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.
Tags: conductive heat, radiant barrier, radiant barrier foil, radiant heat, radiant heat energy
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
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
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?
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”.
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?
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!
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
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
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?
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.
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
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.
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.
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.
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
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.
Nero,
I don’t quite really understand what you mean by “assuming it was installed foil-side facing the studs” Take a look at this page regarding radiant barrier putting behind vinyl and hardi board type sidings?
i was referring to a foil backed OSB like (https://www.gp.com/build/product.aspx?pid=5857)
as exterior sheathing, installed with the foil side inward towards the air gap created by the studded interior wall.
my apologies, sometimes i mess up the lingo.
until again, nero
Nero,
If you’re putting the foil facing inside, toward the inside of the home, then you run the risk of eliminating the necessary air gap when you fill the cavity with insulation. Insulation doesn’t count as an air gap. To do this application properly you would need to ensure the foil doesn’t come in contact with anything for at least 3/4″. Photos and tips are on this page: How to Install Radiant Barrier Foil Inside Walls.
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
Gary,
Here is everything you need to know about Installing Radiant Barrier In A Cathedral Ceiling to keep the heat out. Video and instructions. Let me know if you need more info.