisable to install Radiant Barrier on top of the existing slats? Would the Radiant Barrier work or it will cook up the shingles?

      • Ed Fritz says:

        It will not cook the shingles, but wherever the foil decking has contact with the wood slats you will loose the benefit of a radiant barrier. No space = No Radiant Barrier.

    19. Travis Shepherd says:

      I’m in Southern California (climate zone 3) and doing a reroof. 75% of the ceiling is vaulted with no attic (2×6 or 2×8 rafters with drywall). Currently the rafters have R11 mineral wool and are vented with soffits to a ridge vent. Sheathing is 1″x6″ solid shiplap. It gets very hot upstairs in the summer (particularly in the south-facing room).

      Since I am redoing the roof, I also want to improve the insulation. What is the best way to reduce heat gain in the summer?

      I’m currently looking at the following (some or all, no particular order):
      -Use cool roofing shingles to reflect most energy gain
      -Create an unvented roof (seal the soffits and ridge vent) and add air membrane over sheathing
      -Replace R11 mineral wool with R23/R30 based on rafter size (2×6 and 2×8)
      -Add 2-3″ of polyiso foam over sheathing

      I feel like the cool roofing shingles and air barrier will have the most impact. Is updating the mineral wool or the rigid foam overkill?

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